This document is the user manual for the Yasm assembler. It is intended as both an introduction and a general-purpose reference for all Yasm users.
Yasm is a BSD-licensed assembler that is designed from the ground up to allow for multiple assembler syntaxes to be supported (e.g. NASM, GNU AS, etc.) in addition to multiple output object formats and multiple instruction sets. Its modular architecture allows additional object formats, debug formats, and syntaxes to be added relatively easily.
Yasm started life in 2001 as a rewrite of the NASM (Netwide) x86 assembler under the BSD license. Since then, it has matched and exceeded NASM's capabilities, incorporating features such as supporting the 64-bit AMD64 architecture, parsing GNU AS syntax, and generating STABS, DWARF2, and CodeView 8 debugging information.
Yasm is licensed under the 2-clause and 3-clause "revised" BSD licenses, with one exception: the Bit::Vector module used by the mainline version of Yasm to implement its large integer and machine-independent floating point support is triple-licensed under the Artistic license, GPL, and LGPL. The "yasm-nextgen" codebase uses a different BSD-licensed implementation and is thus entirely under BSD-equivalent licenses. The full text of the licenses are provided in the Yasm source distribution.
This user manual is licensed under the 2-clause BSD license.
3. Material Covered in this Book
This book is intended to be a user's manual for Yasm, serving as both an introduction and a general-purpose reference. While mentions may be made in various sections of Yasm's implementation (usually to explain the reasons behind bugs or unusual aspects to various features), this book will not go into depth explaining how Yasm does its job; for an in-depth discussion of Yasm's internals, see The Design and Implementation of the Yasm Assembler.
yasm [ -f format ] [ -o outfile ] [ other options ...] {infile}
The yasm command assembles the file infile and directs output to the file outfile if specified. If outfile is not specified, yasm will derive a default output file name from the name of its input file, usually by appending .o or .obj, or by removing all extensions for a raw binary file. Failing that, the output file name will be yasm.out.
If called with an infile of "-", yasm assembles the standard input and directs output to the file outfile, or yasm.out if no outfile is specified.
If errors or warnings are discovered during execution, Yasm outputs the error message to stderr (usually the terminal). If no errors or warnings are encountered, Yasm does not output any messages.
Many options may be given in one of two forms: either a dash followed by a single letter, or two dashes followed by a long option name. Options are listed in alphabetical order.
1.3.1.1. -a arch or --arch=arch: Select target architecture
Selects the target architecture. The default architecture is "x86", which supports both the IA-32 and derivatives and AMD64 instruction sets. To print a list of available architectures to standard output, use "help" as arch. SeeSection 1.4 for a list of supported architectures.
1.3.1.2. -f format or --oformat=format: Select object format
Selects the output object format. The default object format is "bin", which is a flat format binary with no relocation. To print a list of available object formats to standard output, use "help" as format. See Section 1.6 for a list of supported object formats.
1.3.1.3. -g debug or --dformat=debug: Select debugging format
Selects the debugging format for debug information. Debugging information can be used by a debugger to associate executable code back to the source file or get data structure and type information. Available debug formats vary between different object formats; yasm will error when an invalid combination is selected. The default object format is selected by the object format. To print a list of available debugging formats to standard output, use "help" as debug. SeeSection 1.7 for a list of supported debugging formats.
1.3.1.4. -h or --help: Print a summary of options
Prints a summary of invocation options. All other options are ignored, and no output file is generated.
1.3.1.5. -L list or --lformat=list: Select list file format
Selects the format/style of the output list file. List files typically intermix the original source with the machine code generated by the assembler. The default list format is "nasm", which mimics the NASM list file format. To print a list of available list file formats to standard output, use "help" as list.
1.3.1.6. -l listfile or --list=listfile: Specify list filename
Specifies the name of the output list file. If this option is not used, no list file is generated.
1.3.1.7. -m machine or --machine=machine: Select target machine architecture
Selects the target machine architecture. Essentially a subtype of the selected architecture, the machine type selects between major subsets of an architecture. For example, for the "x86" architecture, the two available machines are"x86", which is used for the IA-32 and derivative 32-bit instruction set, and "amd64", which is used for the 64-bit instruction set. This differentiation is required to generate the proper object file for relocatable object formats such as COFF and ELF. To print a list of available machines for a given architecture to standard output, use "help" as machine and the given architecture using -a arch. See Part VI for more details.
1.3.1.8. -o filename or --objfile=filename: Specify object filename
Specifies the name of the output file, overriding any default name generated by Yasm.
1.3.1.9. -p parser or --parser=parser: Select parser
Selects the parser (the assembler syntax). The default parser is "nasm", which emulates the syntax of NASM, the Netwide Assembler. Another available parser is "gas", which emulates the syntax of GNU AS. To print a list of available parsers to standard output, use "help" as parser. See Section 1.5 for a list of supported parsers.
1.3.1.10. -r preproc or --preproc=preproc: Select preprocessor
Selects the preprocessor to use on the input file before passing it to the parser. Preprocessors often provide macro functionality that is not included in the main parser. The default preprocessor is "nasm", which is an imported version of the actual NASM preprocessor. A "raw" preprocessor is also available, which simply skips the preprocessing step, passing the input file directly to the parser. To print a list of available preprocessors to standard output, use "help" as preproc.
1.3.1.11. --version: Get the Yasm version
This option causes Yasm to prints the version number of Yasm as well as a license summary to standard output. All other options are ignored, and no output file is generated.
-W options have two contrary forms: -W?name? and -Wno-name. Only the non-default forms are shown here.
The warning options are handled in the order given on the command line, so if -w is followed by -Worphan-labels, all warnings are turned off except for orphan-labels.
1.3.2.1. -w: Inhibit all warning messages
This option causes Yasm to inhibit all warning messages. As discussed above, this option may be followed by other options to re-enable specified warnings.
1.3.2.2. -Werror: Treat warnings as errors
This option causes Yasm to treat all warnings as errors. Normally warnings do not prevent an object file from being generated and do not result in a failure exit status from yasm, whereas errors do. This option makes warnings equivalent to errors in terms of this behavior.
1.3.2.3. -Wno-unrecognized-char: Do not warn on unrecognized input characters
Causes Yasm to not warn on unrecognized characters found in the input. Normally Yasm will generate a warning for any non-ASCII character found in the input file.
1.3.2.4. -Worphan-labels: Warn on labels lacking a trailing colon
When using the NASM-compatible parser, causes Yasm to warn about labels found alone on a line without a trailing colon. While these are legal labels in NASM syntax, they may be unintentional, due to typos or macro definition ordering.
1.3.2.5. -X style: Change error/warning reporting style
Selects a specific output style for error and warning messages. The default is "gnu" style, which mimics the output of gcc. The "vc" style is also available, which mimics the output of Microsoft's Visual Studio compiler.
This option is available so that Yasm integrates more naturally into IDE environments such as Visual Studio or Emacs, allowing the IDE to correctly recognize the error/warning message as such and link back to the offending line of source code.
While these preprocessor options theoretically will affect any preprocessor, the only preprocessor currently in Yasm is the "nasm" preprocessor.
1.3.3.1. -D macro[=value]: Pre-define a macro
Pre-defines a single-line macro. The value is optional (if no value is given, the macro is still defined, but to an empty value).
1.3.3.2. -e or --preproc-only: Only preprocess
Stops assembly after the preprocessing stage; preprocessed output is sent to the specified output name or, if no output name is specified, the standard output. No object file is produced.
1.3.3.3. -I path: Add include file path
Adds directory path to the search path for include files. The search path defaults to only including the directory in which the source file resides.
1.3.3.4. -P filename: Pre-include a file
Pre-includes file filename, making it look as though filename was prepended to the input. Can be useful for prepending multi-line macros that the -D can't support.
1.3.3.5. -U macro: Undefine a macro
Undefines a single-line macro (may be either a built-in macro or one defined earlier in the command line with -D (see Section 1.3.3.1).
1.4. Supported Target Architectures
Yasm supports the following instruction set architectures (ISAs). For more details see Part VI.
x86
The "x86" architecture supports the IA-32 instruction set and derivatives (including 16-bit and non-Intel instructions) and the AMD64 instruction set. It consists of two machines: "x86" (for the IA-32 and derivatives) and"amd64" (for the AMD64 and derivatives). The default machine for the "x86" architecture is the "x86" machine.
1.5. Supported Parsers (Syntaxes)
Yasm parses the following assembler syntaxes:
nasm
NASM syntax is the most full-featured syntax supported by Yasm. Yasm is nearly 100% compatible with NASM for 16-bit and 32-bit x86 code. Yasm additionally supports 64-bit AMD64 code with Yasm extensions to the NASM syntax. For more details see Part II.
gas
The GNU Assembler (GAS) is the de-facto cross-platform assembler for modern Unix systems, and is used as the backend for the GCC compiler. Yasm's support for GAS syntax is moderately good, although immature: not all directives are supported, and only 32-bit x86 and AMD64 architectures are supported. There is also no support for the GAS preprocessor. Despite these limitations, Yasm's GAS syntax support is good enough to handle essentially all x86 and AMD64 GCC compiler output. For more details see Part III.
Yasm supports the following object formats. More details can be found in Part IV.
bin
The "bin" object format produces a flat-format, non-relocatable binary file. It is appropriate for producing DOS .COM executables or things like boot blocks. It supports only 3 sections and those sections are written in a predefined order to the output file.
coff
The COFF object format is an older relocatable object format used on older Unix and compatible systems, and also (more recently) on the DJGPP development system for DOS.
dbg
The "dbg" object format is not a "real" object format; the output file it creates simply describes the sequence of calls made to it by Yasm and the final object and symbol table information in a human-readable text format (that in a normal object format would get processed into that object format's particular binary representation). This object format is not intended for real use, but rather for debugging Yasm's internals.
elf
The ELF object format really comes in three flavors: "elf32" (for 32-bit targets), "elf64" (for 64-bit targets), and "elfx32" (for x32 targets). ELF is a standard object format in common use on modern Unix and compatible systems (e.g. Linux, FreeBSD). ELF has complex support for relocatable and shared objects.
macho
The Mach-O object format really comes in two flavors: "macho32" (for 32-bit targets) and "macho64" (for 64-bit targets). Mach-O is used as the object format on MacOS X. As Yasm currently only supports x86 and AMD64 instruction sets, it can only generate Mach-O objects for Intel-based Macs.
rdf
The RDOFF2 object format is a simple multi-section format originally designed for NASM. It supports segment references but not WRT references. It was designed primarily for simplicity and has minimalistic headers for ease of loading and linking. A complete toolchain (linker, librarian, and loader) is distributed with NASM.
win32
The Win32 object format produces object files compatible with Microsoft compilers (such as Visual Studio) that target the 32-bit x86 Windows platform. The object format itself is an extended version of COFF.
win64
The Win64 object format produces object files compatible with Microsoft compilers that target the 64-bit "x64" Windows platform. This format is very similar to the win32 object format, but produces 64-bit objects.
xdf
The XDF object format is essentially a simplified version of COFF. It's a multi-section relocatable format that supports 64-bit physical and virtual addresses.
1.7. Supported Debugging Formats
Yasm supports generation of source-level debugging information in the following formats. More details can be found in Part V.
cv8
The CV8 debug format is used by Microsoft Visual Studio 2005 (version 8.0) and is completely undocumented, although it bears strong similarities to earlier CodeView formats. Yasm's support for the CV8 debug format is currently limited to generating assembly-level line number information (to allow some level of source-level debugging). The CV8 debug information is stored in the .debug$S and .debug$T sections of the Win64 object file.
dwarf2
The DWARF 2 debug format is a complex, well-documented standard for debugging information. It was created to overcome shortcomings in STABS, allowing for much more detailed and compact descriptions of data structures, data variable movement, and complex language structures such as in C. The debugging information is stored in sections (just like normal program sections) in the object file. Yasm supports full pass-through of DWARF2 debugging information (e.g. from a Ccompiler), and can also generate assembly-level line number information.
null
The "null" debug format is a placeholder; it adds no debugging information to the output file.
stabs
The STABS debug format is a poorly documented, semi-standard format for debugging information in COFF and ELF object files. The debugging information is stored as part of the object file's symbol table and thus is limited in complexity and scope. Despite this, STABS is a common debugging format on older Unix and compatible systems, as well as DJGPP.
Chapter 2. VSYASM - Yasm for Microsoft Visual Studio 2010
Table of Contents
2.2. Alternative Integration Steps
The build system used in Microsoft Visual Studio 2010 is based on MSBUILD, Microsoft's dedicated build management tool, a change that requires that external tools are integrated into the development environment in a new way. VSYASMhas been developed to facilitate Yasm integration with Visual Studio 2010 in a robust and efficient manner. The main difference between VSYASM and other versions is that it is capable of assembling multiple source code files given on a single command line.
When assembling a single file VSYASM behaves in the same way as the normal yasm tool. The only change in this case is that VSYASM doesn't offer the pre-process only mode.
If however the VSYASM command line includes multiple source files, any output, list and map paths given on the command line are resolved to their directory components alone and each source code file is then assembled using these directories for the relevant outputs. Before assembly starts, any non-existent directories needed for VSYASM outputs are recursively created. The assembly process itself stops if any file being assembled generates errors.
The -E file command line switch can be used to send error reports to a file, in which case this file will also include the command line used to invoke VSYASM. This provides a way to check that VSYASM is being called correctly from the controlling Visual Studio build process.
Firstly, the VSYASM executable file (vsyasm.exe) should be added to the Visual Studio directory holding the C tools. This is typically at:
C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin
Secondly, the three files--vsyasm.xml, vsyasm.props and vsyasm.targets--should be added into the project directory of the project in which VSYASM is being used (an alternative will be explained later).
Thirdly, to add Yasm support to a project after the project has been opened in the IDE, right click on the project in the solution explorer and select "Build Customisations…". If vsyasm is offered as an option in the resulting list you can then select it; if not, use the "Find Existing…" button and the resulting file dialogue to navigate to the vsyasm.targets that you put in the project directory, select it to add it to the list and then select it from the list.
Once you have done this, right clicking on the project in the solution explorer and selecting "Properties" will bring up a dialogue with a new item "Yasm Assembler" that will allow you to configure Yasm for building any assembler files added to the project.
2.2. Alternative Integration Steps
If you have many projects that use VSYASM, you can put the three files mentioned above into MSBUILD's build customisation directory which is typically at:
C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\BuildCustomizations
VSYASM will then always be available in the Build Customisations dialogue. An alternative way of doing this is to put these files in a convenient location and then add the path to this location to the "Build Customisations Search Path" item under "VC++ Project Settings" in the Visual Studio 2010 Options dialogue.
In a Visual Studio project with assembler source code files, Yasm settings are entered in the "Yasm Assembler" item in the projects Property Dialogue. The items available correspond with those available on Yasm's command line and are mostly self explanatory but one item--"Object Filename"--does need further explanation.
If the "Object Filename" item refers to a directory (the default), MSBUILD will collect all the assembler files in the project together as a batch and invoke VSYASM in multiple file mode. In order to assemble files one at a time it is necessary to change this to the name of an output file such as, for example, "$(IntDir)%(Filename).obj".
The chapters in this part of the book document the NASM-compatible syntax accepted by the Yasm "nasm" parser and preprocessor.
3.1. Layout of a NASM Source Line
Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see Chapter 5) some combination of the four fields
label: instruction operands ; comment
As usual, most of these fields are optional; the presence or absence of any combination of a label, an instruction and a comment is allowed. Of course, the operand field is either required or forbidden by the presence and nature of the instruction field.
NASM uses backslash (\) as the line continuation character; if a line ends with backslash, the next line is considered to be a part of the backslash-ended line.
NASM places no restrictions on white space within a line: labels may have white space before them, or instructions may have no space before them, or anything. The colon after a label is also optional. Note that this means that if you intend to code lodsb alone on a line, and type lodab by accident, then that's still a valid source line which does nothing but define a label. Running NASM with the command-line option -w+orphan-labels will cause it to warn you if you define a label alone on a line without a trailing colon.
Valid characters in labels are letters, numbers, _, $, #, @, ~, ., and ?. The only characters which may be used as the first character of an identifier are letters, . (with special meaning: see Section 3.9), _ and ?. An identifier may also be prefixed with a $ to indicate that it is intended to be read as an identifier and not a reserved word; thus, if some other module you are linking with defines a symbol called eax, you can refer to $eax in NASM code to distinguish the symbol from the register.
The instruction field may contain any machine instruction: Pentium and P6 instructions, FPU instructions, MMX instructions and even undocumented instructions are all supported. The instruction may be prefixed by LOCK, REP, REPE/REPZ orREPNE/REPNZ, in the usual way. Explicit address-size and operand-size prefixes A16, A32, O16 and O32 are provided. You can also use the name of a segment register as an instruction prefix: coding es mov [bx],ax is equivalent to coding mov [es:bx],ax. We recommend the latter syntax, since it is consistent with other syntactic features of the language, but for instructions such as LODSB, which has no operands and yet can require a segment override, there is no clean syntactic way to proceed apart from es lodsb.
An instruction is not required to use a prefix: prefixes such as CS, A32, LOCK or REPE can appear on a line by themselves, and NASM will just generate the prefix bytes.
In addition to actual machine instructions, NASM also supports a number of pseudo-instructions, described in Section 3.2.
Instruction operands may take a number of forms: they can be registers, described simply by the register name (e.g. AX, BP, EBX, CR0): NASM does not use the gas-style syntax in which register names must be prefixed by a % sign), or they can be effective addresses (see Section 3.3), constants (Section 3.5) or expressions (Section 3.6).
fadd st1 ; this sets st0 := st0 + st1
fadd st0, st1 ; so does this
fadd st1, st0 ; this sets st1 := st1 + st0
fadd to st1 ; so does this
Almost any floating-point instruction that references memory must use one of the prefixes DWORD, QWORD, TWORD, DDQWORD, or OWORD to indicate what size of ((memory operand)) it refers to.
Pseudo-instructions are things which, though not real x86 machine instructions, are used in the instruction field anyway because that's the most convenient place to put them. The current pseudo-instructions are DB, DW, DD, DQ, DT, DDQ,DO, their uninitialized counterparts RESB, RESW, RESD, RESQ, REST, RESDDQ, and RESO, the INCBIN command, the EQU command, and the TIMES prefix.
3.2.1. DB and Friends: Declaring Initialized Data
DB, DW, DD, DQ, DT, DDQ, and DO are used to declare initialized data in the output file. They can be invoked in a wide range of ways:
db 0x55 ; just the byte 0x55
db 0x55,0x56,0x57 ; three bytes in succession
db 'a',0x55 ; character constants are OK
db 'hello',13,10,'$' ; so are string constants
dw 0x1234 ; 0x34 0x12
dw 'a' ; 0x41 0x00 (it's just a number)
dw 'ab' ; 0x41 0x42 (character constant)
dw 'abc' ; 0x41 0x42 0x43 0x00 (string)
dd 0x12345678 ; 0x78 0x56 0x34 0x12
dq 0x1122334455667788 ; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11
ddq 0x112233445566778899aabbccddeeff00
; 0x00 0xff 0xee 0xdd 0xcc 0xbb 0xaa 0x99
; 0x88 0x77 0x66 0x55 0x44 0x33 0x22 0x11
do 0x112233445566778899aabbccddeeff00 ; same as previous
dd 1.234567e20 ; floating-point constant
dq 1.234567e20 ; double-precision float
dt 1.234567e20 ; extended-precision float
DT does not accept numeric constants as operands, and DDQ does not accept float constants as operands. Any size larger than DD does not accept strings as operands.
3.2.2. RESB and Friends: Declaring Uninitialized Data
RESB, RESW, RESD, RESQ, REST, RESDQ, and RESO are designed to be used in the BSS section of a module: they declare uninitialised storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. NASM does not support the MASM/TASM syntax of reserving uninitialised space by writing DW ? or similar things: this is what it does instead. The operand to a RESB-type pseudo-instruction is a critical expression: seeSection 3.8.
