# PDP-8 Instruction Format

PDP-8 machine instructions are 12-bit values; each instruction occupies exactly one 12-bit memory word. There are eight basic instructions, separated in three classes:

## Memory Reference Instructions (MRIs)

| Opcode |  | Page Bit | Page Address
Bits | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
| Address Mode Bit

Mnemonic | Octal | Opcode | Description
--- | --- | --- | ---
AND | 0xxx | 000 | Logical AND
TAD | 1xxx | 001 | Two’s complemend add
ISZ | 2xxx | 010 | Increment and skip if zero
DCA | 3xxx | 011 | Deposit and clear AC
JMS | 4xxx | 100 | Jump to subroutine
JMP | 5xxx | 101 | Jump

## Input Output Transfer Instructions (IOTs)

| Opcode 110 | I/O Device Address | I/O Operation
Bits | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11

## Operate Instructions (OPRs)

| Opcode 111 |  | Microoperation
Bits | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
| Group

Note that EAE instructions are special OPRs. Some of them require a direct operand in an extension word of the instruction.