16x16 Unsigned Multiplication Algorithm

16x16 Unsigned Multiply Routine shows the sequence to do a 16x16 unsigned multiplication. 16x16 Unsigned Multiply Algorithm shows the algorithm that is used. The 32-bit result is stored in four registers.

16x16 Unsigned Multiply Algorithm

RES3:RES0=ARG1H:ARG1L•ARG2H:ARG2L=(ARG1H•ARG2H•216)+(ARG1H•ARG2L•28)+(ARG1L•ARG2H•28)+(ARG1L•ARG2L)

16x16 Unsigned Multiply Routine


        MOVF    ARG1L, W
        MULWF   ARG2L           ; ARG1L * ARG2L → PRODH:PRODL
        MOVFF   PRODH, RES1     ;
        MOVFF   PRODL, RES0     ;
;
        MOVF    ARG1H, W        ;
        MULWF   ARG2H           ; ARG1H * ARG2H → PRODH:PRODL
        MOVFF   PRODH, RES3     ;
        MOVFF   PRODL, RES2     ;
;
        MOVF    ARG1L, W
        MULWF   ARG2H           ; ARG1L * ARG2H → PRODH:PRODL
        MOVF    PRODL, W        ;
        ADDWF   RES1, F         ; Add cross products
        MOVF    PRODH, W        ;
        ADDWFC  RES2, F         ;
        CLRF    WREG            ;
        ADDWFC  RES3, F         ;
;
        MOVF    ARG1H, W        ;
        MULWF   ARG2L           ; ARG1H * ARG2L → PRODH:PRODL
        MOVF    PRODL, W        ;
        ADDWF   RES1, F         ; Add cross products
        MOVF    PRODH, W        ; 
        ADDWFC  RES2, F         ;
        CLRF    WREG            ;
        ADDWFC  RES3, F         ;