dos33fsprogs/basic/appleiibot/mul16_slow.s

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2020-11-10 18:37:14 +00:00
; from http://nparker.llx.com/a2/mult.html
;=====================
;=====================
; 16x16 -> 32 multiply
;=====================
;=====================
mul16_slow:
lda #0 ; Initialize RESULT to 0
sta RESULT+2
ldx #16 ; There are 16 bits in NUM2
L1:
lsr NUM2+1 ; Get low bit of NUM2 into C
ror NUM2
bcc L2 ; 0 or 1?
tay ; If 1, add NUM1 (hi byte of RESULT is in A)
clc
lda NUM1
adc RESULT+2
sta RESULT+2
tya
adc NUM1+1
L2:
ror ; "Stairstep" shift
ror RESULT+2
ror RESULT+1
ror RESULT
dex
bne L1
sta RESULT+3
rts