Registers A1 and B6

Fetch leaves CDC machine addresses in registers so that they need not be recomputed from a PDP-8 format address in a subsequent store. For example, the ISZ (increment and skip if zero) instruction handler computes the physical address parameters from X3 only once.

     ISZ      FETCH X3,X4
              SX4    X4+B1
              BX4    X0*X4
              STORE ,X4
              ...
Between these to macro expansions A1 contains the address of a word block and B6 contains the shift count required to isolate the desired word within the block. See other register assignments.

PDP-8 Simulator