For example:
buffer: resb 64 ; reserve 64 bytes
wordvar: resw 1 ; reserve a word
realarray resq 10 ; array of ten reals
3.2.3. INCBIN: Including External Binary Files
INCBIN includes a binary file verbatim into the output file. This can be handy for (for example) including graphics and sound data directly into a game executable file. However, it is recommended to use this for only small pieces of data. It can be called in one of these three ways:
incbin "file.dat" ; include the whole file
incbin "file.dat",1024 ; skip the first 1024 bytes
incbin "file.dat",1024,512 ; skip the first 1024, and
; actually include at most 512
3.2.4. EQU: Defining Constants
EQU defines a symbol to a given constant value: when EQU is used, the source line must contain a label. The action of EQU is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example,
message db 'hello, world'
msglen equ $-message
defines msglen to be the constant 12. msglen may not then be redefined later. This is not a preprocessor definition either: the value of msglen is evaluated once, using the value of $ (see Section 3.6 for an explanation of $) at the point of definition, rather than being evaluated wherever it is referenced and using the value of $ at the point of reference. Note that the operand to an EQU is also a critical expression (Section 3.8).
3.2.5. TIMES: Repeating Instructions or Data
The TIMES prefix causes the instruction to be assembled multiple times. This is partly present as NASM's equivalent of the DUP syntax supported by MASM-compatible assemblers, in that you can code
zerobuf: times 64 db 0
or similar things; but TIMES is more versatile than that. The argument to TIMES is not just a numeric constant, but a numeric expression, so you can do things like
buffer: db 'hello, world'
times 64-$+buffer db ' '
which will store exactly enough spaces to make the total length of buffer up to 64. Finally, TIMES can be applied to ordinary instructions, so you can code trivial unrolled loops in it:
times 100 movsb
Note that there is no effective difference between times 100 resb 1 and resb 100, except that the latter will be assembled about 100 times faster due to the internal structure of the assembler.
The operand to TIMES, like that of EQU and those of RESB and friends, is a critical expression (Section 3.8).
Note also that TIMES can't be applied to macros: the reason for this is that TIMES is processed after the macro phase, which allows the argument to TIMES to contain expressions such as 64-$+buffer as above. To repeat more than one line of code, or a complex macro, use the preprocessor %rep directive.
An effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. For example:
wordvar dw 123
mov ax,[wordvar]
mov ax,[wordvar+1]
mov ax,[es:wordvar+bx]
Anything not conforming to this simple system is not a valid memory reference in NASM, for example es:wordvar[bx].
More complicated effective addresses, such as those involving more than one register, work in exactly the same way:
mov eax,[ebx*2+ecx+offset]
mov ax,[bp+di+8]
NASM is capable of doing algebra on these effective addresses, so that things which don't necessarily look legal are perfectly all right:
mov eax,[ebx*5] ; assembles as [ebx*4+ebx]
mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]
Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses [eax*2+0] and [eax+eax], and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset.
NASM has a hinting mechanism which will cause [eax+ebx] and [ebx+eax] to generate different opcodes; this is occasionally useful because [esi+ebp] and [ebp+esi] have different default segment registers.
However, you can force NASM to generate an effective address in a particular form by the use of the keywords BYTE, WORD, DWORD and NOSPLIT. If you need [eax+3] to be assembled using a double-word offset field instead of the one byte NASM will normally generate, you can code [dword eax+3]. Similarly, you can force NASM to use a byte offset for a small value which it hasn't seen on the first pass (see Section 3.8 for an example of such a code fragment) by using [byte eax+offset]. As special cases, [byte eax] will code [eax+0] with a byte offset of zero, and [dword eax] will code it with a double-word offset of zero. The normal form, [eax], will be coded with no offset field.
The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don't specify that it is a dword offset, NASM will cause the high word of the offset to be lost.
Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to be absent and space to be saved; in fact, it will also split [eax*2+offset] into [eax+eax+offset]. You can combat this behaviour by the use of theNOSPLIT keyword: [nosplit eax*2] will force [eax*2+0] to be generated literally.
In BITS 64 mode, displacements, for the most part, remain 32 bits and are sign extended prior to use. The exception is one restricted form of the mov instruction: between an AL, AX, EAX, or RAX register and a 64-bit absolute address (no registers are allowed in the effective address, and the address cannot be RIP-relative). In NASM syntax, use of the 64-bit absolute form requires QWORD. Examples in NASM syntax:
mov eax, [1] ; 32 bit, with sign extension
mov al, [rax-1] ; 32 bit, with sign extension
mov al, [qword 0x1122334455667788] ; 64-bit absolute
mov al, [0x1122334455667788] ; truncated to 32-bit (warning)
3.3.2. RIP Relative Addressing
In 64-bit mode, a new form of effective addressing is available to make it easier to write position-independent code. Any memory reference may be made RIP relative (RIP is the instruction pointer register, which contains the address of the location immediately following the current instruction).
In NASM syntax, there are two ways to specify RIP-relative addressing:
mov dword [rip+10], 1
can also be a symbolic constant, and will be treated the same way. On the other hand,
mov dword [symb wrt rip], 1
stores the value 1 into the address of symbol symb. This is distinctly different than the behavior of:
mov dword [symb+rip], 1
which takes the address of the end of the instruction, adds the address of symb to it, then stores the value 1 there. If symb is a variable, this will not store the value 1 into the symb variable!
Yasm also supports the following syntax for RIP-relative addressing. The REL keyword makes it produce RIP-relative addresses, while the ABS keyword makes it produce non-RIP-relative addresses:
mov [rel sym], rax ; RIP-relative
mov [abs sym], rax ; not RIP-relative
The behavior of mov [sym], rax depends on a mode set by the DEFAULT directive (see Section 5.2), as follows. The default mode at Yasm start-up is always ABS, and in REL mode, use of registers, a FS or GS segment override, or an explicit ABSoverride will result in a non-RIP-relative effective address.
default rel
mov [sym], rbx ; RIP-relative
mov [abs sym], rbx ; not RIP-relative (explicit override)
mov [rbx+1], rbx ; not RIP-relative (register use)
mov [fs:sym], rbx ; not RIP-relative (fs or gs use)
mov [ds:sym], rbx ; RIP-relative (segment, but not fs or gs)
mov [rel sym], rbx ; RIP-relative (redundant override)
default abs
mov [sym], rbx ; not RIP-relative
mov [abs sym], rbx ; not RIP-relative
mov [rbx+1], rbx ; not RIP-relative
mov [fs:sym], rbx ; not RIP-relative
mov [ds:sym], rbx ; not RIP-relative
mov [rel sym], rbx ; RIP-relative (explicit override)
Immediate operands in NASM may be 8 bits, 16 bits, 32 bits, and even 64 bits in size. The immediate size can be directly specified through the use of the BYTE, WORD, or DWORD keywords, respectively.
64 bit immediate operands are limited to direct 64-bit register move instructions in BITS 64 mode. For all other instructions in 64-bit mode, immediate values remain 32 bits; their value is sign-extended into the upper 32 bits of the target register prior to being used. The exception is the mov instruction, which can take a 64-bit immediate when the destination is a 64-bit register.
All unsized immediate values in BITS 64 in Yasm default to 32-bit size for consistency. In order to get a 64-bit immediate with a label, specify the size explicitly with the QWORD keyword. For ease of use, Yasm will also try to recognize 64-bit values and change the size to 64 bits automatically for these cases.
Examples in NASM syntax:
add rax, 1 ; optimized down to signed 8-bit
add rax, dword 1 ; force size to 32-bit
add rax, 0xffffffff ; sign-extended 32-bit
add rax, -1 ; same as above
add rax, 0xffffffffffffffff ; truncated to 32-bit (warning)
mov eax, 1 ; 5 byte
mov rax, 1 ; 5 byte (optimized to signed 32-bit)
mov rax, qword 1 ; 10 byte (forced 64-bit)
mov rbx, 0x1234567890abcdef ; 10 byte
mov rcx, 0xffffffff ; 10 byte (does not fit in signed 32-bit)
mov ecx, -1 ; 5 byte, equivalent to above
mov rcx, sym ; 5 byte, 32-bit size default for symbols
mov rcx, qword sym ; 10 byte, override default size
A caution for users using both Yasm and NASM 2.x: the handling of mov reg64, unsized immediate is different between Yasm and NASM 2.x; YASM follows the above behavior, while NASM 2.x does the following:
add rax, 0xffffffff ; sign-extended 32-bit immediate
add rax, -1 ; same as above
add rax, 0xffffffffffffffff ; truncated 32-bit (warning)
add rax, sym ; sign-extended 32-bit immediate
mov eax, 1 ; 5 byte (32-bit immediate)
mov rax, 1 ; 10 byte (64-bit immediate)
mov rbx, 0x1234567890abcdef ; 10 byte instruction
mov rcx, 0xffffffff ; 10 byte instruction
mov ecx, -1 ; 5 byte, equivalent to above
mov ecx, sym ; 5 byte (32-bit immediate)
mov rcx, sym ; 10 byte (64-bit immediate)
mov rcx, qword sym ; 10 byte, same as above
NASM understands four different types of constant: numeric, character, string and floating-point.
A numeric constant is simply a number. NASM allows you to specify numbers in a variety of number bases, in a variety of ways: you can suffix H, Q or O, and B for hex, octal, and binary, or you can prefix 0x for hex in the style of C, or you can prefix $ for hex in the style of Borland Pascal. Note, though, that the $ prefix does double duty as a prefix on identifiers (see Section 3.1), so a hex number prefixed with a $ sign must have a digit after the $ rather than a letter.
Some examples:
mov ax,100 ; decimal
mov ax,0a2h ; hex
mov ax,$0a2 ; hex again: the 0 is required
mov ax,0xa2 ; hex yet again
mov ax,777q ; octal
mov ax,777o ; octal again
mov ax,10010011b ; binary
A character constant consists of up to four characters enclosed in either single or double quotes. The type of quote makes no difference to NASM, except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa.
A character constant with more than one character will be arranged with little-endian order in mind: if you code
mov eax,'abcd'
then the constant generated is not 0x61626364, but 0x64636261, so that if you were then to store the value into memory, it would read abcd rather than dcba. This is also the sense of character constants understood by the Pentium's CPUIDinstruction.
String constants are only acceptable to some pseudo-instructions, namely the DB family and INCBIN.
A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent:
db 'hello' ; string constant
db 'h','e','l','l','o' ; equivalent character constants
And the following are also equivalent:
dd 'ninechars' ; doubleword string constant
dd 'nine','char','s' ; becomes three doublewords
db 'ninechars',0,0,0 ; and really looks like this
Note that when used as an operand to db, a constant like 'ab' is treated as a string constant despite being short enough to be a character constant, because otherwise db 'ab' would have the same effect as db 'a', which would be silly. Similarly, three-character or four-character constants are treated as strings when they are operands to dw.
3.5.4. Floating-Point Constants
Floating-point constants are acceptable only as arguments to DW, DD, DQ and DT. They are expressed in the traditional form: digits, then a period, then optionally more digits, then optionally an E followed by an exponent. The period is mandatory, so that NASM can distinguish between dd 1, which declares an integer constant, and dd 1.0 which declares a floating-point constant.
Some examples:
dw -0.5 ; IEEE half precision
dd 1.2 ; an easy one
dq 1.e10 ; 10,000,000,000
dq 1.e+10 ; synonymous with 1.e10
dq 1.e-10 ; 0.000 000 000 1
dt 3.141592653589793238462 ; pi
NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.
Expressions in NASM are similar in syntax to those in C.
NASM does not guarantee the size of the integers used to evaluate expressions at compile time: since NASM can compile and run on 64-bit systems quite happily, don't assume that expressions are evaluated in 32-bit registers and so try to make deliberate use of ((integer overflow)). It might not always work. The only thing NASM will guarantee is what's guaranteed by ANSI C: you always have at least 32 bits to work in.
NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$).
The arithmetic operators provided by NASM are listed here, in increasing order of precedence.
The | operator gives a bitwise OR, exactly as performed by the OR machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.
3.6.2. ^: Bitwise XOR Operator
^ provides the bitwise XOR operation.
3.6.3. &: Bitwise AND Operator
& provides the bitwise AND operation.
3.6.4. << and >>: Bit Shift Operators
<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.
3.6.5. + and -: Addition and Subtraction Operators
The + and - operators do perfectly ordinary addition and subtraction.
3.6.6. *, /, //, % and %%: Multiplication and Division
* is the multiplication operator. / and // are both division operators: / is unsigned division and // is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.
NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.
Since the % character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.
3.6.7. Unary Operators: +, -, ~ and SEG
The highest-priority operators in NASM's expression grammar are those which only apply to one argument. - negates its operand, + does nothing (it's provided for symmetry with -), ~ computes the one's complement of its operand, andSEG provides the segment address of its operand (explained in more detail in Section 3.6.8).
When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the SEG operator to perform this function.
The SEG operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code
mov ax, seg symbol
mov es, ax
mov bx, symbol
will load es:bx with a valid pointer to the symbol symbol.
Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the WRT(With Reference To) keyword. So you can do things like
mov ax, weird_seg ; weird_seg is a segment base
mov es, ax
mov bx, symbol wrt weird_seg
to load es:bx with a different, but functionally equivalent, pointer to the symbol symbol.
NASM supports far (inter-segment) calls and jumps by means of the syntax call segment:offset, where segment and offset both represent immediate values. So to call a far procedure, you could code either of
call (seg procedure):procedure
call weird_seg:(procedure wrt weird_seg)
(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)
NASM supports the syntax call far procedure as a synonym for the first of the above usages. JMP works identically to CALL in these examples.
To declare a far pointer to a data item in a data segment, you must code
dw symbol, seg symbol
NASM supports no convenient synonym for this, though you can always invent one using the macro processor.
3.7. STRICT: Inhibiting Optimization
When assembling with the optimizer set to level 2 or higher, NASM will use size specifiers (BYTE, WORD, DWORD, QWORD, or TWORD), but will give them the smallest possible size. The keyword STRICT can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,
push dword 33
is encoded in three bytes 66 6A 21, whereas
push strict dword 33
is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00.
A limitation of NASM is that it is a two-pass assembler; unlike TASM and others, it will always do exactly two assembly passes. Therefore it is unable to cope with source files that are complex enough to require three or more passes.
The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,
times (label-$) db 0
label: db 'Where am I?'
The argument to TIMES in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the TIMES line when it first sees it. It will just as firmly reject the slightlyparadoxical code
times (label-$+1) db 0
label: db 'NOW where am I?'
in which any value for the TIMES argument is by definition wrong!
NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the TIMES prefix is a critical expression; for the same reason, the arguments to the RESB family of pseudo-instructions are also critical expressions.
Critical expressions can crop up in other contexts as well: consider the following code.
mov ax, symbol1
symbol1 equ symbol2
symbol2:
On the first pass, NASM cannot determine the value of symbol1, because symbol1 is defined to be equal to symbol2 which NASM hasn't seen yet. On the second pass, therefore, when it encounters the line mov ax,symbol1, it is unable to generate the code for it because it still doesn't know the value of symbol1. On the next line, it would see the EQU again and be able to determine the value of symbol1, but by then it would be too late.
NASM avoids this problem by defining the right-hand side of an EQU statement to be a critical expression, so the definition of symbol1 would be rejected in the first pass.
There is a related issue involving forward references: consider this code fragment.
mov eax, [ebx+offset]
offset equ 10
NASM, on pass one, must calculate the size of the instruction mov eax,[ebx+offset] without knowing the value of offset. It has no way of knowing that offset is small enough to fit into a one-byte offset field and that it could therefore get away with generating a shorter form of the effective-address encoding; for all it knows, in pass one, offset could be a symbol in the code segment, and it might need the full four-byte form. So it is forced to compute the size of the instruction to accommodate a four-byte address part. In pass two, having made this decision, it is now forced to honour it and keep the instruction large, so the code generated in this case is not as small as it could have been. This problem can be solved by defining offset before using it, or by forcing byte size in the effective address by coding [byte ebx+offset].
NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:
label1 ; some code
.loop ; some more code
jne .loop
ret
label2 ; some code
.loop ; some more code
jne .loop
ret
In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.
NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write
label3 ; some more code
; and some more
jmp label1.loop
Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code
label1: ; a non-local label
.local: ; this is really label1.local
..@foo: ; this is a special symbol
label2: ; another non-local label
.local: ; this is really label2.local
jmp ..@foo ; this will jump three lines up
NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format.
Chapter 4. The NASM Preprocessor
Table of Contents
4.1.1. The Normal Way: %define
4.1.2. Enhancing %define: %xdefine
4.1.3. Concatenating Single Line Macro Tokens: %+
4.1.4. Undefining macros: %undef
4.1.5. Preprocessor Variables: %assign
4.2. String Handling in Macros
4.3.1. Overloading Multi-Line Macros
4.3.3. Greedy Macro Parameters
4.3.4. Default Macro Parameters
4.3.5. %0: Macro Parameter Counter
4.3.6. %rotate: Rotating Macro Parameters
4.3.7. Concatenating Macro Parameters
4.3.8. Condition Codes as Macro Parameters
4.3.9. Disabling Listing Expansion
4.4.1. %ifdef: Testing Single-Line Macro Existence
4.4.2. %ifmacro: Testing Multi-Line Macro Existence
4.4.3. %ifctx: Testing the Context Stack
4.4.4. %if: Testing Arbitrary Numeric Expressions
4.4.5. %ifidn and %ifidni: Testing Exact Text Identity
4.4.6. %ifid, %ifnum, %ifstr: Testing Token Types
4.4.7. %error: Reporting User-Defined Errors
4.7.1. %push and %pop: Creating and Removing Contexts
4.7.3. Context-Local Single-Line Macros
4.7.4. %repl: Renaming a Context
4.7.5. Example Use of the Context Stack: Block IFs
4.8.1. __YASM_MAJOR__, etc: Yasm Version
4.8.2. __FILE__ and __LINE__: File Name and Line Number
4.8.3. __YASM_OBJFMT__ and __OUTPUT_FORMAT__: Output Object Format Keyword
4.8.4. STRUC and ENDSTRUC: Declaring Structure Data Types
4.8.5. ISTRUC, AT and IEND: Declaring Instances of Structures
4.8.6. ALIGN and ALIGNB: Data Alignment
The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:
%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \
THIS_VALUE
will work like a single-line macro without the backslash-newline sequence.
4.1.1. The Normal Way: %define
Single-line macros are defined using the %define preprocessor directive. The definitions work in a similar way to C; so you can do things like
%define ctrl 0x1F &
%define param(a,b) ((a)+(a)*(b))
mov byte [param(2,ebx)], ctrl 'D'
which will expand to
mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
When the expansion of a single-line macro contains tokens which invoke another macro, the expansion is performed at invocation time, not at definition time. Thus the code
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
will evaluate in the expected way to mov ax,1+2*8, even though the macro b wasn't defined at the time of definition of a.
Macros defined with %define are case sensitive: after %define foo bar, only foo will expand to bar: Foo or FOO will not. By using %idefine instead of %define (the "i" stands for "insensitive") you can define all the case variants of a macro at once, so that %idefine foo bar would cause foo, Foo, FOO, fOO and so on all to expand to bar.
There is a mechanism which detects when a macro call has occurred as a result of a previous expansion of the same macro, to guard against circular references and infinite loops. If this happens, the preprocessor will only expand the first occurrence of the macro. Hence, if you code
%define a(x) 1+a(x)
mov ax,a(3)
the macro a(3) will expand once, becoming 1+a(3), and will then expand no further. This behaviour can be useful.
You can overload single-line macros: if you write
%define foo(x) 1+x
%define foo(x,y) 1+x*y
the preprocessor will be able to handle both types of macro call, by counting the parameters you pass; so foo(3) will become 1+3 whereas foo(ebx,2) will become 1+ebx*2. However, if you define
%define foo bar
then no other definition of foo will be accepted: a macro with no parameters prohibits the definition of the same name as a macro with parameters, and vice versa.
