The basic introduction to MIX language and machine

Reference:the Mix computer, the mix Introduction sets,

The basic Info storage unit in MIX computer are the byte, which stores positive values in the range of 0 to 63. In MIX, Byte is only have 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 is numbered from 1 through 5, the sign of a number was 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 firs T 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 instructi Ons.

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 is abbreviated as E, G, L

/ o

Input-output block device, each device labelled as UN, ranging from 0 to 20. and u0 to U7 was magnetic tape units, U8 through, disks and drums, U16 was a card reader, U17 is a card writer, U18 I s 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 inst Rutions in a internal register called location counter (known as PC on other computer)

Instruction structure

MIX instructions is codified as words with the following subfield Sturcture:

Subfield	fspec	Description
address	(0:2)	the first and bytes plus sign is the  address  field. Combined with the INDEX field, denotes the memory address to being used by the instruction.
index	(3:3)	the third byte is The index , normally used for indexing the ADDRESS3.
mod	(4:4)	byte four are used either as an O Peration 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 is the Address field, combined with the index field (for address stored in INDEX register Refe Red by INDEX Byte)

For given a instruction, ' add ' stands for the mem address after combining the address of field and and ' cont ' is t He 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 ' to MOD 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.

The basic introduction to MIX language and machine