dos33fsprogs/hellmood_memories/multiply_s8x8.s
2020-05-17 00:17:25 -04:00

54 lines
758 B
ArmAsm

;=================================================
; M1 * M2
multiply_s8x8:
lda M2
eor M1 ; calc if we need to adjust at end
; (++ vs +- vs -+ vs --)
php ; save status on stack
; if M1 negative, negate it
lda M1
bpl m1_positive
eor #$ff
clc
adc #0
m1_positive:
sta M1
; if M2 negative, naegate it
lda M2
bpl m2_positive
eor #$ff
clc
adc #0
m2_positive:
sta M2
;==================
; unsigned multiply
jsr multiply_u8x8
; done, high result in factor2, low result in factor1
; adjust to be signed
; if m1 and m2 positive, good
; if m1 and m2 negative, good
; otherwise, negate result
plp ; restore saved pos/neg value
bpl done_result
negate_result:
sec
lda #0
sbc M1
lda #0
sbc M2
done_result:
sta M2
rts