This doesn't prevent single-line macros being redefined: you can perfectly well define a macro with
%define foo bar
and then re-define it later in the same source file with
%define foo baz
Then everywhere the macro foo is invoked, it will be expanded according to the most recent definition. This is particularly useful when defining single-line macros with %assign (see Section 4.1.5).
You can pre-define single-line macros using the "-D" option on the Yasm command line: see Section 1.3.3.1.
4.1.2. Enhancing %define: %xdefine
To have a reference to an embedded single-line macro resolved at the time that it is embedded, as opposed to when the calling macro is expanded, you need a different mechanism to the one offered by %define. The solution is to use%xdefine, or its case-insensitive counterpart %xidefine.
Suppose you have the following code:
%define isTrue 1
%define isFalse isTrue
%define isTrue 0
val1: db isFalse
%define isTrue 1
val2: db isFalse
In this case, val1 is equal to 0, and val2 is equal to 1. This is because, when a single-line macro is defined using %define, it is expanded only when it is called. As isFalse expands to isTrue, the expansion will be the current value ofisTrue. The first time it is called that is 0, and the second time it is 1.
If you wanted isFalse to expand to the value assigned to the embedded macro isTrue at the time that isFalse was defined, you need to change the above code to use %xdefine.
%xdefine isTrue 1
%xdefine isFalse isTrue
%xdefine isTrue 0
val1: db isFalse
%xdefine isTrue 1
val2: db isFalse
Now, each time that isFalse is called, it expands to 1, as that is what the embedded macro isTrue expanded to at the time that isFalse was defined.
4.1.3. Concatenating Single Line Macro Tokens: %+
Individual tokens in single line macros can be concatenated, to produce longer tokens for later processing. This can be useful if there are several similar macros that perform similar functions.
As an example, consider the following:
%define BDASTART 400h ; Start of BIOS data area
struc tBIOSDA ; its structure
.COM1addr RESW 1
.COM2addr RESW 1
; ..and so on
endstruc
Now, if we need to access the elements of tBIOSDA in different places, we can end up with:
mov ax,BDASTART + tBIOSDA.COM1addr
mov bx,BDASTART + tBIOSDA.COM2addr
This will become pretty ugly (and tedious) if used in many places, and can be reduced in size significantly by using the following macro:
; Macro to access BIOS variables by their names (from tBDA):
%define BDA(x) BDASTART + tBIOSDA. %+ x
Now the above code can be written as:
mov ax,BDA(COM1addr)
mov bx,BDA(COM2addr)
Using this feature, we can simplify references to a lot of macros (and, in turn, reduce typing errors).
4.1.4. Undefining macros: %undef
Single-line macros can be removed with the %undef command. For example, the following sequence:
%define foo bar
%undef foo
mov eax, foo
will expand to the instruction mov eax, foo, since after %undef the macro foo is no longer defined.
Macros that would otherwise be pre-defined can be undefined on the command-line using the "-U" option on the Yasm command line: see Section 1.3.3.5.
4.1.5. Preprocessor Variables: %assign
An alternative way to define single-line macros is by means of the %assign command (and its case-insensitive counterpart %iassign, which differs from %assign in exactly the same way that %idefine differs from %define).
%assign is used to define single-line macros which take no parameters and have a numeric value. This value can be specified in the form of an expression, and it will be evaluated once, when the %assign directive is processed.
Like %define, macros defined using %assign can be re-defined later, so you can do things like
%assign i i+1
to increment the numeric value of a macro.
%assign is useful for controlling the termination of %rep preprocessor loops: see Section 4.5 for an example of this.
The expression passed to %assign is a critical expression (see Section 3.8), and must also evaluate to a pure number (rather than a relocatable reference such as a code or data address, or anything involving a register).
4.2. String Handling in Macros
It's often useful to be able to handle strings in macros. NASM supports two simple string handling macro operators from which more complex operations can be constructed.
The %strlen macro is like %assign macro in that it creates (or redefines) a numeric value to a macro. The difference is that with %strlen, the numeric value is the length of a string. An example of the use of this would be:
%strlen charcnt 'my string'
In this example, charcnt would receive the value 8, just as if an %assign had been used. In this example, 'my string' was a literal string but it could also have been a single-line macro that expands to a string, as in the following example:
%define sometext 'my string'
%strlen charcnt sometext
As in the first case, this would result in charcnt being assigned the value of 8.
Individual letters in strings can be extracted using %substr. An example of its use is probably more useful than the description:
%substr mychar 'xyz' 1 ; equivalent to %define mychar 'x'
%substr mychar 'xyz' 2 ; equivalent to %define mychar 'y'
%substr mychar 'xyz' 3 ; equivalent to %define mychar 'z'
In this example, mychar gets the value of 'y'. As with %strlen (see Section 4.2.1), the first parameter is the single-line macro to be created and the second is the string. The third parameter specifies which character is to be selected. Note that the first index is 1, not 0 and the last index is equal to the value that %strlen would assign given the same string. Index values out of range result in an empty string.
Multi-line macros are much more like the type of macro seen in MASM and TASM: a multi-line macro definition in NASM looks something like this.
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
This defines a C-like function prologue as a macro: so you would invoke the macro with a call such as
myfunc: prologue 12
which would expand to the three lines of code
myfunc: push ebp
mov ebp,esp
sub esp,12
after the macro name in the %macro line defines the number of parameters the macro prologue expects to receive. The use of %1 inside the macro definition refers to the first parameter to the macro call. With a macro taking more than one parameter, subsequent parameters would be referred to as %2, %3 and so on.
Multi-line macros, like single-line macros, are case-sensitive, unless you define them using the alternative directive %imacro.
If you need to pass a comma as part of a parameter to a multi-line macro, you can do that by enclosing the entire parameter in braces. So you could code things like
%macro silly 2
%2: db %1
%endmacro
silly 'a', letter_a ; letter_a: db 'a'
silly 'ab', string_ab ; string_ab: db 'ab'
silly {13,10}, crlf ; crlf: db 13,10
4.3.1. Overloading Multi-Line Macros
As with single-line macros, multi-line macros can be overloaded by defining the same macro name several times with different numbers of parameters. This time, no exception is made for macros with no parameters at all. So you could define
%macro prologue 0
push ebp
mov ebp,esp
%endmacro
to define an alternative form of the function prologue which allocates no local stack space.
Sometimes, however, you might want to "overload" a machine instruction; for example, you might want to define
%macro push 2
push %1
push %2
%endmacro
so that you could code
push ebx ; this line is not a macro call
push eax,ecx ; but this one is
Ordinarily, NASM will give a warning for the first of the above two lines, since push is now defined to be a macro, and is being invoked with a number of parameters for which no definition has been given. The correct code will still be generated, but the assembler will give a warning. This warning can be disabled by the use of the -wno-macro-params command-line option (see Section 1.3.2).
NASM allows you to define labels within a multi-line macro definition in such a way as to make them local to the macro call: so calling the same macro multiple times will use a different label each time. You do this by prefixing %% to the label name. So you can invent an instruction which executes a RET if the Z flag is set by doing this:
%macro retz 0
jnz %%skip
ret
%%skip:
%endmacro
You can call this macro as many times as you want, and every time you call it NASM will make up a different "real" name to substitute for the label %%skip. The names NASM invents are of the form ..@2345.skip, where the number 2345 changes with every macro call. The ..@ prefix prevents macro-local labels from interfering with the local label mechanism, as described in Section 3.9. You should avoid defining your own labels in this form (the ..@ prefix, then a number, then another period) in case they interfere with macro-local labels.
4.3.3. Greedy Macro Parameters
Occasionally it is useful to define a macro which lumps its entire command line into one parameter definition, possibly after extracting one or two smaller parameters from the front. An example might be a macro to write a text string to a file in MS-DOS, where you might want to be able to write
writefile [filehandle],"hello, world",13,10
NASM allows you to define the last parameter of a macro to be greedy, meaning that if you invoke the macro with more parameters than it expects, all the spare parameters get lumped into the last defined one along with the separating commas. So if you code:
%macro writefile 2+
jmp %%endstr
%%str: db %2
%%endstr:
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
then the example call to writefile above will work as expected: the text before the first comma, [filehandle], is used as the first macro parameter and expanded when %1 is referred to, and all the subsequent text is lumped into %2 and placed after the db.
The greedy nature of the macro is indicated to NASM by the use of the + sign after the parameter count on the %macro line.
If you define a greedy macro, you are effectively telling NASM how it should expand the macro given any number of parameters from the actual number specified up to infinity; in this case, for example, NASM now knows what to do when it sees a call to writefile with 2, 3, 4 or more parameters. NASM will take this into account when overloading macros, and will not allow you to define another form of writefile taking 4 parameters (for example).
Of course, the above macro could have been implemented as a non-greedy macro, in which case the call to it would have had to look like
writefile [filehandle], {"hello, world",13,10}
NASM provides both mechanisms for putting ((commas in macro parameters)), and you choose which one you prefer for each macro definition.
See Section 5.3.3 for a better way to write the above macro.
4.3.4. Default Macro Parameters
NASM also allows you to define a multi-line macro with a range of allowable parameter counts. If you do this, you can specify defaults for omitted parameters. So, for example:
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
This macro (which makes use of the writefile macro defined in Section 4.3.3) can be called with an explicit error message, which it will display on the error output stream before exiting, or it can be called with no parameters, in which case it will use the default error message supplied in the macro definition.
In general, you supply a minimum and maximum number of parameters for a macro of this type; the minimum number of parameters are then required in the macro call, and then you provide defaults for the optional ones. So if a macro definition began with the line
%macro foobar 1-3 eax,[ebx+2]
then it could be called with between one and three parameters, and %1 would always be taken from the macro call. %2, if not specified by the macro call, would default to eax, and %3 if not specified would default to [ebx+2].
You may omit parameter defaults from the macro definition, in which case the parameter default is taken to be blank. This can be useful for macros which can take a variable number of parameters, since the %0 token (see Section 4.3.5) allows you to determine how many parameters were really passed to the macro call.
This defaulting mechanism can be combined with the greedy-parameter mechanism; so the die macro above could be made more powerful, and more useful, by changing the first line of the definition to
%macro die 0-1+ "Painful program death has occurred.",13,10
The maximum parameter count can be infinite, denoted by *. In this case, of course, it is impossible to provide a full set of default parameters. Examples of this usage are shown in Section 4.3.6.
4.3.5. %0: Macro Parameter Counter
For a macro which can take a variable number of parameters, the parameter reference %0 will return a numeric constant giving the number of parameters passed to the macro. This can be used as an argument to %rep (see Section 4.5) in order to iterate through all the parameters of a macro. Examples are given in Section 4.3.6.
4.3.6. %rotate: Rotating Macro Parameters
Unix shell programmers will be familiar with the shift shell command, which allows the arguments passed to a shell script (referenced as $1, $2 and so on) to be moved left by one place, so that the argument previously referenced as $2becomes available as $1, and the argument previously referenced as $1 is no longer available at all.
NASM provides a similar mechanism, in the form of %rotate. As its name suggests, it differs from the Unix shift in that no parameters are lost: parameters rotated off the left end of the argument list reappear on the right, and vice versa.
%rotate is invoked with a single numeric argument (which may be an expression). The macro parameters are rotated to the left by that many places. If the argument to %rotate is negative, the macro parameters are rotated to the right.
So a pair of macros to save and restore a set of registers might work as follows:
%macro multipush 1-*
%rep %0
push %1
%rotate 1
%endrep
%endmacro
This macro invokes the PUSH instruction on each of its arguments in turn, from left to right. It begins by pushing its first argument, %1, then invokes %rotate to move all the arguments one place to the left, so that the original second argument is now available as %1. Repeating this procedure as many times as there were arguments (achieved by supplying %0 as the argument to %rep) causes each argument in turn to be pushed.
Note also the use of * as the maximum parameter count, indicating that there is no upper limit on the number of parameters you may supply to the multipush macro.
It would be convenient, when using this macro, to have a POP equivalent, which didn't require the arguments to be given in reverse order. Ideally, you would write the multipush macro call, then cut-and-paste the line to where the pop needed to be done, and change the name of the called macro to multipop, and the macro would take care of popping the registers in the opposite order from the one in which they were pushed.
This can be done by the following definition:
%macro multipop 1-*
%rep %0
%rotate -1
pop %1
%endrep
%endmacro
This macro begins by rotating its arguments one place to the right, so that the original last argument appears as %1. This is then popped, and the arguments are rotated right again, so the second-to-last argument becomes %1. Thus the arguments are iterated through in reverse order.
4.3.7. Concatenating Macro Parameters
NASM can concatenate macro parameters on to other text surrounding them. This allows you to declare a family of symbols, for example, in a macro definition. If, for example, you wanted to generate a table of key codes along with offsets into the table, you could code something like
%macro keytab_entry 2
keypos%1 equ $-keytab
db %2
%endmacro
keytab:
keytab_entry F1,128+1
keytab_entry F2,128+2
keytab_entry Return,13
which would expand to
keytab:
keyposF1 equ $-keytab
db 128+1
keyposF2 equ $-keytab
db 128+2
keyposReturn equ $-keytab
db 13
You can just as easily concatenate text on to the other end of a macro parameter, by writing %1foo.
(giving the number of the macro parameter) from the second (literal text to be concatenated to the parameter).
This concatenation can also be applied to other preprocessor in-line objects, such as macro-local labels (Section 4.3.2) and context-local labels (Section 4.7.2). In all cases, ambiguities in syntax can be resolved by enclosing everything after the % sign and before the literal text in braces: so %{%foo}bar concatenates the text bar to the end of the real name of the macro-local label %%foo. (This is unnecessary, since the form NASM uses for the real names of macro-local labels means that the two usages %{%foo}bar and %%foobar would both expand to the same thing anyway; nevertheless, the capability is there.)
4.3.8. Condition Codes as Macro Parameters
NASM can give special treatment to a macro parameter which contains a condition code. For a start, you can refer to the macro parameter %1 by means of the alternative syntax %+1, which informs NASM that this macro parameter is supposed to contain a condition code, and will cause the preprocessor to report an error message if the macro is called with a parameter which is not a valid condition code.
Far more usefully, though, you can refer to the macro parameter by means of %-1, which NASM will expand as the inverse condition code. So the retz macro defined in Section 4.3.2 can be replaced by a general conditional-return macro like this:
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
This macro can now be invoked using calls like retc ne, which will cause the conditional-jump instruction in the macro expansion to come out as JE, or retc po which will make the jump a JPE.
The %+1 macro-parameter reference is quite happy to interpret the arguments CXZ and ECXZ as valid condition codes; however, %-1 will report an error if passed either of these, because no inverse condition code exists.
4.3.9. Disabling Listing Expansion
When NASM is generating a listing file from your program, it will generally expand multi-line macros by means of writing the macro call and then listing each line of the expansion. This allows you to see which instructions in the macro expansion are generating what code; however, for some macros this clutters the listing up unnecessarily.
NASM therefore provides the .nolist qualifier, which you can include in a macro definition to inhibit the expansion of the macro in the listing file. The .nolist qualifier comes directly after the number of parameters, like this:
%macro foo 1.nolist
Or like this:
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
Similarly to the C preprocessor, NASM allows sections of a source file to be assembled only if certain conditions are met. The general syntax of this feature looks like this:
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
The %else clause is optional, as is the %elif clause. You can have more than one %elif clause as well.
4.4.1. %ifdef: Testing Single-Line Macro Existence
Beginning a conditional-assembly block with the line %ifdef MACRO will assemble the subsequent code if, and only if, a single-line macro called MACRO is defined. If not, then the %elif and %else blocks (if any) will be processed instead.
For example, when debugging a program, you might want to write code such as
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
Then you could use the command-line option -D DEBUG to create a version of the program which produced debugging messages, and remove the option to generate the final release version of the program.
You can test for a macro not being defined by using %ifndef instead of %ifdef. You can also test for macro definitions in %elif blocks by using %elifdef and %elifndef.
4.4.2. %ifmacro: Testing Multi-Line Macro Existence
The %ifmacro directive operates in the same way as the %ifdef directive, except that it checks for the existence of a multi-line macro.
For example, you may be working with a large project and not have control over the macros in a library. You may want to create a macro with one name if it doesn't already exist, and another name if one with that name does exist.
The %ifmacro is considered true if defining a macro with the given name and number of arguments would cause a definitions conflict. For example:
%ifmacro MyMacro 1-3
%error "MyMacro 1-3" causes a conflict with an existing macro.
%else
%macro MyMacro 1-3
; insert code to define the macro
%endmacro
%endif
This will create the macro MyMacro 1-3 if no macro already exists which would conflict with it, and emits a warning if there would be a definition conflict.
You can test for the macro not existing by using the %ifnmacro instead of %ifmacro. Additional tests can be performed in %elif blocks by using %elifmacro and %elifnmacro.
4.4.3. %ifctx: Testing the Context Stack
The conditional-assembly construct %ifctx ctxname will cause the subsequent code to be assembled if and only if the top context on the preprocessor's context stack has the name ctxname. As with %ifdef, the inverse and %elif forms %ifnctx,%elifctx and %elifnctx are also supported.
For more details of the context stack, see Section 4.7. For a sample use of %ifctx, see Section 4.7.5.
4.4.4. %if: Testing Arbitrary Numeric Expressions
The conditional-assembly construct %if expr will cause the subsequent code to be assembled if and only if the value of the numeric expression expr is non-zero. An example of the use of this feature is in deciding when to break out of a%rep preprocessor loop: see Section 4.5 for a detailed example.
The expression given to %if, and its counterpart %elif, is a critical expression (see Section 3.8).
%if extends the normal NASM expression syntax, by providing a set of relational operators which are not normally available in expressions. The operators =, <, >, <=, >= and <> test equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively. The C-like forms == and != are supported as alternative forms of = and <>. In addition, low-priority logical operators &&, ^^ and || are provided, supplying logical AND, logical XOR andlogical OR. These work like the C logical operators (although C has no logical XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that ^^, for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false.
4.4.5. %ifidn and %ifidni: Testing Exact Text Identity
The construct %ifidn text1,text2 will cause the subsequent code to be assembled if and only if text1 and text2, after expanding single-line macros, are identical pieces of text. Differences in white space are not counted.
%ifidni is similar to %ifidn, but is case-insensitive.
For example, the following macro pushes a register or number on the stack, and allows you to treat IP as a real register:
%macro pushparam 1
%ifidni %1,ip
call %%label
%%label:
%else
push %1
%endif
%endmacro
Like most other %if constructs, %ifidn has a counterpart %elifidn, and negative forms %ifnidn and %elifnidn. Similarly, %ifidni has counterparts %elifidni, %ifnidni and %elifnidni.
4.4.6. %ifid, %ifnum, %ifstr: Testing Token Types
Some macros will want to perform different tasks depending on whether they are passed a number, a string, or an identifier. For example, a string output macro might want to be able to cope with being passed either a string constant or a pointer to an existing string.
The conditional assembly construct %ifid, taking one parameter (which may be blank), assembles the subsequent code if and only if the first token in the parameter exists and is an identifier. %ifnum works similarly, but tests for the token being a numeric constant; %ifstr tests for it being a string.
For example, the writefile macro defined in Section 4.3.3 can be extended to take advantage of %ifstr in the following fashion:
%macro writefile 2-3+
%ifstr %2
jmp %%endstr
%if %0 = 3
%%str: db %2,%3
%else
%%str: db %2
%endif
%%endstr: mov dx,%%str
mov cx,%%endstr-%%str
%else
mov dx,%2
mov cx,%3
%endif
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
Then the writefile macro can cope with being called in either of the following two ways:
writefile [file], strpointer, length
writefile [file], "hello", 13, 10
In the first, strpointer is used as the address of an already-declared string, and length is used as its length; in the second, a string is given to the macro, which therefore declares it itself and works out the address and length for itself.
