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