COMP16
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
         PLA
         STA   :SUBT      ; SUBTRAHEND
         PLA
         STA   :SUBT+1
         PLA
         STA   :MINU      ; MINUEND
         PLA
         STA   :MINU+1
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
         LDA   :MINU
         CMP   :SUBT      ; COMPARE LOW BYTES
         BEQ   :EQUAL
         LDA   :MINU+1
         SBC   :SUBT+1    ; COMPARE HIGH BYTES
         ORA   #1         ; MAKE Z=0, SINCE LOW
                          ; BYTES ARE NOT EQUAL
         BVS   :OVFLOW    ; MUST HANDLE OVERFLOW
         RTS              ; EXIT
:EQUAL
         LDA   :MINU+1
         SBC   :SUBT+1    ; UPPER BYTES
         BVS   :OVFLOW
         RTS              ; RETURN W FLAGS SET
:OVFLOW
         EOR   #$80       ; COMPLEMENT N FLAG
         ORA   #1         ; IF OVERFLOW THEN THE
         RTS
:MINU    DS    2
:SUBT    DS    2