6502: multiply using shift+add, no lookup tables

65mul.asm

; done in michal kowalski's 6502 macroassembler & simulator
putc	=	$e001
puth	=	$e003
		*=$100
		lda	#$10
		ldx	#8
		jsr	pmul
		lda	#3
		ldx	#3
		jsr	pmul
		lda	#4
		ldx	#4
		jsr	pmul
		lda	#$10
		ldx	#3
		jsr	pmul
		lda	#4
		ldx	#$20
		jsr	pmul
		lda	#$FF
		ldx	#1
		jsr	pmul
		brk
m_prem	ldy	#0
		sty	sad
		sty	ffs
		iny
		sty	why
		dey
; @ start: A=1 X=2
		sta	die
m_main	txa
		bit	why
		beq	no_bit
; add shifted var
		lda	die
		ldx	sad
m_add	asl
		dex
		bne	m_add
		adc	ffs
		sta	ffs
no_bit	inc	sad
		asl	why
		lda	why
		asl
		cmp	#0
		bne	m_main
		lda	ffs
		ldx	#2
		stx	ffs
		rts
pmul	jsr	pax	; print problem
		jsr	mul	; calc
		jsr	pv	; print answer
		rts
pax		sta	puth
		ldy	#'*'
		sty	putc
		stx	puth
		rts
pv		ldy	#'='
		sty	putc
		sta	puth
		ldy	#$0D
		sty	putc
		ldy	#$0A
		sty	putc
		rts
mul		sta	why		; can't do a-x directly, why
		cpx	why		; hi/lo order check
		bcs	m_lh_o	;
		bpl	m_hl_o	; WTF
		bmi	m_hl_o	;
m_lh_o	txa
		ldx	why
m_hl_o	cpx	#4		; too simple to waste cycles on
		bmi	m_ft_i	; there has to be 1 branch
		bcc	m_prem	; instruction for this
		bne	m_prem	; instruction for this
m_ft_i	tay
		txa
		asl
		asl
		sta	why
		tya
		jmp	god
		*=$1C0
god	.db	$4C
why	.db	0
ffs	.db	2
sad	.db 0
die .db 0
	*=$200
m_ft_0 ; fast ops table
		lda	#0	; 0x0
		rts		; 0x2
		nop		; 0x3
; if * 1
		rts
		nop
		nop
		nop
; if * 2
		asl
		rts
		nop
		nop
; if * 3
		jmp	m_ft_3
		nop
m_ft_4	asl
		asl
		rts
		nop
; hate me
m_ft_3	tax
		asl
		stx	why
		adc	why
		rts

last edited 9/15/2025

any real world work done on this CPU that was made in assembly is a miracle in itself