dos33fsprogs/tfv/tfv_multiply.s

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2017-09-07 05:32:31 +00:00
; http://www.llx.com/~nparker/a2/mult.html
; MULTIPLY NUM1H:NUM1L * NUM2H:NUM2L
2017-09-07 05:40:00 +00:00
; NUM2 is zeroed out
; result is in RESULT3:RESULT2:RESULT1:RESULT0
2017-09-07 05:32:31 +00:00
NUM1L: .byte 0
NUM1H: .byte 0
NUM2L: .byte 0
NUM2H: .byte 0
RESULT: .byte 0,0,0,0
NEGATE: .byte 0
; If we have 2k to spare we should check out
; http://codebase64.org/doku.php?id=base:seriously_fast_multiplication
multiply:
lda #$0
sta NEGATE
; Handle Signed
lda NUM1H
bpl check_num2
inc NEGATE
clc
lda NUM1L
eor #$ff
adc #$1
sta NUM1L
lda NUM1H
eor #$ff
adc #$0
sta NUM1H
check_num2:
lda NUM2H
bpl unsigned_multiply
inc NEGATE
clc
lda NUM2L
eor #$ff
adc #$1
sta NUM2L
lda NUM2H
eor #$ff
adc #$0
sta NUM2H
unsigned_multiply:
lda #0 ; Initialize RESULT to 0
sta RESULT+2
ldx #16 ; 16x16 multiply
multiply_mainloop:
lsr NUM2H ; Shift right 16-bit NUM2
ror NUM2L ; low bit goes into carry
bcc shift_output ; 0 or 1?
tay ; If 1, add NUM1 (hi byte of RESULT is in A)
clc
lda NUM1L
adc RESULT+2
sta RESULT+2
tya
adc NUM1H
shift_output:
ror A ; "Stairstep" shift
ror RESULT+2
ror RESULT+1
ror RESULT
dex
bne multiply_mainloop
sta RESULT+3
;; Negate if necessary
lda NEGATE
and #$1
beq positive
clc
lda RESULT+0
eor #$ff
adc #$1
sta RESULT+0
lda RESULT+1
eor #$ff
adc #$0
sta RESULT+1
lda RESULT+2
eor #$ff
adc #$0
sta RESULT+2
lda RESULT+3
eor #$ff
adc #$0
sta RESULT+3
positive:
rts