reference: The MIX Computer, The MIX Introduction sets,

The basic info storage unit in MIX computer is the byte, which stores positive values in the range of 0 to 63. In MIX, Byte only has 6 bytes, and the word Byte refer to MIX 6-byte.

A MIX word is defined as set of 5 bytes and plus a sign. The bytes within a word are numbered from 1 to 5, The sign of a number is denoted by index 0. Graphincally.

 -----------------------------------------------

|   0   |   1   |   2   |   3   |   4   |   5   |

 -----------------------------------------------

|  +/-  | byte  | byte  | byte  | byte  | byte  |

 -----------------------------------------------

We can refer to subfields within a word using a field specification or fspec of the form "(L:R)", where L denotes the first byte, and R the last byte of the subfield.

Registers:

rA

A register, general purpose register holding a word usually serving as the operadn of the arithmetic and storing instructions.

rX

X register, general purpose register holding a word, acting as an extension or a replacement of 'rA'

rJ

jump register stores positive 2-bytes values, representing a jump addresss.

rI1, rI2, rI3, rI4, rI5, rI6

OV

overflow toggle( a bit with valuess on and off), denoted as OV

CM

comparision indicator, having 3 values: EQUAL, GREATER or LESS, possible value are abbreviated as E, G, L

I/O

input-output block device, each device labelled as un, ranging from 0 to 20. And u0 to u7 are magnetic tape units, u8 through 15 are disks and drums, u16 is a card reader, u17 is a card writer, u18 is a line printer and, u19 is a typewirter terminal and u20 a paper tape.

CPU

MIX owes a virtual CPU which controls the above components, able to execute a rich set of instructions. As von Neumann computer, the CPU fetches binary instructions from memory sequentially, and stores the add of the next instrutions in an internal register called location counter(known as PC in other computer)

Instruction structure

MIX instructions are codified as words with the following subfield sturcture:

Subfield	fspec	Description
ADDRESS	(0:2)	The first two bytes plus sign are the address field. Combined with the INDEX field, denotes the memory address to be used by the instruction.
INDEX	(3:3)	The third byte is the index, normally used for indexing the address3.
MOD	(4:4)	Byte four is used either as an operation code modifier or as a field specification.
OPCODE	(5:5)	The last (least significant) byte in the word denotes the operation code.

The first 2 bytes plus sign are the address field, combined with the INDEX field(for address stored in index register refered by INDEX byte)

For given a instruction, 'add' stands for the mem address after combining the address in address field and and 'cont' is the contents of the subfield indicated by MOD of the memory cell with address.

[rI2] = + 00 63

[31] = - 10 11 00 11 22

Given instruction ‘I = - 00 32 02 11 10’,  we have:

ADDRESS = - 00 32

INDEX = 02 = 2

MOD = 11 = (1:3)

OPCODE = 10

add = ADDRESS + [rI2] = 63 + (-32) = 31

cont = [M](MOD) = (- 10 11 00 11 22)(1:3)

While  "MOD" means we use left padding to the bytes selected by ‘MOD’ to obtain a complete word as the result.

IF we use mnemonic name assigned to this MIX instruction, we can rewrite the above instruction as:

LD2 -32, 2(1:3)

or, for a generic instruction:

　　MNEMONIC ADDRESSS, INDEX(MOD)

note:

MOD equals to (0:5) means the whole word.