Memory Fetch Macro

All fetches from simulated PDP-8 memory go through an instance of this macro. (See STORE.)

FETCH    MACRO MA,MB       FETCH FROM MEMORY
          MX6    -10
          BX1    X6*MA
          AX1    6
          SB6    X1
          BX1    -X6*MA
          SA1    X1+MEMORY
          A&MB   X1,B6
          B&MB   X0*MB
          ENDM

Four PDP-8 words are blocked into one CDC word. The macro first separates the PDP-8 address (MA) into two parts, a 10-bit block number (in X1) and a 2-bit word within block (in B6) scaled to be a multiple of 16.

The proper block is fetched from MEMORY. Then the desired word is extracted and returned in the desired X register (MB)..

Memory Fetch Routine

FORTRAN code gains access to the fetch logic through this one adaptor routine.

FETCH    PS
          RJ     RESREG
          FETCH X3,X4
          RJ     SAVREG
          EQ     FETCH

PDP-8 Simulator