PDP-10 Instruction Format

PDP-10 machine code instructions are stored in 36-bit words with the
following format:

Bits   Length  Contents
0-8    9       OP: operation code
9-12   4       AC: accumulator
13     1       I: indirect flag
14-17  4       X: index
18-35  18      Y: address

(Input/output instructions have a different format, but are only allowed
in privileged programs.)


Instruction fields are represented in assembly language statements as 
follows:

  OM              (AC, I, Y, X are zero)
  OM Y            (AC, I, X are zero)
  OM AC,          (I, Y, X are zero)
  OM AC,Y         (I and X are zero)
  OM AC,Y(X)      (I is zero)
  OM AC,@Y        (I is 1, X is zero)
  OM AC,@Y(X)     (AC, I, Y, X all non-zero)

(OM: operation code mnemonic name)