MIT PDP-10 'Info' file converted to Hypertext 'html' format by Henry Baker

Instruction Formats

The PDP-10 is a word oriented machine. Words contain 36 data bits, numbered (left to right) 0 to 35. Every machine instruction is one word. The program counter or PC is a register which contains the address of the next word to be used as an instruction; after a normal instruction, the PC is incremented by one so that successive instructions come from successive words.

There are two formats for machine instructions. Most instructions have the format:

 000000000 0111 1 1111 112222222222333333
 012345678 9012 3 4567 890123456789012345
 ________________________________________
|         |    | |    |                  |
|   OP    | AC |I| X  |        Y         |
|_________|____|_|____|__________________|

Input-output instructions (which are illegal in user mode) have the format: (These instructions are not discussed any farther by this file).

 000 0000000 111 1 1111 112222222222333333
 012 3456789 012 3 4567 890123456789012345
 _________________________________________
|   |       |   | |    |                  |
|111|  DEV  |IOP|I| X  |        Y         |
|___|_______|___|_|____|__________________|

In the diagrams above the field names are

        OP  = operation code
        AC  = accumulator field
        I   = indirect bit
        X   = index field
        Y   = address field
        DEV = device code
        IOP = input-output operation code

Some example intructions are:

        MOVE 1,@100             ;MOVE is the OP.  AC is 1.
                                ;@ sets the I bit.
                                ;X is zero, Y is 100.

        HRRZ 17,1(3)            ;HRRZ is the OP. AC is 17,
                                ;Y = 1, X = 3, I = 0

        SOS FOO                 ;SOS is OP, FOO is symbolic
                                ;for the Y field.  AC, X, I are 0.

The address field, the index field, and the indirect bit are all used in effective address computation. See also Addr-Comp.