Operation

8x8 Unsigned Multiply Routine shows the instruction sequence for an 8x8 unsigned multiplication. Only one instruction is required when one of the arguments is already loaded in the WREG register. 8x8 Signed Multiply Routine shows the sequence to do an 8x8 signed multiplication. To account for the sign bits of the arguments, each argument’s Most Significant bit (MSb) is tested and the appropriate subtractions are done.

Copy
        MOVF    ARG1, W   ;

        MULWF   ARG2      ; ARG1 * ARG2 -> PRODH:PRODL

Copy
        MOVF    ARG1, W
        MULWF   ARG2       ; ARG1 * ARG2 -> PRODH:PRODL
        BTFSC   ARG2, SB   ; Test Sign Bit
        SUBWF   PRODH, F   ; PRODH = PRODH - ARG1
        MOVF    ARG2, W
        BTFSC   ARG1, SB   ; Test Sign Bit
        SUBWF   PRODH, F   ; PRODH = PRODH - ARG2