Note the use of %if inside the %ifstr: this is to detect whether the macro was passed two arguments (so the string would be a single string constant, and db %2 would be adequate) or more (in which case, all but the first two would be lumped together into %3, and db %2,%3 would be required).
The usual %elifXXX, %ifnXXX and %elifnXXX versions exist for each of %ifid, %ifnum and %ifstr.
4.4.7. %error: Reporting User-Defined Errors
The preprocessor directive %error will cause NASM to report an error if it occurs in assembled code. So if other users are going to try to assemble your source files, you can ensure that they define the right macros by means of code like this:
%ifdef SOME_MACRO
; do some setup
%elifdef SOME_OTHER_MACRO
; do some different setup
%else
%error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
%endif
Then any user who fails to understand the way your code is supposed to be assembled will be quickly warned of their mistake, rather than having to wait until the program crashes on being run and then not knowing what went wrong.
NASM's TIMES prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level: %rep.
The directives %rep and %endrep (%rep takes a numeric argument, which can be an expression; %endrep takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor:
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
This will generate a sequence of 64 INC instructions, incrementing every word of memory from [table] to [table+126].
For more complex termination conditions, or to break out of a repeat loop part way along, you can use the %exitrep directive to terminate the loop, like this:
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
fib_number equ ($-fibonacci)/2
This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to %rep. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi-user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing.
Using, once again, a very similar syntax to the C preprocessor, the NASM preprocessor lets you include other source files into your code. This is done by the use of the %include directive:
%include "macros.mac"
will include the contents of the file macros.mac into the source file containing the %include directive.
Include files are first searched for relative to the directory containing the source file that is performing the inclusion, and then relative to any directories specified on the Yasm command line using the -I option (seeSection 1.3.3.3), in the order given on the command line (any relative paths on the Yasm command line are relative to the current working directory, e.g. where Yasm is being run from). While this search strategy does not match traditional NASM behavior, it does match the behavior of most C compilers and better handles relative pathnames.
The standard C idiom for preventing a file being included more than once is just as applicable in the NASM preprocessor: if the file macros.mac has the form
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%endif
then including the file more than once will not cause errors, because the second time the file is included nothing will happen because the macro MACROS_MAC will already be defined.
You can force a file to be included even if there is no %include directive that explicitly includes it, by using the -P option on the Yasm command line (see Section 1.3.3.4).
Having labels that are local to a macro definition is sometimes not quite powerful enough: sometimes you want to be able to share labels between several macro calls. An example might be a REPEAT … UNTIL loop, in which the expansion of the REPEAT macro would need to be able to refer to a label which the UNTIL macro had defined. However, for such a macro you would also want to be able to nest these loops.
The NASM preprocessor provides this level of power by means of a context stack. The preprocessor maintains a stack of contexts, each of which is characterised by a name. You add a new context to the stack using the %push directive, and remove one using %pop. You can define labels that are local to a particular context on the stack.
4.7.1. %push and %pop: Creating and Removing Contexts
The %push directive is used to create a new context and place it on the top of the context stack. %push requires one argument, which is the name of the context. For example:
%push foobar
This pushes a new context called foobar on the stack. You can have several contexts on the stack with the same name: they can still be distinguished.
The directive %pop, requiring no arguments, removes the top context from the context stack and destroys it, along with any labels associated with it.
Just as the usage %%foo defines a label which is local to the particular macro call in which it is used, the usage %$foo is used to define a label which is local to the context on the top of the context stack. So the REPEAT and UNTILexample given above could be implemented by means of:
%macro repeat 0
%push repeat
%$begin:
%endmacro
%macro until 1
j%-1 %$begin
%pop
%endmacro
and invoked by means of, for example,
mov cx,string
repeat
add cx,3
scasb
until e
which would scan every fourth byte of a string in search of the byte in AL.
If you need to define, or access, labels local to the context below the top one on the stack, you can use %$$foo, or %$$$foo for the context below that, and so on.
4.7.3. Context-Local Single-Line Macros
The NASM preprocessor also allows you to define single-line macros which are local to a particular context, in just the same way:
%define %$localmac 3
will define the single-line macro %$localmac to be local to the top context on the stack. Of course, after a subsequent %push, it can then still be accessed by the name %$$localmac.
4.7.4. %repl: Renaming a Context
If you need to change the name of the top context on the stack (in order, for example, to have it respond differently to %ifctx), you can execute a %pop followed by a %push; but this will have the side effect of destroying all context-local labels and macros associated with the context that was just popped.
The NASM preprocessor provides the directive %repl, which replaces a context with a different name, without touching the associated macros and labels. So you could replace the destructive code
%pop
%push newname
with the non-destructive version %repl newname.
4.7.5. Example Use of the Context Stack: Block IFs
This example makes use of almost all the context-stack features, including the conditional-assembly construct %ifctx, to implement a block IF statement as a set of macros.
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
This code is more robust than the REPEAT and UNTIL macros given in Section 4.7.2, because it uses conditional assembly to check that the macros are issued in the right order (for example, not calling endif before if) and issues a %errorif they're not.
In addition, the endif macro has to be able to cope with the two distinct cases of either directly following an if, or following an else. It achieves this, again, by using conditional assembly to do different things depending on whether the context on top of the stack is if or else.
The else macro has to preserve the context on the stack, in order to have the %$ifnot referred to by the if macro be the same as the one defined by the endif macro, but has to change the context's name so that endif will know there was an intervening else. It does this by the use of %repl.
A sample usage of these macros might look like:
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
mov ax,cx
endif
endif
The block-IF macros handle nesting quite happily, by means of pushing another context, describing the inner if, on top of the one describing the outer if; thus else and endif always refer to the last unmatched if or else.
Yasm defines a set of standard macros in the NASM preprocessor which are already defined when it starts to process any source file. If you really need a program to be assembled with no pre-defined macros, you can use the %cleardirective to empty the preprocessor of everything.
Most user-level NASM syntax directives (see Chapter 5) are implemented as macros which invoke primitive directives; these are described in Chapter 5. The rest of the standard macro set is described here.
4.8.1. __YASM_MAJOR__, etc: Yasm Version
The single-line macros __YASM_MAJOR__, __YASM_MINOR__, and __YASM_SUBMINOR__ expand to the major, minor, and subminor parts of the version number of Yasm being used. In addition, __YASM_VER__ expands to a string representation of the Yasm version and __YASM_VERSION_ID__ expands to a 32-bit BCD-encoded representation of the Yasm version, with the major version in the most significant 8 bits, followed by the 8-bit minor version and 8-bit subminor version, and 0 in the least significant 8 bits. For example, under Yasm 0.5.1, __YASM_MAJOR__ would be defined to be 0, __YASM_MINOR__ would be defined as 5, __YASM_SUBMINOR__ would be defined as 1, __YASM_VER__ would be defined as "0.5.1", and __YASM_VERSION_ID__ would be defined as 000050100h.
In addition, the single line macro __YASM_BUILD__ expands to the Yasm "build" number, typically the Subversion changeset number. It should be seen as less significant than the subminor version, and is generally only useful in discriminating between Yasm nightly snapshots or pre-release (e.g. release candidate) Yasm versions.
4.8.2. __FILE__ and __LINE__: File Name and Line Number
Like the C preprocessor, the NASM preprocessor allows the user to find out the file name and line number containing the current instruction. The macro __FILE__ expands to a string constant giving the name of the current input file (which may change through the course of assembly if %include directives are used), and __LINE__ expands to a numeric constant giving the current line number in the input file.
These macros could be used, for example, to communicate debugging information to a macro, since invoking __LINE__ inside a macro definition (either single-line or multi-line) will return the line number of the macro call, rather thandefinition. So to determine where in a piece of code a crash is occurring, for example, one could write a routine stillhere, which is passed a line number in EAX and outputs something like "line 155: still here". You could then write a macro
%macro notdeadyet 0
push eax
mov eax, __LINE__
call stillhere
pop eax
%endmacro
and then pepper your code with calls to notdeadyet until you find the crash point.
4.8.3. __YASM_OBJFMT__ and __OUTPUT_FORMAT__: Output Object Format Keyword
__YASM_OBJFMT__, and its NASM-compatible alias __OUTPUT_FORMAT__, expand to the object format keyword specified on the command line with -f keyword (see Section 1.3.1.2). For example, if yasm is invoked with -f elf, __YASM_OBJFMT__ expands toelf.
These expansions match the option given on the command line exactly, even when the object formats are equivalent. For example, -f elf and -f elf32 are equivalent specifiers for the 32-bit ELF format, and -f elf -m amd64 and -f elf64 are equivalent specifiers for the 64-bit ELF format, but __YASM_OBJFMT__ would expand to elf and elf32 for the first two cases, and elf and elf64 for the second two cases.
4.8.4. STRUC and ENDSTRUC: Declaring Structure Data Types
The NASM preprocessor is sufficiently powerful that data structures can be implemented as a set of macros. The macros STRUC and ENDSTRUC are used to define a structure data type.
STRUC takes one parameter, which is the name of the data type. This name is defined as a symbol with the value zero, and also has the suffix _size appended to it and is then defined as an EQU giving the size of the structure. Once STRUChas been issued, you are defining the structure, and should define fields using the RESB family of pseudo-instructions, and then invoke ENDSTRUC to finish the definition.
For example, to define a structure called mytype containing a longword, a word, a byte and a string of bytes, you might code
struc mytype
mt_long: resd 1
mt_word: resw 1
mt_byte: resb 1
mt_str: resb 32
endstruc
The above code defines six symbols: mt_long as 0 (the offset from the beginning of a mytype structure to the longword field), mt_word as 4, mt_byte as 6, mt_str as 7, mytype_size as 39, and mytype itself as zero.
The reason why the structure type name is defined at zero is a side effect of allowing structures to work with the local label mechanism: if your structure members tend to have the same names in more than one structure, you can define the above structure like this:
struc mytype
.long: resd 1
.word: resw 1
.byte: resb 1
.str: resb 32
endstruc
This defines the offsets to the structure fields as mytype.long, mytype.word, mytype.byte and mytype.str.
Since NASM syntax has no intrinsic structure support, does not support any form of period notation to refer to the elements of a structure once you have one (except the above local-label notation), so code such as mov ax,[mystruc.mt_word]is not valid. mt_word is a constant just like any other constant, so the correct syntax is mov ax,[mystruc+mt_word] or mov ax,[mystruc+mytype.word].
4.8.5. ISTRUC, AT and IEND: Declaring Instances of Structures
Having defined a structure type, the next thing you typically want to do is to declare instances of that structure in your data segment. The NASM preprocessor provides an easy way to do this in the ISTRUC mechanism. To declare a structure of type mytype in a program, you code something like this:
mystruc: istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
The function of the AT macro is to make use of the TIMES prefix to advance the assembly position to the correct point for the specified structure field, and then to declare the specified data. Therefore the structure fields must be declared in the same order as they were specified in the structure definition.
If the data to go in a structure field requires more than one source line to specify, the remaining source lines can easily come after the AT line. For example:
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
Depending on personal taste, you can also omit the code part of the AT line completely, and start the structure field on the next line:
at mt_str
db 'hello, world'
db 13,10,0
4.8.6. ALIGN and ALIGNB: Data Alignment
The ALIGN and ALIGNB macros provide a convenient way to align code or data on a word, longword, paragraph or other boundary. The syntax of the ALIGN and ALIGNB macros is
align 4 ; align on 4-byte boundary
align 16 ; align on 16-byte boundary
align 16,nop ; equivalent to previous line
align 8,db 0 ; pad with 0s rather than NOPs
align 4,resb 1 ; align to 4 in the BSS
alignb 4 ; equivalent to previous line
Both macros require their first argument to be a power of two; they both compute the number of additional bytes required to bring the length of the current section up to a multiple of that power of two, and output either NOP fill or apply the TIMES prefix to their second argument to perform the alignment.
If the second argument is not specified, the default for ALIGN is NOP, and the default for ALIGNB is RESB 1. ALIGN treats a NOP argument specially by generating maximal NOP fill instructions (not necessarily NOP opcodes) for the currentBITS setting, whereas ALIGNB takes its second argument literally. Otherwise, the two macros are equivalent when a second argument is specified. Normally, you can just use ALIGN in code and data sections and ALIGNB in BSS sections, and never need the second argument except for special purposes.
ALIGN and ALIGNB, being simple macros, perform no error checking: they cannot warn you if their first argument fails to be a power of two, or if their second argument generates more than one byte of code. In each of these cases they will silently do the wrong thing.
ALIGNB (or ALIGN with a second argument of RESB 1) can be used within structure definitions:
struc mytype2
mt_byte: resb 1
alignb 2
mt_word: resw 1
alignb 4
mt_long: resd 1
mt_str: resb 32
endstruc
This will ensure that the structure members are sensibly aligned relative to the base of the structure.
A final caveat: ALIGNB works relative to the beginning of the section, not the beginning of the address space in the final executable. Aligning to a 16-byte boundary when the section you're in is only guaranteed to be aligned to a 4-byte boundary, for example, is a waste of effort. Again, Yasm does not check that the section's alignment characteristics are sensible for the use of ALIGNB. ALIGN is more intelligent and does adjust the section alignment to be the maximum specified alignment.
Chapter 5. NASM Assembler Directives
Table of Contents
5.1. Specifying Target Processor Mode
5.1.2. USE16, USE32, and USE64
5.2. DEFAULT: Change the assembler defaults
5.3. Changing and Defining Sections
5.3.2. Standardized Section Names
5.4. ABSOLUTE: Defining Absolute Labels
5.5. EXTERN: Importing Symbols
5.6. GLOBAL: Exporting Symbols
5.7. COMMON: Defining Common Data Areas
5.8. CPU: Defining CPU Dependencies
NASM's directives come in two types: user-level directives and primitive directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms.
Primitive directives are enclosed in square brackets; user-level directives are not.
In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These format-specific directives are documented along with the formats that implement them, in Part IV.
5.1. Specifying Target Processor Mode
The BITS directive specifies whether Yasm should generate code designed to run on a processor operating in 16-bit mode, 32-bit mode, or 64-bit mode. The syntax is BITS 16, BITS 32, or BITS 64.
In most cases, you should not need to use BITS explicitly. The coff, elf32, macho32, and win32 object formats, which are designed for use in 32-bit operating systems, all cause Yasm to select 32-bit mode by default. The elf64, macho64, andwin64 object formats, which are designed for use in 64-bit operating systems, both cause Yasm to select 64-bit mode by default. The xdf object format allows you to specify each segment you define as USE16, USE32, or USE64, and Yasm will set its operating mode accordingly, so the use of the BITS directive is once again unnecessary.
The most likely reason for using the BITS directive is to write 32-bit or 64-bit code in a flat binary file; this is because the bin object format defaults to 16-bit mode in anticipation of it being used most frequently to write DOS .COMprograms, DOS .SYS device drivers and boot loader software.
You do not need to specify BITS 32 merely in order to use 32-bit instructions in a 16-bit DOS program; if you do, the assembler will generate incorrect code because it will be writing code targeted at a 32-bit platform, to be run on a 16-bit one. However, it is necessary to specify BITS 64 to use 64-bit instructions and registers; this is done to allow use of those instruction and register names in 32-bit or 16-bit programs, although such use will generate a warning.
When Yasm is in BITS 16 mode, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In BITS 32 mode, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working in 16-bit addresses need an 0x67.
When Yasm is in BITS 64 mode, 32-bit instructions usually require no prefixes, and most uses of 64-bit registers or data size requires a REX prefix. Yasm automatically inserts REX prefixes where necessary. There are also 8 more general and SSE registers, and 16-bit addressing is no longer supported. The default address size is 64 bits; 32-bit addressing can be selected with the 0x67 prefix. The default operand size is still 32 bits, however, and the 0x66 prefix selects 16-bit operand size. The REX prefix is used both to select 64-bit operand size, and to access the new registers. A few instructions have a default 64-bit operand size.
When the REX prefix is used, the processor does not know how to address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead, it is possible to access the the low 8-bits of the SP, BP SI, and DI registers as SPL, BPL, SIL, and DIL, respectively; but only when the REX prefix is used.
The BITS directive has an exactly equivalent primitive form, [BITS 16], [BITS 32], and [BITS 64]. The user-level form is a macro which has no function other than to call the primitive form.
5.1.2. USE16, USE32, and USE64
The USE16, USE32, and USE64 directives can be used in place of BITS 16, BITS 32, and BITS 64 respectively for compatibility with other assemblers.
5.2. DEFAULT: Change the assembler defaults
The DEFAULT directive changes the assembler defaults. Normally, Yasm defaults to a mode where the programmer is expected to explicitly specify most features directly. However, sometimes this is not desirable if a certain behavior is very commmonly used.
Currently, the only DEFAULT that is settable is whether or not registerless effective addresses in 64-bit mode are RIP-relative or not. By default, they are absolute unless overridden with the REL specifier (see Section 3.3). However, ifDEFAULT REL is specified, REL is default, unless overridden with the ABS specifier, a FS or GS segment override is used, or another register is part of the effective address.
The special handling of FS and GS overrides are due to the fact that these segments are the only segments which can have non-0 base addresses in 64-bit mode, and thus are generally used as thread pointers or other special functions. With a non-zero base address, generating RIP-relative addresses for these forms would be extremely confusing. Other segment registers such as DS always have a base address of 0, so RIP-relative access still makes sense.
DEFAULT REL is disabled with DEFAULT ABS. The default mode of the assembler at start-up is DEFAULT ABS.
5.3. Changing and Defining Sections
The SECTION directive (((SEGMENT)) is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence SECTION may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist.
5.3.2. Standardized Section Names
The Unix object formats, and the bin object format, all support the standardised section names .text, .data and .bss for the code, data and uninitialised-data sections. The obj format, by contrast, does not recognise these section names as being special, and indeed will strip off the leading period of any section name that has one.
The SECTION directive is unusual in that its user-level form functions differently from its primitive form. The primitive form, [SECTION xyz], simply switches the current target section to the one given. The user-level form, SECTION xyz, however, first defines the single-line macro __SECT__ to be the primitive [SECTION] directive which it is about to issue, and then issues it. So the user-level directive
SECTION .text
expands to the two lines
%define __SECT__ [SECTION .text]
[SECTION .text]
Users may find it useful to make use of this in their own macros. For example, the writefile macro defined in the NASM Manual can be usefully rewritten in the following more sophisticated form:
%macro writefile 2+
[section .data]
%%str: db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
This form of the macro, once passed a string to output, first switches temporarily to the data section of the file, using the primitive form of the SECTION directive so as not to modify __SECT__. It then declares its string in the data section, and then invokes __SECT__ to switch back to whichever section the user was previously working in. It thus avoids the need, in the previous version of the macro, to include a JMP instruction to jump over the data, and also does not fail if, in a complicated OBJ format module, the user could potentially be assembling the code in any of several separate code sections.
5.4. ABSOLUTE: Defining Absolute Labels
The ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the RESB family.
ABSOLUTE is used as follows:
ABSOLUTE 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf resw 16
This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.
The user-level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro when it is invoked.
STRUC and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).
ABSOLUTE doesn't have to take an absolute constant as an argument: it can take an expression (actually, a critical expression: see Section 3.8) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this:
org 100h ; it's a .COM program
jmp setup ; setup code comes last
; the resident part of the TSR goes here
setup: ; now write the code that installs the TSR here
absolute setup
runtimevar1 resw 1
runtimevar2 resd 20
tsr_end:
This defines some variables "on top of" the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol "tsr_end" can be used to calculate the total size of the part of the TSR that needs to be made resident.
5.5. EXTERN: Importing Symbols
EXTERN is similar to the MASM directive EXTRN and the C keyword extern: it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one. Not every object-file format can support external variables: the bin format cannot.
