base:24bit_multiplication_24bit_product
24 bit multiplication (signed or unsigned)
; Signed or unsigned 24-bit multiply (24-bit product) ; by Neils ; DASM format PROCESSOR 6502 factor1 EQU $61 factor2 EQU $64 product EQU $67 MAC twoscomplement lda {1}+2 eor #$ff sta {1}+2 lda {1}+1 eor #$ff sta {1}+1 lda {1} eor #$ff clc adc #$01 sta {1} ENDM ; An example program that multiplies -981 by 1340 ; Benchmark: 812 cycles (with this input) SEG PROGRAM ORG $C000 ; factor1 := -981 lda #$2b ldx #$fc ldy #$ff sta factor1 stx factor1+1 sty factor1+2 ; factor2 := 1340 lda #$3c ldx #$05 ldy #$00 sta factor2 stx factor2+1 sty factor2+2 ; result := factor1 * factor2 jsr signed_mul24 rts ; Signed 24-bit multiply routine ; Clobbers a, x, factor1, factor2 signed_mul24 SUBROUTINE ldx #$00 ; .x will hold the sign of product lda factor1+2 bpl .skip ; if factor1 is negative twoscomplement factor1 ; then factor1 := -factor1 inx ; and switch sign .skip lda factor2+2 bpl .skip2 ; if factor2 is negative twoscomplement factor2 ; then factor2 := -factor2 inx ; and switch sign .skip2 jsr unsigned_mul24 ; do unsigned multiplication txa and #$01 ; if .x is odd beq .q twoscomplement product ; then product := -product .q rts ; Unsigned 24-bit multiply routine ; Clobbers a, factor1, factor2 unsigned_mul24 SUBROUTINE lda #$00 ; set product to zero sta product sta product+1 sta product+2 .loop lda factor2 ; while factor2 != 0 bne .nz lda factor2+1 bne .nz lda factor2+2 bne .nz rts .nz lda factor2 ; if factor2 is odd and #$01 beq .skip lda factor1 ; product += factor1 clc adc product sta product lda factor1+1 adc product+1 sta product+1 lda factor1+2 adc product+2 sta product+2 ; end if .skip asl factor1 ; << factor1 rol factor1+1 rol factor1+2 lsr factor2+2 ; >> factor2 ror factor2+1 ror factor2 jmp .loop ; end while
base/24bit_multiplication_24bit_product.txt · Last modified: 2018-02-11 22:54 by neils