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33 lines
553 B
ArmAsm
33 lines
553 B
ArmAsm
; from http://nparker.llx.com/a2/mult.html
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;=====================
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;=====================
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; 16x16 -> 32 multiply
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;=====================
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;=====================
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mul16_slow:
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lda #0 ; Initialize RESULT to 0
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sta RESULT+2
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ldx #16 ; There are 16 bits in NUM2
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L1:
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lsr NUM2+1 ; Get low bit of NUM2 into C
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ror NUM2
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bcc L2 ; 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 NUM1
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adc RESULT+2
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sta RESULT+2
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tya
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adc NUM1+1
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L2:
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ror ; "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 L1
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sta RESULT+3
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rts
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