The EXTERN directive takes as many arguments as you like. Each argument is the name of a symbol:
extern _printf
extern _sscanf, _fscanf
Some object-file formats provide extra features to the EXTERN directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the obj format allows you to declare that the default segment base of an external should be the group dgroup by means of the directive
extern _variable:wrt dgroup
The primitive form of EXTERN differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level.
You can declare the same variable as EXTERN more than once: NASM will quietly ignore the second and later redeclarations. You can't declare a variable as EXTERN as well as something else, though.
5.6. GLOBAL: Exporting Symbols
GLOBAL is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to it, then in order to prevent linker errors, some other module must actually define the symbol and declare it as GLOBAL. Some assemblers use the name PUBLIC for this purpose.
The GLOBAL directive applying to a symbol must appear before the definition of the symbol.
GLOBAL uses the same syntax as EXTERN, except that it must refer to symbols which are defined in the same module as the GLOBAL directive. For example:
global _main
_main: ; some code
GLOBAL, like EXTERN, allows object formats to define private extensions by means of a colon. The elf object format, for example, lets you specify whether global data items are functions or data:
global hashlookup:function, hashtable:data
Like EXTERN, the primitive form of GLOBAL differs from the user-level form only in that it can take only one argument at a time.
5.7. COMMON: Defining Common Data Areas
The COMMON directive is used to declare common variables. A common variable is much like a global variable declared in the uninitialised data section, so that
common intvar 4
is similar in function to
global intvar
section .bss
intvar resd 1
The difference is that if more than one module defines the same common variable, then at link time those variables will be merged, and references to intvar in all modules will point at the same piece of memory.
Like GLOBAL and EXTERN, COMMON supports object-format specific extensions. For example, the obj format allows common variables to be NEAR or FAR, and the elf format allows you to specify the alignment requirements of a common variable:
common commvar 4:near ; works in OBJ
common intarray 100:4 ; works in ELF: 4 byte aligned
Once again, like EXTERN and GLOBAL, the primitive form of COMMON differs from the user-level form only in that it can take only one argument at a time.
5.8. CPU: Defining CPU Dependencies
The CPU directive restricts assembly to those instructions which are available on the specified CPU. See Part VI for CPU options for various architectures.
All options are case insensitive. Instructions will be enabled only if they apply to the selected cpu or lower.
The chapters in this part of the book document the GNU AS-compatible syntax accepted by the Yasm "gas" parser.
Table of Contents
Chapter 6. TBD
To be written.
The chapters in this part of the book document Yasm's support for various object file formats.
Chapter 7. bin: Flat-Form Binary Output
The bin "object format" does not produce object files: the output file produced contains only the section data; no headers or relocations are generated. The output can be considered "plain binary", and is useful for operating system and boot loader development, generating MS-DOS .COM executables and .SYS device drivers, and creating images for embedded target environments (e.g. Flash ROM).
The bin object format supports an unlimited number of named sections. See Section 7.2 for details. The only restriction on these sections is that their storage locations in the output file cannot overlap.
When used with the x86 architecture, the bin object format starts Yasm in 16-bit mode. In order to write native 32-bit or 64-bit code, an explicit BITS 32 or BITS 64 directive is required respectively.
bin produces an output file with no extension by default; it simply strips the extension from the input file name. Thus the default output filename for the input file foo.asm is simply foo.
bin provides the ORG directive in NASM syntax to allow setting of the memory address at which the output file is initially loaded. The ORG directive may only be used once (as the output file can only be initially loaded into a single location). If ORG is not specified, ORG 0 is used by default.
This makes the operation of NASM-syntax ORG very different from the operation of ORG in other assemblers, which typically simply move the assembly location to the value given. bin provides a more powerful alternative in the form of extensions to the SECTION directive; see Section 7.2 for details.
When combined with multiple sections, ORG also has the effect of defaulting the LMA of the first section to the ORG value to make the output file as small as possible. If this is not the desired behavior, explicitly specify a LMA for all sections via either START or FOLLOWS qualifiers in the SECTION directive.
7.2. bin Extensions to the SECTION Directive
The bin object format allows the use of multiple sections of arbitrary names. It also extends the SECTION (or SEGMENT) directive to allow complex ordering of the segments both in the output file or initial load address (also known as LMA) and at the ultimate execution address (the virtual address or VMA).
The VMA is the execution address. Yasm calculates absolute memory references within a section assuming that the program code is at the VMA while being executed. The LMA, on the other hand, specifies where a section is initially loaded, as well as its location in the output file.
Often, VMA will be the same as LMA. However, they may be different if the program or another piece of code copies (relocates) a section prior to execution. A typical example of this in an embedded system would be a piece of code stored in ROM, but is copied to faster RAM prior to execution. Another example would be overlays: sections loaded on demand from different file locations to the same execution location.
The bin extensions to the SECTION directive allow flexible specification of both VMA and LMA, including alignment constraints. As with other object formats, additional attributes may be added after the section name. The available attributes are listed in Table 7.1.
Table 7.1. bin Section Attributes
|
Attribute |
Indicates the section |
|
progbits |
is stored in the disk image, as opposed to allocated and initialized at load. |
|
nobits |
is allocated and initialized at load (the opposite of progbits). Only one of progbits or nobits may be specified; they are mutually exclusive attributes. |
|
start=address |
has an LMA starting at address. If a LMA alignment constraint is given, it is checked against the provided address and a warning is issued if address does not meet the alignment constraint. |
|
follows=sectname |
should follow the section named sectname in the output file (LMA). If a LMA alignment constraint is given, it is respected and a gap is inserted such that the section meets its alignment requirement. Note that as LMA overlap is not allowed, typically only one section may follow another. |
|
align=n |
requires a LMA alignment of n bytes. The value n must always be a power of 2. LMA alignment defaults to 4 if not specified. |
|
vstart=address |
has an VMA starting at address. If a VMA alignment constraint is given, it is checked against the provided address and a warning is issued if address does not meet the alignment constraint. |
|
vfollows=sectname |
should follow the section named sectname in the output file (VMA). If a VMA alignment constraint is given, it is respected and a gap is inserted such that the section meets its alignment requirement. VMA overlap is allowed, so more than one section may follow another (possibly useful in the case of overlays). |
|
valign=n |
requires a VMA alignment of n bytes. The value n must always be a power of 2. VMA alignment defaults to the LMA alignment if not specified. |
Only one of start or follows may be specified for a section; the same restriction applies to vstart and vfollows.
Unless otherwise specified via the use of follows or start, Yasm by default assumes the implicit ordering given by the order of the sections in the input file. A section named .text is always the first section. Any code which comes before an explicit SECTION directive goes into the .text section. The .text section attributes may be overridden by giving an explicit SECTION .text directive with attributes.
Also, unless otherwise specified, Yasm defaults to setting VMA=LMA. If just "valign` is specified, Yasm just takes the LMA and aligns it to the required alignment. This may have the effect of pushing following sections" VMAs to non-LMA addresses as well, to avoid VMA overlap.
Yasm treats nobits sections in a special way in order to minimize the size of the output file. As nobits sections can be 0-sized in the LMA realm, but cannot be if located between two other sections (due to the VMA=LMA default), Yasm moves all nobits sections with unspecified LMA to the end of the output file, where they can savely have 0 LMA size and thus not take up any space in the output file. If this behavior is not desired, a nobits section LMA (just like aprogbits section) may be specified using either the follows or start section attribute.
To facilitate writing code that copies itself from one location to another (e.g. from its LMA to its VMA during execution), the bin object format provides several special symbols for every defined section. Each special symbol begins with section. followed by the section name. The supported special bin symbols are:
section.sectname.start
Set to the LMA address of the section named sectname.
section.sectname.vstart
Set to the VMA address of the section named sectname.
section.sectname.length
Set to the length of the section named sectname. The length is considered the runtime length, so "nobits` sections" length is their runtime length, not 0.
Map files may be generated in bin via the use of the [MAP] directive. The map filename may be specified either with a command line option (--mapfile=filename) or in the [MAP] directive. If a map is requested but no output filename is given, the map output goes to standard output by default.
If no [MAP] directive is given in the input file, no map output is generated. If [MAP] is given with no options, a brief map is generated. The [MAP] directive accepts the following options to control what is included in the map file. More than one option may be specified. Any option other than the ones below is interpreted as the output filename.
brief
Includes the input and output filenames, origin (ORG value), and a brief section summary listing the VMA and LMA start and stop addresses and the section length of every section.
sections , segments
Includes a detailed list of sections, including the VMA and LMA alignment, any "follows" settings, as well as the VMA and LMA start addresses and the section length.
symbols
Includes a detailed list of all EQU values and VMA and LMA symbol locations, grouped by section.
all
All of the above.
Chapter 8. coff: Common Object File Format
Chapter 9. elf32: Executable and Linkable Format 32-bit Object Files
Table of Contents
9.3.1. IDENT: Add file identification
9.3.4. WEAK: Create weak symbol
9.4. ELF Extensions to the GLOBAL Directive
9.5. ELF Extensions to the COMMON Directive
9.6. elf32 Special Symbols and WRT
The Executable and Linkable Object Format is the primary object format for many operating systems including FreeBSD or GNU/Linux. It appears in three forms:
- Shared object files (.so)
- Relocatable object files (.o)
- Executable files (no convention)
Yasm only directly supports relocatable object files. Other tools, such as the GNU Linker ld, help turn relocatable object files into the other formats. Yasm supports generation of both 32-bit and 64-bit ELF files, called elf32 andelf64. An additional format, called elfx32, is a 32-bit ELF file that supports 64-bit execution (instructions and registers) while limiting pointer sizes to 32-bit.
Yasm defaults to BITS 32 mode when outputting to the elf32 object format.
ELF supports two debugging formats: stabs (see Chapter 20) and dwarf2 (see Chapter 19). Different debuggers understand these different formats; the newer debug format is dwarf2, so try that first.
ELF's section-based output supports attributes on a per-section basis. These attributes include alloc, exec, write, progbits, and align. Except for align, they can each be negated in NASM syntax by prepending "no", e.g., "noexec". The attributes are later read by the operating system to select the proper behavior for each section, with the meanings shown in Table 9.1.
Table 9.1. ELF Section Attributes
|
Attribute |
Indicates the section |
|
alloc |
is loaded into memory at runtime. This is true for code and data sections, and false for metadata sections. |
|
exec |
has permission to be run as executable code. |
|
write |
is writable at runtime. |
|
progbits |
is stored in the disk image, as opposed to allocated and initialized at load. |
|
align=n |
requires a memory alignment of n bytes. The value n must always be a power of 2. |
In NASM syntax, the attribute nobits is provided as an alias for noprogbits.
The standard primary sections have attribute defaults according their expected use, and any unknown section gets its own defaults, as shown in Table 9.2.
Table 9.2. ELF Standard Sections
|
Section |
alloc |
exec |
write |
progbits |
align |
|
.bss |
alloc |
write |
4 |
||
|
.data |
alloc |
write |
progbits |
4 |
|
|
.rodata |
alloc |
progbits |
4 |
||
|
.text |
alloc |
exec |
progbits |
16 |
|
|
.comment |
progbits |
0 |
|||
|
unknown |
alloc |
progbits |
1 |
ELF adds additional assembler directives to define weak symbols (WEAK), set symbol size (SIZE), and indicate whether a symbol is specifically a function or an object (TYPE). ELF also adds a directive to assist in identifying the source file or version, IDENT.
9.3.1. IDENT: Add file identification
The IDENT directive allows adding arbitrary string data to an ELF object file that will be saved in the object and executable file, but will not be loaded into memory like data in the .data section. It is often used for saving version control keyword information from tools such as cvs or svn into files so that the source revision the object was created with can be read using the ident command found on most Unix systems.
The directive takes one or more string parameters. Each parameter is saved in sequence as a 0-terminated string in the .comment section of the object file. Multiple uses of the IDENT directive are legal, and the strings will be saved into the .comment section in the order given in the source file.
In NASM syntax, no wrapper macro is provided for IDENT, so it must be wrapped in square brackets. Example use in NASM syntax:
[ident "$Id$"]
ELF's symbol table has the capability of storing a size for a symbol. This is commonly used for functions or data objects. While the size can be specificed directly for COMMON symbols, the SIZE directive allows for specifying the size of any symbol, including local symbols.
The directive takes two parameters; the first parameter is the symbol name, and the second is the size. The size may be a constant or an expression. Example:
func:
ret
.end:
size func func.end-func
ELF's symbol table has the capability of indicating whether a symbol is a function or data. While this can be specified directly in the GLOBAL directive (see Section 9.4), the TYPE directive allows specifying the symbol type for any symbol, including local symbols.
The directive takes two parameters; the first parameter is the symbol name, and the second is the symbol type. The symbol type must be either function or object. An unrecognized type will cause a warning to be generated. Example of use:
func:
ret
type func function
section .data
var dd 4
type var object
9.3.4. WEAK: Create weak symbol
ELF allows defining certain symbols as "weak". Weak symbols are similar to global symbols, except during linking, weak symbols are only chosen after global and local symbols during symbol resolution. Unlike global symbols, multiple object files may declare the same weak symbol, and references to a symbol get resolved against a weak symbol only if no global or local symbols have the same name.
This functionality is primarily useful for libraries that want to provide common functions but not come into conflict with user programs. For example, libc has a syscall (function) called "read". However, to implement a threaded process using POSIX threads in user-space, libpthread needs to supply a function also called "read" that provides a blocking interface to the programmer, but actually does non-blocking calls to the kernel. To allow an application to be linked to both libc and libpthread (to share common code), libc needs to have its version of the syscall with a non-weak name like "_sys_read" with a weak symbol called "read". If an application is linked against libc only, the linker won't find a non-weak symbol for "read", so it will use the weak one. If the same application is linked against libc and libpthread, then the linker will link "read" calls to the symbol in libpthread, ignoring the weak one in libc, regardless of library link order. If libc used a non-weak name, which "read" function the program ended up with might depend on a variety of factors; a weak symbol is a way to tell the linker that a symbol is less important resolution-wise.
The WEAK directive takes a single parameter, the symbol name to declare weak. Example:
weakfunc:
strongfunc:
ret
weak weakfunc
global strongfunc
9.4. ELF Extensions to the GLOBAL Directive
ELF object files can contain more information about a global symbol than just its address: they can contain the size of the symbol and its type as well. These are not merely debugger conveniences, but are actually necessary when the program being written is a ((shared library)). Yasm therefore supports some extensions to the NASM syntax GLOBAL directive (see Section 5.6), allowing you to specify these features. Yasm also provides the ELF-specific directives inSection 9.3 to allow specifying this information for non-global symbols.
You can specify whether a global variable is a function or a data object by suffixing the name with a colon and the word function or data. (((object)) is a synonym for data.) For example:
global hashlookup:function, hashtable:data
exports the global symbol hashlookup as a function and hashtable as a data object.
Optionally, you can control the ELF visibility of the symbol. Just add one of the visibility keywords: default, internal, hidden, or protected. The default is default, of course. For example, to make hashlookup hidden:
global hashlookup:function hidden
You can also specify the size of the data associated with the symbol, as a numeric expression (which may involve labels, and even forward references) after the type specifier. Like this:
global hashtable:data (hashtable.end - hashtable)
hashtable:
db this,that,theother ; some data here
.end:
This makes Yasm automatically calculate the length of the table and place that information into the ELF symbol table. The same information can be given more verbosely using the TYPE (see Section 9.3.3) and SIZE (see Section 9.3.2) directives as follows:
global hashtable
type hashtable object
size hashtable hashtable.end - hashtable
hashtable:
db this,that,theother ; some data here
.end:
Declaring the type and size of global symbols is necessary when writing shared library code.
9.5. ELF Extensions to the COMMON Directive
ELF also allows you to specify alignment requirements on common variables. This is done by putting a number (which must be a power of two) after the name and size of the common variable, separated (as usual) by a colon. For example, an array of doublewords would benefit from 4-byte alignment:
common dwordarray 128:4
This declares the total size of the array to be 128 bytes, and requires that it be aligned on a 4-byte boundary.
9.6. elf32 Special Symbols and WRT
The ELF specification contains enough features to allow position-independent code (PIC) to be written, which makes ELF shared libraries very flexible. However, it also means Yasm has to be able to generate a variety of strange relocation types in ELF object files, if it is to be an assembler which can write PIC.
Since ELF does not support segment-base references, the WRT operator is not used for its normal purpose; therefore Yasm's elf32 output format makes use of WRT for a different purpose, namely the PIC-specific relocation types.
elf32 defines five special symbols which you can use as the right-hand side of the WRT operator to obtain PIC relocation types. They are ..gotpc, ..gotoff, ..got, ..plt and ..sym. Their functions are summarized here:
..gotpc
Referring to the symbol marking the global offset table base using wrt ..gotpc will end up giving the distance from the beginning of the current section to the global offset table. (((_GLOBAL_OFFSET_TABLE_)) is the standard symbol name used to refer to the GOT.) So you would then need to add $$ to the result to get the real address of the GOT.
..gotoff
Referring to a location in one of your own sections using wrt ..gotoff will give the distance from the beginning of the GOT to the specified location, so that adding on the address of the GOT would give the real address of the location you wanted.
..got
Referring to an external or global symbol using wrt ..got causes the linker to build an entry in the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.
..plt
Referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
..sym
Referring to a symbol name using wrt ..sym causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.
Chapter 10. elf64: Executable and Linkable Format 64-bit Object Files
Table of Contents
10.1. elf64 Special Symbols and WRT
The elf64 object format is the 64-bit version of the Executable and Linkable Object Format. As it shares many similarities with elf32, only differences between elf32 and elf64 will be described in this chapter. For details on elf32, seeChapter 9.
Yasm defaults to BITS 64 mode when outputting to the elf64 object format.
elf64 supports the same debug formats as elf32, however, the stabs debug format is limited to 32-bit addresses, so dwarf2 (see Chapter 19) is the recommended debugging format.
elf64 also supports the exact same sections, section attributes, and directives as elf32. See Section 9.2 for more details on section attributes, and Section 9.3 for details on the additional directives ELF provides.
10.1. elf64 Special Symbols and WRT
The primary difference between elf32 and elf64 (other than 64-bit support in general) is the differences in shared library handling and position-independent code. As BITS 64 enables the use of RIP-relative addressing, most variable accesses can be relative to RIP, allowing easy relocation of the shared library to a different memory address.
While RIP-relative addressing is available, it does not handle all possible variable access modes, so special symbols are still required, as in elf32. And as with elf32, the elf64 output format makes use of WRT for utilizing the PIC-specific relocation types.
elf64 defines four special symbols which you can use as the right-hand side of the WRT operator to obtain PIC relocation types. They are ..gotpcrel, ..got, ..plt and ..sym. Their functions are summarized here:
..gotpcrel
While RIP-relative addressing allows you to encode an instruction pointer relative data reference to foo with [rel foo], it's sometimes necessary to encode a RIP-relative reference to a linker-generated symbol pointer for symbol foo; this is done using wrt ..gotpcrel, e.g. [rel foo wrt ..gotpcrel]. Unlike in elf32, this relocation, combined with RIP-relative addressing, makes it possible to load an address from the ((global offset table)) using a single instruction. Note that since RIP-relative references are limited to a signed 32-bit displacement, the GOT size accessible through this method is limited to 2 GB.
..got
As in elf32, referring to an external or global symbol using wrt ..got causes the linker to build an entry in the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.
..plt
As in elf32, referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
..sym
As in elf32, referring to a symbol name using wrt ..sym causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.
Chapter 11. elfx32: ELF 32-bit Object Files for 64-bit Processors
Table of Contents
11.1. elfx32 Special Symbols and WRT
The elfx32 object format is the 32-bit version of the Executable and Linkable Object Format for 64-bit execution. Similar to elf64, it allows for use of 64-bit registers and instructions, but like elf32, limits pointers to 32 bits in size. As it shares many similarities with elf32 and elf64, only differences between these formats and elfx32 will be described in this chapter. For details on elf32, see Chapter 9; for details on elf64, see Chapter 10. Operating system support for elfx32 is currently less common than for elf64.
