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X：处理器的排名，例如：8086、80286、80386等等

inc.asm

; Segment Attribute//段属性定义
DA_32    equ    0x4000
DA_DR    equ    0x90
DA_DRW   equ    0x92
DA_DRWA  equ    0x93
DA_C     equ    0x98
DA_CR    equ    0x9A
DA_CCO   equ    0x9C
DA_CCOR  equ    0x9E

; Selector Attribute//选择子属性定义
SA_RPL0    equ    0
SA_RPL1    equ    1
SA_RPL2    equ    2
SA_RPL3    equ    3

SA_TIG    equ    0
SA_TIL    equ    4

; 描述符
; usage: Descriptor Base, Limit, Attr
;        Base:  dd
;        Limit: dd (low 20 bits available)
;        Attr:  dw (lower 4 bits of higher byte are always 0)
%macro Descriptor 3	                          ; 段基址， 段界限， 段属性
    dw    %2 & 0xFFFF                         ; 段界限1
    dw    %1 & 0xFFFF                         ; 段基址1
    db    (%1 >> 16) & 0xFF                   ; 段基址2
    dw    ((%2 >> 8) & 0xF00) | (%3 & 0xF0FF) ; 属性1 + 段界限2 + 属性2
    db    (%1 >> 24) & 0xFF                   ; 段基址3
%endmacro                                     ; 共 8 字节

loader.asm

%include "inc.asm"

org 0x9000

jmp CODE16_SEGMENT

[section .gdt]
; GDT definition
; 
GDT_ENTRY       :     Descriptor    0,            0,           0
CODE32_DESC     :     Descriptor    0,    Code32SegLen  - 1,   DA_C + DA_32
; GDT end

GdtLen    equ   $ - GDT_ENTRY

GdtPtr:
          dw   GdtLen - 1
          dd   0
          
          
; GDT Selector

Code32Selector    equ (0x0001 << 3) + SA_TIG + SA_RPL0

; end of [section .gdt]

//实模式代码段
[section .s16]
[bits 16]
CODE16_SEGMENT:
    mov ax, cs
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7c00
    
    ; initialize GDT for 32 bits code segment
    mov eax, 0
    mov ax, cs
    shl eax, 4//左移4位
    add eax, CODE32_SEGMENT
    mov word [CODE32_DESC + 2], ax
    shr eax, 16
    mov byte [CODE32_DESC + 4], al
    mov byte [CODE32_DESC + 7], ah
    
    ; initialize GDT pointer struct
    mov eax, 0
    mov ax, ds
    shl eax, 4
    add eax, GDT_ENTRY
    mov dword [GdtPtr + 2], eax

    ; 1. load GDT
    lgdt [GdtPtr]
    
    ; 2. close interrupt
    cli //关中断
    
    ; 3. open A20
    in al, 0x92
    or al, 00000010b
    out 0x92, al
    
    ; 4. enter protect mode//通知处理器进入保护模式
    mov eax, cr0
    or eax, 0x01
    mov cr0, eax
    
    ; 5. jump to 32 bits code
    jmp dword Code32Selector : 0

[section .s32]
[bits 32]
CODE32_SEGMENT:
    mov eax, 0
    jmp CODE32_SEGMENT

Code32SegLen    equ    $ - CODE32_SEGMENT

makefile

.PHONY : all clean rebuild

BOOT_SRC := boot.asm
BOOT_OUT := boot

LOADER_SRC  := loader.asm
INCLUDE_SRC := inc.asm
LOADER_OUT  := loader

IMG := data.img
IMG_PATH := /mnt/hgfs

RM := rm -fr

all : $(IMG) $(BOOT_OUT) $(LOADER_OUT)
	@echo "Build Success ==> D.T.OS!"

$(IMG) :
	bximage $@ -q -fd -size=1.44
	
$(BOOT_OUT) : $(BOOT_SRC)
	nasm $^ -o $@
	dd if=$@ of=$(IMG) bs=512 count=1 conv=notrunc
	
$(LOADER_OUT) : $(LOADER_SRC) $(INCLUDE_SRC)
	nasm $< -o $@
	sudo mount -o loop $(IMG) $(IMG_PATH)
	sudo cp $@ $(IMG_PATH)/$@
	sudo umount $(IMG_PATH)
	
clean :
	$(RM) $(IMG) $(BOOT_OUT) $(LOADER_OUT)
	
rebuild :
	@$(MAKE) clean
	@$(MAKE) all