cc65/libsrc/runtime/mul8.s

;
; Ullrich von Bassewitz, 2009-08-17
;
; CC65 runtime: multiplication for ints. Short versions.
;

        .export         tosumula0, tosmula0
        .export         mul8x16, mul8x16a
        .import         popsreg
        .importzp       sreg, ptr4


;---------------------------------------------------------------------------
; 8x16 routine with external entry points used by the 16x16 routine in mul.s

tosmula0:
tosumula0:
        sta     ptr4
mul8x16:jsr     popsreg         ; Get left operand

        lda     #0              ; Clear byte 1
        ldy     #8              ; Number of bits
        ldx     sreg+1          ; Get into register for speed
        beq     mul8x8          ; Do 8x8 multiplication if high byte zero
mul8x16a:
        sta     ptr4+1          ; Clear byte 2

        lsr     ptr4            ; Get first bit into carry
@L0:    bcc     @L1

        clc
        adc     sreg
        pha
        txa                     ; hi byte of left op
        adc     ptr4+1
        sta     ptr4+1
        pla

@L1:    ror     ptr4+1
        ror     a
        ror     ptr4
        dey
        bne     @L0
        tax
        lda     ptr4            ; Load the result
        rts

;---------------------------------------------------------------------------
; 8x8 multiplication routine

mul8x8:
        lsr     ptr4            ; Get first bit into carry
@L0:    bcc     @L1
        clc
        adc     sreg
@L1:    ror
        ror     ptr4
        dey
        bne     @L0
        tax
        lda     ptr4            ; Load the result
        rts                     ; Done