Yasm defaults to BITS 64 mode when outputting to the elfx32 object format.
elfx32 supports the same debug formats, sections, section attributes, and directives as elf32 and elf64. See Section 9.2 for more details on section attributes, and Section 9.3 for details on the additional directives ELF provides.
11.1. elfx32 Special Symbols and WRT
Due to the availability of RIP-relative addressing, elfx32 shared library handling and position-independent code is essentially identical to elf64.
As in elf64, elfx32 defines four special symbols which you can use as the right-hand side of the WRT operator to obtain PIC relocation types. They are ..gotpcrel, ..got, ..plt and ..sym and have the same functionality as they do in elf64. Their functions are summarized here:
..gotpcrel
While RIP-relative addressing allows you to encode an instruction pointer relative data reference to foo with [rel foo], it's sometimes necessary to encode a RIP-relative reference to a linker-generated symbol pointer for symbol foo; this is done using wrt ..gotpcrel, e.g. [rel foo wrt ..gotpcrel]. As in elf64, this relocation, combined with RIP-relative addressing, makes it possible to load an address from the ((global offset table)) using a single instruction. Note that since RIP-relative references are limited to a signed 32-bit displacement, the GOT size accessible through this method is limited to 2 GB.
..got
As in elf64, referring to an external or global symbol using wrt ..got causes the linker to build an entry in the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol.
..plt
As in elf64, referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.
..sym
As in elf64, referring to a symbol name using wrt ..sym causes Yasm to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker.
Chapter 12. macho32: Mach 32-bit Object File Format
Chapter 13. macho64: Mach 64-bit Object File Format
Chapter 14. rdf: Relocatable Dynamic Object File Format
Chapter 15. win32: Microsoft Win32 Object Files
Table of Contents
15.1. win32 Extensions to the SECTION Directive
15.2. win32: Safe Structured Exception Handling
The win32 object format generates Microsoft Win32 object files for use on the 32-bit native Windows XP (and Vista) platforms. Object files produced using this object format may be linked with 32-bit Microsoft linkers such as Visual Studio in order to produce 32-bit PE executables.
The win32 object format provides a default output filename extension of .obj.
Note that although Microsoft say that Win32 object files follow the COFF (Common Object File Format) standard, the object files produced by Microsoft Win32 compilers are not compatible with COFF linkers such as DJGPP's, and vice versa. This is due to a difference of opinion over the precise semantics of PC-relative relocations. To produce COFF files suitable for DJGPP, use the coff output format; conversely, the coff format does not produce object files that Win32 linkers can generate correct output from.
15.1. win32 Extensions to the SECTION Directive
The win32 object format allows you to specify additional information on the SECTION directive line, to control the type and properties of sections you declare. Section types and properties are generated automatically by Yasm for thestandard section names .text, .data and .bss, but may still be overridden by these qualifiers.
The available qualifiers are:
code or text
Defines the section to be a code section. This marks the section as readable and executable, but not writable, and also indicates to the linker that the type of the section is code.
data or bss
Defines the section to be a data section, analogously to code. Data sections are marked as readable and writable, but not executable. data declares an initialized data section, whereas bss declares an uninitialized data section.
rdata
Declares an initialized data section that is readable but not writable. Microsoft compilers use this section to place constants in it.
info
Defines the section to be an informational section, which is not included in the executable file by the linker, but may (for example) pass information to the linker. For example, declaring an info-type section called .drectve causes the linker to interpret the contents of the section as command-line options.
align=n
Specifies the alignment requirements of the section. The maximum you may specify is 8192: the Win32 object file format contains no means to request a greater section alignment. If alignment is not explicitly specified, the defaults are 16-byte alignment for code sections, 8-byte alignment for rdata sections and 4-byte alignment for data (and BSS) sections. Informational sections get a default alignment of 1 byte (no alignment), though the value does not matter. The alignment must be a power of 2.
Other qualifiers are supported which control specific section flags: discard, cache, page, share, execute, read, write, and base. Each of these sets the similarly-named section flag, while prefixing them with no clears the corresponding section flag; e.g. nodiscard clears the discard flag.
The defaults assumed by Yasm if you do not specify the above qualifiers are:
section .text code align=16
section .data data align=4
section .rdata rdata align=8
section .rodata rdata align=8
section .rdata$ rdata align=8
section .bss bss align=4
section .drectve info
section .comment info
Any other section name is treated by default like .text.
15.2. win32: Safe Structured Exception Handling
Among other improvements in Windows XP SP2 and Windows Server 2003 Microsoft introduced the concept of "safe structured exception handling." The general idea is to collect handlers' entry points in a designated read-only table and have each entry point verified against this table for exceptions prior to control being passed to the handler. In order for an executable to be created with a safe exception handler table, each object file on the linker command line must contain a special symbol named @feat.00. If any object file passed to the linker does not have this symbol, then the exception handler table is omitted from the executable and thus the run-time checks will not be performed for the application. By default, the table is omitted from the executable silently if this happens and therefore can be easily overlooked. A user can instruct the linker to refuse to produce an executable without this table by passing the/safeseh command line option.
As of version 1.1.0, Yasm adds this special symbol to win32 object files so its output does not fail to link with /safeseh.
Yasm also has directives to support registering custom exception handlers. The safeseh directive instructs the assembler to produce appropriately formatted input data for the safe exception handler table. A typical use case is given inExample 15.1.
Example 15.1. Win32 safeseh Example
section .text
extern _MessageBoxA@16
safeseh handler ; register handler as "safe handler"
handler:
push DWORD 1 ; MB_OKCANCEL
push DWORD caption
push DWORD text
push DWORD 0
call _MessageBoxA@16
sub eax,1 ; incidentally suits as return value
; for exception handler
ret
global _main
_main:
push DWORD handler
push DWORD [fs:0]
mov DWORD [fs:0],esp ; engage exception handler
xor eax,eax
mov eax,DWORD[eax] ; cause exception
pop DWORD [fs:0] ; disengage exception handler
add esp,4
ret
text: db 'OK to rethrow, CANCEL to generate core dump',0
caption:db 'SEGV',0
section .drectve info
db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
If an application has a safe exception handler table, attempting to execute any unregistered exception handler will result in immediate program termination. Thus it is important to register each exception handler's entry point with the safeseh directive.
All mentions of linker in this section refer to the Microsoft linker version 7.x and later. The presence of the @feat.00 symbol and the data for the safe exception handler table cause no backward incompatibilities and thus "safeseh" object files generated can still be linked by earlier linker versions or by non-Microsoft linkers.
Chapter 16. win64: PE32+ (Microsoft Win64) Object Files
Table of Contents
16.1. win64 Extensions to the SECTION Directive
16.2. win64 Structured Exception Handling
16.2.1. x64 Stack, Register and Function Parameter Conventions
16.2.3. Frame Function Structure
16.2.5. Yasm Primitives for Unwind Operations
16.2.6. Yasm Macros for Formal Stack Operations
The win64 or x64 object format generates Microsoft Win64 object files for use on the 64-bit native Windows XP x64 (and Vista x64) platforms. Object files produced using this object format may be linked with 64-bit Microsoft linkers such as that in Visual Studio 2005 and 2008 in order to produce 64-bit PE32+ executables.
win64 provides a default output filename extension of .obj.
16.1. win64 Extensions to the SECTION Directive
Like the win32 format, win64 allows you to specify additional information on the SECTION directive line, to control the type and properties of sections you declare.
16.2. win64 Structured Exception Handling
Most functions that make use of the stack in 64-bit versions of Windows must support exception handling even if they make no internal use of such facilities. This is because these operating systems locate exception handlers by using a process called "stack unwinding" that depends on functions providing data that describes how they use the stack.
When an exception occurs the stack is "unwound" by working backwards through the chain of function calls prior to the exception event to determine whether functions have appropriate exception handlers or whether they have saved non-volatile registers whose value needs to be restored in order to reconstruct the execution context of the next higher function in the chain. This process depends on compilers and assemblers providing "unwind data" for functions.
The following sections give details of the mechanisms that are available in Yasm to meet these needs and thereby allow functions written in assembler to comply with the coding conventions used in 64-bit versions of Windows. These Yasm facilities follow those provided in MASM.
16.2.1. x64 Stack, Register and Function Parameter Conventions
Figure 16.1 shows how the stack is typically used in function calls. When a function is called, an 8 byte return address is automatically pushed onto the stack and the function then saves any non-volatile registers that it will use. Additional space can also be allocated for local variables and a frame pointer register can be assigned if needed.
Figure 16.1. x64 Calling Convention
The first four integer function parameters are passed (in left to right order) in the registers RCX, RDX, R8 and R9. Further integer parameters are passed on the stack by pushing them in right to left order (parameters to the left at lower addresses). Stack space is allocated for the four register parameters ("shadow space") but their values are not stored by the calling function so the called function must do this if necessary. The called function effectively owns this space and can use it for any purpose, so the calling function cannot rely on its contents on return. Register parameters occupy the least significant ends of registers and shadow space must be allocated for four register parameters even if the called function doesn't have this many parameters.
The first four floating point parameters are passed in XMM0 to XMM3. When integer and floating point parameters are mixed, the correspondence between parameters and registers is not changed. Hence an integer parameter after two floating point ones will be in R8 with RCX and RDX unused.
When they are passed by value, structures and unions whose sizes are 8, 16, 32 or 64 bits are passed as if they are integers of the same size. Arrays and larger structures and unions are passed as pointers to memory allocated and assigned by the calling function.
The registers RAX, RCX, RDX, R8, R9, R10, R11 are volatile and can be freely used by a called function without preserving their values (note, however, that some may be used to pass parameters). In consequence functions cannot expect these registers to be preserved across calls to other functions.
The registers RBX, RBP, RSI, RDI, R12, R13, R14, R15, and XMM6 to XMM15 are non-volatile and must be saved and restored by functions that use them.
Except for floating point values, which are returned in XMM0, function return values that fit in 64 bits are returned in RAX. Some 128-bit values are also passed in XMM0 but larger values are returned in memory assigned by the calling program and pointed to by an additional "hidden" function parameter that becomes the first parameter and pushes other parameters to the right. This pointer value must also be passed back to the calling program in RAX when the called program returns.
Functions that allocate stack space, call other functions, save non-volatile registers or use exception handling are called "frame functions"; other functions are called "leaf functions".
Frame functions use an area on the stack called a "stack frame" and have a defined prologue in which this is set up. Typically they save register parameters in their shadow locations (if needed), save any non-volatile registers that they use, allocate stack space for local variables, and establish a register as a stack frame pointer. They must also have one or more defined epilogues that free any allocated stack space and restore non-volatile registers before returning to the calling function.
Unless stack space is allocated dynamically, a frame function must maintain the 16 byte alignment of the stack pointer whilst outside its prologue and epilogue code (except during calls to other functions). A frame function that dynamically allocates stack space must first allocate any fixed stack space that it needs and then allocate and set up a register for indexed access to this area. The lower base address of this area must be 16 byte aligned and the register must be provided irrespective of whether the function itself makes explicit use of it. The function is then free to leave the stack unaligned during execution although it must re-establish the 16 byte alignment if or when it calls other functions.
Leaf functions do not require defined prologues or epilogues but they must not call other functions; nor can they change any non-volatile register or the stack pointer (which means that they do not maintain 16 byte stack alignment during execution). They can, however, exit with a jump to the entry point of another frame or leaf function provided that the respective stacked parameters are compatible.
These rules are summarized in Table 16.1 (function code that is not part of a prologue or an epilogue are referred to in the table as the function's body).
Table 16.1. Function Structured Exception Handling Rules
|
Function needs or can: |
Frame Function with Frame Pointer Register |
Frame Function without Frame Pointer Register |
Leaf Function |
|
prologue and epilogue(s) |
yes |
yes |
no |
|
use exception handling |
yes |
yes |
no |
|
allocate space on the stack |
yes |
yes |
no |
|
save or push registers onto the stack |
yes |
yes |
no |
|
use non-volatile registers (after saving) |
yes |
yes |
no |
|
use dynamic stack allocation |
yes |
no |
no |
|
change stack pointer in function body |
yes [a] |
no |
no |
|
unaligned stack pointer in function body |
yes [a] |
no |
yes |
|
make calls to other functions |
yes |
yes |
no |
|
make jumps to other functions |
no |
no |
yes [b] |
|
[a] but 16 byte stack alignment must be re-established when any functions are called. [b] but the function parameters in registers and on the stack must be compatible. |
|||
16.2.3. Frame Function Structure
As already indicated, frame functions must have a well defined structure including a prologue and one or more epilogues, each of a specific form. The code in a function that is not part of its prologue or its one or more epilogues will be referred to here as the function's body.
A typical function prologue has the form:
mov [rsp+8],rcx ; store parameter in shadow space if necessary
push r14 ; save any non-volatile registers to be used
push r13 ;
sub rsp,size ; allocate stack for local variables if needed
lea r13,[bias+rsp] ; use r13 as a frame pointer with an offset
When a frame pointer is needed the programmer can choose which register is used ("bias" will be explained later). Although it does not have to be used for access to the allocated space, it must be assigned in the prologue and remain unchanged during the execution of the body of the function.
If a large amount of stack space is used it is also necessary to call __chkstk with size in RAX prior to allocating this stack space in order to add memory pages to the stack if needed (see the Microsoft Visual Studio 2005 documentation for further details).
The matching form of the epilogue is:
lea rsp,[r13-bias] ; this is not part of the official epilogue
add rsp,size ; the official epilogue starts here
pop r13
pop r14
ret
The following can also be used provided that a frame pointer register has been established:
lea rsp,[r13+size-bias]
pop r13
pop r14
ret
These are the only two forms of epilogue allowed. It must start either with an add rsp,const instruction or with lea rsp,[const+fp_register]; the first form can be used either with or without a frame pointer register but the second form requires one. These instructions are then followed by zero or more 8 byte register pops and a return instruction (which can be replaced with a limited set of jump instructions as described in Microsoft documentation). Epilogue forms are highly restricted because this allows the exception dispatch code to locate them without the need for unwind data in addition to that provided for the prologue.
The data on the location and length of each function prologue, on any fixed stack allocation and on any saved non-volatile registers is recorded in special sections in the object code. Yasm provides macros to create this data that will now be described (with examples of the way they are used).
There are two types of stack frame that need to be considered in creating unwind data.
The first, shown at left in Figure 16.2, involves only a fixed allocation of space on the stack and results in a stack pointer that remains fixed in value within the function's body except during calls to other functions. In this type of stack frame the stack pointer value at the end of the prologue is used as the base for the offsets in the unwind primitives and macros described later. It must be 16 byte aligned at this point.
Figure 16.2. x64 Detailed Stack Frame
In the second type of frame, shown in Figure 16.2, stack space is dynamically allocated with the result that the stack pointer value is statically unpredictable and cannot be used as a base for unwind offsets. In this situation a frame pointer register must be used to provide this base address. Here the base for unwind offsets is the lower end of the fixed allocation area on the stack, which is typically the value of the stack pointer when the frame register is assigned. It must be 16 byte aligned and must be assigned before any unwind macros with offsets are used.
In order to allow the maximum amount of data to be accessed with single byte offsets (-128 to \+127) from the frame pointer register, it is normal to offset its value towards the centre of the allocated area (the "bias" introduced earlier). The identity of the frame pointer register and this offset, which must be a multiple of 16 bytes, is recorded in the unwind data to allow the stack frame base address to be calculated from the value in the frame register.
16.2.5. Yasm Primitives for Unwind Operations
Here are the low level facilities Yasm provides to create unwind data.
proc_frame name
Generates a function table entry in .pdata and unwind information in .xdata for a function's structured exception handling data.
[pushreg reg]
Generates unwind data for the specified non-volatile register. Use only for non-volatile integer registers; for volatile registers use an [allocstack 8] instead.
[setframe reg, offset]
Generates unwind data for a frame register and its stack offset. The offset must be a multiple of 16 and be less than or equal to 240.
[allocstack size]
Generates unwind data for stack space. The size must be a multiple of 8.
[savereg reg, offset]
Generates unwind data for the specified register and offset; the offset must be positive multiple of 8 relative to the base of the procedure's frame.
[savexmm128 reg, offset]
Generates unwind data for the specified XMM register and offset; the offset must be positive multiple of 16 relative to the base of the procedure's frame.
[pushframe code]
Generates unwind data for a 40 or 48 byte (with an optional error code) frame used to store the result of a hardware exception or interrupt.
[endprolog]
Signals the end of the prologue; must be in the first 255 bytes of the function.
endproc_frame
Used at the end of functions started with proc_frame.
Example 16.1 shows how these primitives are used (this is based on an example provided in Microsoft Visual Studio 2005 documentation).
Example 16.1. Win64 Unwind Primitives
PROC_FRAME sample
db 0x48 ; emit a REX prefix to enable hot-patching
push rbp ; save prospective frame pointer
[pushreg rbp] ; create unwind data for this rbp register push
sub rsp,0x40 ; allocate stack space
[allocstack 0x40] ; create unwind data for this stack allocation
lea rbp,[rsp+0x20] ; assign the frame pointer with a bias of 32
[setframe rbp,0x20] ; create unwind data for a frame register in rbp
movdqa [rbp],xmm7 ; save a non-volatile XMM register
[savexmm128 xmm7, 0x20] ; create unwind data for an XMM register save
mov [rbp+0x18],rsi ; save rsi
[savereg rsi,0x38] ; create unwind data for a save of rsi
mov [rsp+0x10],rdi ; save rdi
[savereg rdi, 0x10] ; create unwind data for a save of rdi
[endprolog]
; We can change the stack pointer outside of the prologue because we
; have a frame pointer. If we didn't have one this would be illegal.
; A frame pointer is needed because of this stack pointer modification.
sub rsp,0x60 ; we are free to modify the stack pointer
mov rax,0 ; we can unwind this access violation
mov rax,[rax]
movdqa xmm7,[rbp] ; restore the registers that weren't saved
mov rsi,[rbp+0x18] ; with a push; this is not part of the
mov rdi,[rbp-0x10] ; official epilog
lea rsp,[rbp+0x20] ; This is the official epilog
pop rbp
ret
ENDPROC_FRAME
16.2.6. Yasm Macros for Formal Stack Operations
From the descriptions of the YASM primitives given earlier it can be seen that there is a close relationship between each normal stack operation and the related primitive needed to generate its unwind data. In consequence it is sensible to provide a set of macros that perform both operations in a single macro call. Yasm provides the following macros that combine the two operations.
proc_frame name
Generates a function table entry in .pdata and unwind information in .xdata.
alloc_stack n
Allocates a stack area of n bytes.
save_reg reg, loc
Saves a non-volatile register reg at offset loc on the stack.
push_reg reg
Pushes a non-volatile register reg on the stack.
rex_push_reg reg
Pushes a non-volatile register reg on the stack using a 2 byte push instruction.
save_xmm128 reg, loc
Saves a non-volatile XMM register reg at offset loc on the stack.
set_frame reg, loc
Sets the frame register reg to offset loc on the stack.
push_eflags
Pushes the eflags register
push_rex_eflags
Pushes the eflags register using a 2 byte push instruction (allows hot patching).
push_frame code
Pushes a 40 byte frame and an optional 8 byte error code onto the stack.
end_prologue , end_prolog
Ends the function prologue (this is an alternative to [endprolog]).
endproc_frame
Used at the end of funtions started with proc_frame.
Example 16.2 is Example 16.1 using these higher level macros.
