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