Example 16.2. Win64 Unwind Macros
PROC_FRAME sample ; start the prologue
rex_push_reg rbp ; push the prospective frame pointer
alloc_stack 0x40 ; allocate 64 bytes of local stack space
set_frame rbp, 0x20 ; set a frame register to [rsp+32]
save_xmm128 xmm7,0x20 ; save xmm7, rsi & rdi to the local stack space
save_reg rsi, 0x38 ; unwind base address: [rsp_after_entry - 72]
save_reg rdi, 0x10 ; frame register value: [rsp_after_entry - 40]
END_PROLOGUE
sub rsp,0x60 ; we can now change the stack pointer
mov rax,0 ; and unwind this access violation
mov rax,[rax] ; because we have a frame pointer
movdqa xmm7,[rbp] ; restore the registers that weren't saved with
mov rsi,[rbp+0x18] ; a push (not a part of the official epilog)
mov rdi,[rbp-0x10]
lea rsp,[rbp+0x20] ; the official epilogue
pop rbp
ret
ENDPROC_FRAME
Chapter 17. xdf: Extended Dynamic Object Format
The chapters in this part of the book document Yasm's support for various debugging formats.
Chapter 18. cv8: CodeView Debugging Format for VC8
Chapter 19. dwarf2: DWARF2 Debugging Format
Chapter 20. stabs: Stabs Debugging Format
The chapters in this part of the book document Yasm's support for various instruction set architectures.
The x86 architecture is the generic name for a multi-vendor 16-bit, 32-bit, and most recently 64-bit architecture. It was originally developed by Intel in the 8086 series of CPU, extended to 32-bit by Intel in the 80386 CPU, and extended by AMD to 64 bits in the Opteron and Athlon 64 CPU lines. While as of 2007, Intel and AMD are the highest volume manufacturers of x86 CPUs, many other vendors have also manufactured x86 CPUs. Generally the manufacturers have cross-licensed (or copied) major improvements to the architecture, but there are some unique features present in many of the implementations.
The x86 architecture has a variable instruction size that allows for moderate code compression while also allowing for very complex operand combinations as well as a very large instruction set size with many extensions. Instructions generally vary from zero to three operands with only a single memory operand allowed.
Different processors have different recommendations for the NOP (no operation) instructions used for padding in code. Padding is commonly performed to align loop boundaries to maximize performance, and it is key that the padding itself add minimal overhead. While the one-byte NOP 90h is standard across all x86 implementations, more recent generations of processors recommend different variations for longer padding sequences for optimal performance. Most processors that claim a 686 (e.g. Pentium Pro) generation or newer featureset support the "long" NOP opcode 0Fh 1Fh, although this opcode was undocumented until recently. Older processors that do not support these dedicated long NOP opcodes generally recommended alternative longer NOP sequences; while these sequences work as NOPs, they can cause decoding inefficiencies on newer processors.
Because of the various NOP recommendations, the code generated by the Yasm ALIGN directive depends on both the execution mode (BITS) setting and the processor selected by the CPU directive (see Section 21.2.1). Table 21.1 lists the various combinations of generated NOPs.
Table 21.1. x86 NOP Padding Modes
|
BITS |
CPU |
Padding |
|
16 |
Any |
16-bit short NOPs |
|
32 |
None given, or less than 686 |
32-bit short NOPs (no long NOPs) |
|
32 |
686 or newer Intel processor |
Intel guidelines, using long NOPs |
|
32 |
K6 or newer AMD processor |
AMD K10 guidelines, using long NOPs |
|
64 |
None |
Intel guidelines, using long NOPs |
|
64 |
686 or newer Intel processor |
Intel guidelines, using long NOPs |
|
64 |
K6 or newer AMD processor |
AMD K10 guidelines, using long NOPs |
In addition, the above defaults may be overridden by passing one of the options in Table 21.2 to the CPU directive.
Table 21.2. x86 NOP CPU Directive Options
|
Name |
Description |
|
Long NOPs not used |
|
|
Intel guidelines, using long NOPs |
|
|
AMD K10 guidelines, using long NOPs |
21.2. Execution Modes and Extensions
The x86 has been extended in many ways throughout its history, remaining mostly backwards compatible while adding execution modes and large extensions to the instruction set. A modern x86 processor can operate in one of four major modes: 16-bit real mode, 16-bit protected mode, 32-bit protected mode, and 64-bit long mode. The primary difference between real and protected mode is in the handling of segments: in real mode the segments directly address memory as 16-byte pages, whereas in protected mode the segments are instead indexes into a descriptor table that contains the physical base and size of the segment. 32-bit protected mode allows paging and virtual memory as well as a 32-bit rather than a 16-bit offset.
The 16-bit and 32-bit operating modes both allow for use of both 16-bit and 32-bit registers via instruction prefixes that set the operation and address size to either 16-bit or 32-bit, with the active operating mode setting the default operation size and the "other" size being flagged with a prefix. These operation and address sizes also affect the size of immediate operands: for example, an instruction with a 32-bit operation size with an immediate operand will have a 32-bit value in the encoded instruction, excepting optimizations such as sign-extended 8-bit values.
Unlike the 16-bit and 32-bit modes, 64-bit long mode is more of a break from the "legacy" modes. Long mode obsoletes several instructions. It is also the only mode in which 64-bit registers are available; 64-bit registers cannot be accessed from either 16-bit or 32-bit mode. Also, unlike the other modes, most encoded values in long mode are limited to 32 bits in size. A small subset of the MOV instructions allow 64 bit encoded values, but values greater than 32 bits in other instructions must come from a register. Partly due to this limitation, but also due to the wide use of relocatable shared libraries, long mode also adds a new addressing mode: RIP-relative.
The NASM parser allows setting what subsets of instructions and operands are accepted by Yasm via use of the CPU directive (see Section 5.8). As the x86 architecture has a very large number of extensions, both specific feature flags such as "SSE3" and CPU names such as "P4" can be specified. The feature flags have both normal and "no"-prefixed versions to turn on and off a single feature, while the CPU names turn on only the features listed, turning off all other features. Table 21.3 lists the feature flags, and Table 21.4 lists the CPU names Yasm supports. Having both feature flags and CPU names allows for combinations such as CPU P3 nofpu. Both feature flags and CPU names are case insensitive.
Table 21.3. x86 CPU Feature Flags
|
Name |
Description |
|
FPU |
Floating Point Unit (FPU) instructions |
|
MMX |
MMX SIMD instructions |
|
SSE |
Streaming SIMD Extensions (SSE) instructions |
|
SSE2 |
Streaming SIMD Extensions 2 instructions |
|
SSE3 |
Streaming SIMD Extensions 3 instructions |
|
SSSE3 |
Supplemental Streaming SIMD Extensions 3 instructions |
|
SSE4.1 |
Streaming SIMD Extensions 4, Penryn subset (47 instructions) |
|
SSE4.2 |
Streaming SIMD Extensions 4, Nehalem subset (7 instructions) |
|
SSE4 |
All Streaming SIMD Extensions 4 instructions (both SSE4.1 and SSE4.2) |
|
SSE4a |
Streaming SIMD Extensions 4a (AMD) |
|
SSE5 |
Streaming SIMD Extensions 5 |
|
XSAVE |
XSAVE instructions |
|
AVX |
Advanced Vector Extensions instructions |
|
FMA |
Fused Multiply-Add instructions |
|
AES |
Advanced Encryption Standard instructions |
|
CLMUL, PCLMULQDQ |
PCLMULQDQ instruction |
|
3DNow |
3DNow! instructions |
|
Cyrix |
Cyrix-specific instructions |
|
AMD |
AMD-specific instructions (older than K6) |
|
SMM |
System Management Mode instructions |
|
Prot, Protected |
Protected mode only instructions |
|
Undoc, Undocumented |
Undocumented instructions |
|
Obs, Obsolete |
Obsolete instructions |
|
Priv, Privileged |
Privileged instructions |
|
SVM |
Secure Virtual Machine instructions |
|
PadLock |
VIA PadLock instructions |
|
EM64T |
Intel EM64T or better instructions (not necessarily 64-bit only) |
|
Name |
Feature Flags |
Description |
|
8086 |
Priv |
Intel 8086 |
|
186, 80186, i186 |
Priv |
Intel 80186 |
|
286, 80286, i286 |
Priv |
Intel 80286 |
|
386, 80386, i386 |
SMM, Prot, Priv |
Intel 80386 |
|
486, 80486, i486 |
FPU, SMM, Prot, Priv |
Intel 80486 |
|
586, i586, Pentium, P5 |
FPU, SMM, Prot, Priv |
Intel Pentium |
|
686, i686, P6, PPro, PentiumPro |
FPU, SMM, Prot, Priv |
Intel Pentium Pro |
|
P2, Pentium2, Pentium-2, PentiumII, Pentium-II |
MMX, FPU, SMM, Prot, Priv |
Intel Pentium II |
|
P3, Pentium3, Pentium-3, PentiumIII, Pentium-III, Katmai |
SSE, MMX, FPU, SMM, Prot, Priv |
Intel Pentium III |
|
P4, Pentium4, Pentium-4, PentiumIV, Pentium-IV, Williamette |
SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel Pentium 4 |
|
IA64, IA-64, Itanium |
SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel Itanium (x86) |
|
K6 |
3DNow, MMX, FPU, SMM, Prot, Priv |
AMD K6 |
|
Athlon, K7 |
SSE, 3DNow, MMX, FPU, SMM, Prot, Priv |
AMD Athlon |
|
Hammer, Clawhammer, Opteron, Athlon64, Athlon-64 |
SSE2, SSE, 3DNow, MMX, FPU, SMM, Prot, Priv |
AMD Athlon64 and Opteron |
|
Prescott |
SSE3, SSE2, SSE MMX, FPU, SMM, Prot, Priv |
Intel codename Prescott |
|
Conroe, Core2 |
SSSE3, SSE3, SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel codename Conroe |
|
Penryn |
SSE4.1, SSSE3, SSE3, SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel codename Penryn |
|
Nehalem, Corei7 |
XSAVE, SSE4.2, SSE4.1, SSSE3, SSE3, SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel codename Nehalem |
|
Westmere |
CLMUL, AES, XSAVE, SSE4.2, SSE4.1, SSSE3, SSE3, SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel codename Westmere |
|
Sandybridge |
AVX, CLMUL, AES, XSAVE, SSE4.2, SSE4.1, SSSE3, SSE3, SSE2, SSE, MMX, FPU, SMM, Prot, Priv |
Intel codename Sandy Bridge |
|
Venice |
SSE3, SSE2, SSE, 3DNow, MMX, FPU, SMM, Prot, Priv |
AMD codename Venice |
|
K10, Phenom, Family10h |
SSE4a, SSE3, SSE2, SSE, 3DNow, MMX, FPU, SMM, Prot, Priv |
AMD codename K10 |
|
Bulldozer |
SSE5, SSE4a, SSE3, SSE2, SSE, 3DNow, MMX, FPU, SMM, Prot, Priv |
AMD codename Bulldozer |
In order to have access to 64-bit instructions, both a 64-bit capable CPU must be selected, and 64-bit assembly mode must be set (in NASM syntax) by either using BITS 64 (see Section 5.1) or targetting a 64-bit object format such aself64.
The default CPU setting is for the latest processor and all feature flags to be enabled; e.g. all x86 instructions for any processor, including all instruction set extensions and 64-bit instructions.
The 64-bit x86 register set consists of 16 general purpose registers, only 8 of which are available in 16-bit and 32-bit mode. The core eight 16-bit registers are AX, BX, CX, DX, SI, DI, BP, and SP. The least significant 8 bits of the first four of these registers are accessible via the AL, BL, CL, and DL in all execution modes. In 64-bit mode, the least significant 8 bits of the other four of these registers are also accessible; these are named SIL, DIL, SPL, and BPL. The most significant 8 bits of the first four 16-bit registers are also available, although there are some restrictions on when they can be used in 64-bit mode; these are named AH, BH, CH, and DH.
The 80386 extended these registers to 32 bits while retaining all of the 16-bit and 8-bit names that were available in 16-bit mode. The new extended registers are denoted by adding a E prefix; thus the core eight 32-bit registers are named EAX, EBX, ECX, EDX, ESI, EDI, EBP, and ESP. The original 8-bit and 16-bit register names map into the least significant portion of the 32-bit registers.
64-bit long mode further extended these registers to 64 bits in size by adding a R prefix to the 16-bit name; thus the base eight 64-bit registers are named RAX, RBX, etc. Long mode also added eight extra registers named numerically r8through r15. The least significant 32 bits of these registers are available via a d suffix (r8d through r15d), the least significant 16 bits via a w suffix (r8w through r15w), and the least significant 8 bits via a b suffix (r8b throughr15b).
Figure 21.1 summarizes the full 64-bit x86 general purpose register set.
Figure 21.1. x86 General Purpose Registers
Symbols
!=, %if: Testing Arbitrary Numeric Expressions
$
here, Expressions
prefix, Layout of a NASM Source Line, Numeric Constants
$$, Expressions, elf32 Special Symbols and WRT
% operator, *, /, //, % and %%: Multiplication and Division
%$$, Context-Local Labels
%% operator, *, /, //, % and %%: Multiplication and Division
%+, Concatenating Single Line Macro Tokens: %+
%+1, Condition Codes as Macro Parameters
%-1, Condition Codes as Macro Parameters
%0, Default Macro Parameters, %0: Macro Parameter Counter
%assign, Preprocessor Variables: %assign
%clear, Standard Macros
%define, The Normal Way: %define
%elif, Conditional Assembly, %if: Testing Arbitrary Numeric Expressions
%elifctx, %ifctx: Testing the Context Stack
%elifdef, %ifdef: Testing Single-Line Macro Existence
%elifid, %ifid, %ifnum, %ifstr: Testing Token Types
%elifidn, %ifidn and %ifidni: Testing Exact Text Identity
%elifidni, %ifidn and %ifidni: Testing Exact Text Identity
%elifmacro, %ifmacro: Testing Multi-Line Macro Existence
%elifnctx, %ifctx: Testing the Context Stack
%elifndef, %ifdef: Testing Single-Line Macro Existence
%elifnid, %ifid, %ifnum, %ifstr: Testing Token Types
%elifnidn, %ifidn and %ifidni: Testing Exact Text Identity
%elifnidni, %ifidn and %ifidni: Testing Exact Text Identity
%elifnmacro, %ifmacro: Testing Multi-Line Macro Existence
%elifnnum, %ifid, %ifnum, %ifstr: Testing Token Types
%elifnstr, %ifid, %ifnum, %ifstr: Testing Token Types
%elifnum, %ifid, %ifnum, %ifstr: Testing Token Types
%elifstr, %ifid, %ifnum, %ifstr: Testing Token Types
%else, Conditional Assembly
%endrep, Preprocessor Loops
%error, %error: Reporting User-Defined Errors
%exitrep, Preprocessor Loops
%iassign, Preprocessor Variables: %assign
%idefine, The Normal Way: %define
%if, Conditional Assembly, %if: Testing Arbitrary Numeric Expressions
%ifctx, %ifctx: Testing the Context Stack, Example Use of the Context Stack: Block IFs
%ifdef, %ifdef: Testing Single-Line Macro Existence
%ifid, %ifid, %ifnum, %ifstr: Testing Token Types
%ifidn, %ifidn and %ifidni: Testing Exact Text Identity
%ifidni, %ifidn and %ifidni: Testing Exact Text Identity
%ifmacro, %ifmacro: Testing Multi-Line Macro Existence
%ifnctx, %ifctx: Testing the Context Stack
%ifndef, %ifdef: Testing Single-Line Macro Existence
%ifnid, %ifid, %ifnum, %ifstr: Testing Token Types
%ifnidn, %ifidn and %ifidni: Testing Exact Text Identity
%ifnidni, %ifidn and %ifidni: Testing Exact Text Identity
%ifnmacro, %ifmacro: Testing Multi-Line Macro Existence
%ifnnum, %ifid, %ifnum, %ifstr: Testing Token Types
%ifnstr, %ifid, %ifnum, %ifstr: Testing Token Types
%ifnum, %ifid, %ifnum, %ifstr: Testing Token Types
%ifstr, %ifid, %ifnum, %ifstr: Testing Token Types
%imacro, Multi-Line Macros
%include, Including Other Files
%macro, Multi-Line Macros
%pop, The Context Stack, %push and %pop: Creating and Removing Contexts
%push, The Context Stack, %push and %pop: Creating and Removing Contexts
%rep, TIMES: Repeating Instructions or Data, Preprocessor Loops
%repl, %repl: Renaming a Context
%rotate, %rotate: Rotating Macro Parameters
%strlen, String Length: %strlen
%substr, Sub-strings: %substr
%undef, Undefining macros: %undef
%xdefine, Enhancing %define: %xdefine
%xidefine, Enhancing %define: %xdefine
& operator, &: Bitwise AND Operator
&&, %if: Testing Arbitrary Numeric Expressions
* operator, *, /, //, % and %%: Multiplication and Division
+ modifier, Greedy Macro Parameters
+ operator
binary, + and -: Addition and Subtraction Operators
unary, Unary Operators: +, -, ~ and SEG
- operator
binary, + and -: Addition and Subtraction Operators
unary, Unary Operators: +, -, ~ and SEG
--mapfile, Map Files
-f, __YASM_OBJFMT__ and __OUTPUT_FORMAT__: Output Object Format Keyword
..@, Macro-Local Labels
..@ symbol prefix, Local Labels
..got, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
..gotoff, elf32 Special Symbols and WRT
..gotpc, elf32 Special Symbols and WRT
..gotpcrel, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
..plt, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
..sym, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
.COM, bin: Flat-Form Binary Output
.comment, IDENT: Add file identification
.drectve, win32 Extensions to the SECTION Directive
.nolist, Disabling Listing Expansion
.obj, win32: Microsoft Win32 Object Files
.pdata, win64 Structured Exception Handling
.SYS, bin: Flat-Form Binary Output
.xdata, win64 Structured Exception Handling
/ operator, *, /, //, % and %%: Multiplication and Division
// operator, *, /, //, % and %%: Multiplication and Division
16-bit mode
versus 32-bit mode, BITS
32-bit, win32: Microsoft Win32 Object Files
32-bit mode
versus 64-bit mode, BITS
32-bit shared libraries, elf32 Special Symbols and WRT
64-bit, elf64: Executable and Linkable Format 64-bit Object Files, win64: PE32+ (Microsoft Win64) Object Files
64-bit shared libraries, elf64 Special Symbols and WRT
<, %if: Testing Arbitrary Numeric Expressions
<< operator, << and >>: Bit Shift Operators
<=, %if: Testing Arbitrary Numeric Expressions
<>, %if: Testing Arbitrary Numeric Expressions
=, %if: Testing Arbitrary Numeric Expressions
==, %if: Testing Arbitrary Numeric Expressions
>, %if: Testing Arbitrary Numeric Expressions
>=, %if: Testing Arbitrary Numeric Expressions
>> operator, << and >>: Bit Shift Operators
?, RESB and Friends: Declaring Uninitialized Data
[MAP], Map Files
^ operator, ^: Bitwise XOR Operator
^^, %if: Testing Arbitrary Numeric Expressions
__FILE__, __FILE__ and __LINE__: File Name and Line Number
__LINE__, __FILE__ and __LINE__: File Name and Line Number
__OUTPUT_FORMAT__, __YASM_OBJFMT__ and __OUTPUT_FORMAT__: Output Object Format Keyword
__SECT__, The __SECT__ Macro, ABSOLUTE: Defining Absolute Labels
__YASM_BUILD__, __YASM_MAJOR__, etc: Yasm Version
__YASM_MAJOR__, __YASM_MAJOR__, etc: Yasm Version
__YASM_MINOR__, __YASM_MAJOR__, etc: Yasm Version
__YASM_OBJFMT__, __YASM_OBJFMT__ and __OUTPUT_FORMAT__: Output Object Format Keyword
__YASM_SUBMINOR__, __YASM_MAJOR__, etc: Yasm Version
__YASM_VERSION_ID__, __YASM_MAJOR__, etc: Yasm Version
__YASM_VER__, __YASM_MAJOR__, etc: Yasm Version
| operator, |: Bitwise OR Operator
||, %if: Testing Arbitrary Numeric Expressions
~ operator, Unary Operators: +, -, ~ and SEG
A
ABS, RIP Relative Addressing, DEFAULT: Change the assembler defaults
ABSOLUTE, ABSOLUTE: Defining Absolute Labels
addition, + and -: Addition and Subtraction Operators
address-size prefixes, Layout of a NASM Source Line
after % sign, Concatenating Macro Parameters
algebra, Effective Addresses
ALIGN, ALIGN and ALIGNB: Data Alignment, bin Extensions to the SECTION Directive
code, NOP Padding
ALIGNB, ALIGN and ALIGNB: Data Alignment
alignment
code, NOP Padding
in win32 sections, win32 Extensions to the SECTION Directive
of common variables, ELF Extensions to the COMMON Directive
alignment in elf, ELF Extensions to the COMMON Directive
amd64, elf64: Executable and Linkable Format 64-bit Object Files, elfx32: ELF 32-bit Object Files for 64-bit Processors, x86 Architecture
amdnop, NOP Padding
arbitrary numeric expressions, %if: Testing Arbitrary Numeric Expressions
around macro parameters, Multi-Line Macros
Assembler Directives, NASM Assembler Directives
assembly passes, Critical Expressions
AT, ISTRUC, AT and IEND: Declaring Instances of Structures
B
basicnop, NOP Padding
bin, bin: Flat-Form Binary Output
binary, Numeric Constants, + and -: Addition and Subtraction Operators
Binary Files, INCBIN: Including External Binary Files
Binary origin, ORG: Binary Origin
Bit Shift, << and >>: Bit Shift Operators
BITS, BITS
bitwise AND, &: Bitwise AND Operator
bitwise OR, |: Bitwise OR Operator
bitwise XOR, ^: Bitwise XOR Operator
Block IFs, Example Use of the Context Stack: Block IFs
braces
after % sign, Concatenating Macro Parameters
around macro parameters, Multi-Line Macros
C
CALL FAR, SEG and WRT
case sensitive, The Normal Way: %define, Enhancing %define: %xdefine, Preprocessor Variables: %assign
case-insensitive, %ifidn and %ifidni: Testing Exact Text Identity
case-sensitive, Multi-Line Macros
changing sections, Changing and Defining Sections
character constant, DB and Friends: Declaring Initialized Data
Character Constants, Character Constants
circular references, The Normal Way: %define
code, NOP Padding
CodeView, cv8: CodeView Debugging Format for VC8
coff, coff: Common Object File Format
COFF
debugging, stabs: Stabs Debugging Format
colon, Layout of a NASM Source Line
COMMON, COMMON: Defining Common Data Areas
Common Object File Format, coff: Common Object File Format
common variables, COMMON: Defining Common Data Areas
alignment in elf, ELF Extensions to the COMMON Directive
Concatenating Macro Parameters, Concatenating Macro Parameters
Condition Codes as Macro Parameters, Condition Codes as Macro Parameters
Conditional Assembly, Conditional Assembly
conditional-return macro, Condition Codes as Macro Parameters
Constants, Constants
constants, Floating-Point Constants
context stack, %ifctx: Testing the Context Stack
Context Stack, The Context Stack, Example Use of the Context Stack: Block IFs
Context-Local Labels, Context-Local Labels
Context-Local Single-Line Macros, Context-Local Single-Line Macros
counting macro parameters, %0: Macro Parameter Counter
CPU, CPU: Defining CPU Dependencies
CPUID, Character Constants
creating contexts, %push and %pop: Creating and Removing Contexts
critical expression, RESB and Friends: Declaring Uninitialized Data, EQU: Defining Constants, Preprocessor Variables: %assign, ABSOLUTE: Defining Absolute Labels
Critical Expressions, Critical Expressions
cv8, cv8: CodeView Debugging Format for VC8
D
data, ELF Extensions to the GLOBAL Directive
DB, DB and Friends: Declaring Initialized Data, String Constants
DD, DB and Friends: Declaring Initialized Data, String Constants, Floating-Point Constants
DDQ, DB and Friends: Declaring Initialized Data
DDQWORD, Layout of a NASM Source Line
debugging, dwarf2: DWARF2 Debugging Format, stabs: Stabs Debugging Format
Declaring Structure, STRUC and ENDSTRUC: Declaring Structure Data Types
DEFAULT, RIP Relative Addressing, DEFAULT: Change the assembler defaults
default, ELF Extensions to the GLOBAL Directive
Default Macro Parameters, Default Macro Parameters
Defining Sections, Changing and Defining Sections
directives, ELF Directives
Disabling Listing Expansion, Disabling Listing Expansion
division, *, /, //, % and %%: Multiplication and Division
DO, DB and Friends: Declaring Initialized Data
DQ, DB and Friends: Declaring Initialized Data, String Constants, Floating-Point Constants
DT, DB and Friends: Declaring Initialized Data, Floating-Point Constants
DUP, TIMES: Repeating Instructions or Data
DW, DB and Friends: Declaring Initialized Data, String Constants, Floating-Point Constants
DWARF, dwarf2: DWARF2 Debugging Format
dwarf2, dwarf2: DWARF2 Debugging Format
DWORD, Layout of a NASM Source Line
E
effective address, Effective Addresses
effective addresses, Layout of a NASM Source Line
effective-address, Critical Expressions
elf, elf32: Executable and Linkable Format 32-bit Object Files, elf64: Executable and Linkable Format 64-bit Object Files
directives, ELF Directives
elf32, elf32: Executable and Linkable Format 32-bit Object Files
elf64, elf64: Executable and Linkable Format 64-bit Object Files
elfx32, elfx32: ELF 32-bit Object Files for 64-bit Processors
SECTION, ELF Sections
symbol size, SIZE: Set symbol size
symbol type, TYPE: Set symbol type
weak reference, WEAK: Create weak symbol
ELF
32-bit shared libraries, elf32 Special Symbols and WRT
64-bit shared libraries, elf64 Special Symbols and WRT
debugging, dwarf2: DWARF2 Debugging Format, stabs: Stabs Debugging Format
x32 shared libraries, elfx32 Special Symbols and WRT
elf32, elf32: Executable and Linkable Format 32-bit Object Files
elf64, elf64: Executable and Linkable Format 64-bit Object Files
elfx32, elfx32: ELF 32-bit Object Files for 64-bit Processors
ENDSTRUC, STRUC and ENDSTRUC: Declaring Structure Data Types, ABSOLUTE: Defining Absolute Labels
EQU, EQU: Defining Constants, Critical Expressions
exact text identity, %ifidn and %ifidni: Testing Exact Text Identity
Executable and Linkable Format, elf32: Executable and Linkable Format 32-bit Object Files
64-bit, elf64: Executable and Linkable Format 64-bit Object Files
x32, elfx32: ELF 32-bit Object Files for 64-bit Processors
Exporting Symbols, GLOBAL: Exporting Symbols
Expressions, Expressions
Extended Dynamic Object, xdf: Extended Dynamic Object Format
EXTERN, EXTERN: Importing Symbols
F
far pointer, SEG and WRT
Flash, bin: Flat-Form Binary Output
Flat-Form Binary, bin: Flat-Form Binary Output
floating-point, Layout of a NASM Source Line, DB and Friends: Declaring Initialized Data
constants, Floating-Point Constants
FOLLOWS, bin Extensions to the SECTION Directive
format-specific directives, NASM Assembler Directives
forward references, Critical Expressions
FreeBSD, elf32: Executable and Linkable Format 32-bit Object Files
function, TYPE: Set symbol type, ELF Extensions to the GLOBAL Directive
G
gdb, dwarf2: DWARF2 Debugging Format, stabs: Stabs Debugging Format
GLOBAL, GLOBAL: Exporting Symbols, ELF Extensions to the GLOBAL Directive
global offset table, elf32 Special Symbols and WRT
GOT, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
graphics, INCBIN: Including External Binary Files
Greedy Macro Parameters, Greedy Macro Parameters
groups, SEG and WRT
H
here, Expressions
hex, Numeric Constants
hidden, ELF Extensions to the GLOBAL Directive
I
IDENT, IDENT: Add file identification
IEND, ISTRUC, AT and IEND: Declaring Instances of Structures
Immediates, Immediate Operands
Importing Symbols, EXTERN: Importing Symbols
in win32, win32 Extensions to the SECTION Directive
in win32 sections, win32 Extensions to the SECTION Directive
INCBIN, INCBIN: Including External Binary Files, String Constants
Including Other Files, Including Other Files
infinite loop, Expressions
informational section, win32 Extensions to the SECTION Directive
Initialized, DB and Friends: Declaring Initialized Data
Instances of Structures, ISTRUC, AT and IEND: Declaring Instances of Structures
Intel number formats, Floating-Point Constants
intelnop, NOP Padding
internal, ELF Extensions to the GLOBAL Directive
ISTRUC, ISTRUC, AT and IEND: Declaring Instances of Structures
iterating over macro parameters, %rotate: Rotating Macro Parameters
L
label prefix, Local Labels
library, WEAK: Create weak symbol
Linux
elf, elf32: Executable and Linkable Format 32-bit Object Files, elf64: Executable and Linkable Format 64-bit Object Files
x32, elfx32: ELF 32-bit Object Files for 64-bit Processors
little-endian, Character Constants
LMA, bin Extensions to the SECTION Directive
Local Labels, Local Labels
logical AND, %if: Testing Arbitrary Numeric Expressions
logical OR, %if: Testing Arbitrary Numeric Expressions
logical XOR, %if: Testing Arbitrary Numeric Expressions
M
Mac OSX, macho32: Mach 32-bit Object File Format, macho64: Mach 64-bit Object File Format
Mach-O, macho32: Mach 32-bit Object File Format, macho64: Mach 64-bit Object File Format
macho
macho32, macho32: Mach 32-bit Object File Format
macho64, macho64: Mach 64-bit Object File Format
macho32, macho32: Mach 32-bit Object File Format
macho64, macho64: Mach 64-bit Object File Format
macro processor, The NASM Preprocessor
Macro-Local Labels, Macro-Local Labels
macros, TIMES: Repeating Instructions or Data
Map file, Map Files
memory reference, Effective Addresses
Microsoft Visual Studio 2010, VSYASM - Yasm for Microsoft Visual Studio 2010
modulo operators, *, /, //, % and %%: Multiplication and Division
MSBUILD, VSYASM - Yasm for Microsoft Visual Studio 2010
multi-line macro existence, %ifmacro: Testing Multi-Line Macro Existence
Multi-Line Macros, Multi-Line Macros
multi-line macros, Overloading Multi-Line Macros
multiplication, *, /, //, % and %%: Multiplication and Division
multipush, %rotate: Rotating Macro Parameters
N
NOP, NOP Padding
NOSPLIT, Effective Addresses
numeric constant, DB and Friends: Declaring Initialized Data
Numeric Constants, Numeric Constants
O
object, TYPE: Set symbol type
octal, Numeric Constants
of common variables, ELF Extensions to the COMMON Directive
of symbols, SIZE: Set symbol size, TYPE: Set symbol type, ELF Extensions to the GLOBAL Directive
omitted parameters, Default Macro Parameters
one's complement, Unary Operators: +, -, ~ and SEG
operand-size prefixes, Layout of a NASM Source Line
operands, Layout of a NASM Source Line
operators, Expressions
ORG, ORG: Binary Origin
Origin, ORG: Binary Origin
orphan-labels, Layout of a NASM Source Line
overlapping segments, SEG and WRT
overloading
multi-line macros, Overloading Multi-Line Macros
single-line macros, The Normal Way: %define
OWORD, Layout of a NASM Source Line
P
padding, NOP Padding
paradox, Critical Expressions
passes, Critical Expressions
PE, win32: Microsoft Win32 Object Files
PE32+, win64: PE32+ (Microsoft Win64) Object Files
period, Local Labels
PIC, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT
PIC-specific, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
PLT, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
Position-Independent Code, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
pre-define, The Normal Way: %define
precedence, Expressions
preferred, SEG and WRT
prefix, Layout of a NASM Source Line, Numeric Constants
preprocessor, EQU: Defining Constants
Preprocessor Loops, Preprocessor Loops
Preprocessor Variables, Preprocessor Variables: %assign
primitive directives, NASM Assembler Directives
procedure linkage table, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
Processor Mode, Specifying Target Processor Mode
protected, ELF Extensions to the GLOBAL Directive
pseudo-instructions, Pseudo-Instructions
PUBLIC, GLOBAL: Exporting Symbols
pure binary, bin: Flat-Form Binary Output
Q
QWORD, Layout of a NASM Source Line
R
rdf, rdf: Relocatable Dynamic Object File Format
RDOFF, rdf: Relocatable Dynamic Object File Format
REL, RIP Relative Addressing, DEFAULT: Change the assembler defaults
relational operators, %if: Testing Arbitrary Numeric Expressions
Relocatable Dynamic Object File Format, rdf: Relocatable Dynamic Object File Format
relocations
PIC-specific, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT, elfx32 Special Symbols and WRT
removing contexts, %push and %pop: Creating and Removing Contexts
renaming contexts, %repl: Renaming a Context
Repeating, TIMES: Repeating Instructions or Data
repeating code, Preprocessor Loops
RESB, RESB and Friends: Declaring Uninitialized Data, Critical Expressions
RESD, RESB and Friends: Declaring Uninitialized Data
RESDQ, RESB and Friends: Declaring Uninitialized Data
RESO, RESB and Friends: Declaring Uninitialized Data
RESQ, RESB and Friends: Declaring Uninitialized Data
REST, RESB and Friends: Declaring Uninitialized Data
RESW, RESB and Friends: Declaring Uninitialized Data
REX, BITS
Rotating Macro Parameters, %rotate: Rotating Macro Parameters
S
searching for include files, Including Other Files
SECTION, SECTION and SEGMENT, ELF Sections, win32 Extensions to the SECTION Directive
win32 extensions to, win32 Extensions to the SECTION Directive
section alignment
in win32, win32 Extensions to the SECTION Directive
section.length, bin Special Symbols
section.start, bin Special Symbols
section.vstart, bin Special Symbols
SEG, Unary Operators: +, -, ~ and SEG, SEG and WRT
segment address, Unary Operators: +, -, ~ and SEG, SEG and WRT
segment override, Layout of a NASM Source Line
segmentation
x86, Segmentation
segments, SEG and WRT
shift command, %rotate: Rotating Macro Parameters
signed division, *, /, //, % and %%: Multiplication and Division
signed modulo, *, /, //, % and %%: Multiplication and Division
single-line macro existence, %ifdef: Testing Single-Line Macro Existence
Single-line macros, The Normal Way: %define
single-line macros, The Normal Way: %define
size
of symbols, SIZE: Set symbol size, ELF Extensions to the GLOBAL Directive
SIZE, SIZE: Set symbol size
Solaris x86, elf32: Executable and Linkable Format 32-bit Object Files
Solaris x86-64, elf64: Executable and Linkable Format 64-bit Object Files
sound, INCBIN: Including External Binary Files
specifying, SIZE: Set symbol size, TYPE: Set symbol type, ELF Extensions to the GLOBAL Directive
square brackets, Effective Addresses
stabs, stabs: Stabs Debugging Format
Standard Macros, Standard Macros
standard section names, win32 Extensions to the SECTION Directive
standardised section names, Standardized Section Names
STRICT, STRICT: Inhibiting Optimization
string constant, DB and Friends: Declaring Initialized Data
String Constants, String Constants
String Handling in Macros, String Handling in Macros
String Length, String Length: %strlen
STRUC, STRUC and ENDSTRUC: Declaring Structure Data Types, ABSOLUTE: Defining Absolute Labels
structured exceptions, win64 Structured Exception Handling
Sub-strings, Sub-strings: %substr
subtraction, + and -: Addition and Subtraction Operators
switching between sections, Changing and Defining Sections
symbol size, SIZE: Set symbol size
symbol sizes
specifying, SIZE: Set symbol size, ELF Extensions to the GLOBAL Directive
symbol type, TYPE: Set symbol type
symbol types
specifying, TYPE: Set symbol type, ELF Extensions to the GLOBAL Directive
T
testing
arbitrary numeric expressions, %if: Testing Arbitrary Numeric Expressions
context stack, %ifctx: Testing the Context Stack
exact text identity, %ifidn and %ifidni: Testing Exact Text Identity
multi-line macro existence, %ifmacro: Testing Multi-Line Macro Existence
single-line macro existence, %ifdef: Testing Single-Line Macro Existence
token types, %ifid, %ifnum, %ifstr: Testing Token Types
TIMES, TIMES: Repeating Instructions or Data, Critical Expressions
token types, %ifid, %ifnum, %ifstr: Testing Token Types
two-pass assembler, Critical Expressions
TWORD, Layout of a NASM Source Line
type
of symbols, TYPE: Set symbol type, ELF Extensions to the GLOBAL Directive
TYPE, TYPE: Set symbol type
U
unary, Unary Operators: +, -, ~ and SEG
Unary Operators, Unary Operators: +, -, ~ and SEG
Uninitialized, RESB and Friends: Declaring Uninitialized Data
UnixWare, elf32: Executable and Linkable Format 32-bit Object Files
unrolled loops, TIMES: Repeating Instructions or Data
unsigned division, *, /, //, % and %%: Multiplication and Division
unsigned modulo, *, /, //, % and %%: Multiplication and Division
unwind data, win64 Structured Exception Handling
USE16, USE16, USE32, and USE64
USE32, USE16, USE32, and USE64
USE64, USE16, USE32, and USE64
User-Defined Errors, %error: Reporting User-Defined Errors
user-level assembler directives, Standard Macros
user-level directives, NASM Assembler Directives
V
Valid characters, Layout of a NASM Source Line
VALIGN, bin Extensions to the SECTION Directive
version control, IDENT: Add file identification
version number of Yasm, __YASM_MAJOR__, etc: Yasm Version
versus 32-bit mode, BITS
versus 64-bit mode, BITS
VFOLLOWS, bin Extensions to the SECTION Directive
Vista, win32: Microsoft Win32 Object Files
Vista x64, win64: PE32+ (Microsoft Win64) Object Files
Visual Studio, win32: Microsoft Win32 Object Files, win64: PE32+ (Microsoft Win64) Object Files
Visual Studio 2005, cv8: CodeView Debugging Format for VC8
Visual Studio 2008, cv8: CodeView Debugging Format for VC8
Visual Studio 2010, VSYASM - Yasm for Microsoft Visual Studio 2010
VMA, bin Extensions to the SECTION Directive
VSYASM, VSYASM - Yasm for Microsoft Visual Studio 2010
W
WEAK, WEAK: Create weak symbol
weak reference, WEAK: Create weak symbol
win32, win32: Microsoft Win32 Object Files
SECTION, win32 Extensions to the SECTION Directive
Win32, win32: Microsoft Win32 Object Files
win32 extensions to, win32 Extensions to the SECTION Directive
win64, win64: PE32+ (Microsoft Win64) Object Files
Win64, win64: PE32+ (Microsoft Win64) Object Files
Windows
32-bit, win32: Microsoft Win32 Object Files
64-bit, win64: PE32+ (Microsoft Win64) Object Files
Windows XP, win32: Microsoft Win32 Object Files
Windows XP x64, win64: PE32+ (Microsoft Win64) Object Files
WRT, SEG and WRT, elf32 Special Symbols and WRT, elf64 Special Symbols and WRT
X
x32, elfx32: ELF 32-bit Object Files for 64-bit Processors
x32 shared libraries, elfx32 Special Symbols and WRT
x64, win64: PE32+ (Microsoft Win64) Object Files
structured exceptions, win64 Structured Exception Handling
x86, x86 Architecture, Segmentation
xdf, xdf: Extended Dynamic Object Format
Y
Yasm Version, __YASM_MAJOR__, etc: Yasm Version
SRC=http://www.tortall.net/projects/yasm/manual/html/manual.html