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76 lines
1.9 KiB
ArmAsm
76 lines
1.9 KiB
ArmAsm
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; 2010-11-02, Ullrich von Bassewitz
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; 2014-09-10, Greg King
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; CC65 runtime: 8x8 => 16 signed multiplication
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;
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.export imul8x8r16, imul8x8r16m
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.importzp ptr1, ptr3, tmp1
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.macpack generic
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;---------------------------------------------------------------------------
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; 8x8 => 16 signed multiplication routine.
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;
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; multiplicand multiplier product
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; LHS RHS result
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; -------------------------------------------------------------
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; .A (ptr3-low) ptr1-low .XA
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;
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imul8x8r16:
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sta ptr3
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imul8x8r16m:
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ldx #>$0000
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bit ptr3
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bpl @L7
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dex
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@L7: stx ptr3+1 ; Extend sign of Left-Hand Side
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ldy #<$0000 ; Clear .XY accumulator
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ldx #>$0000
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lda ptr1
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bmi NegMult
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bpl @L2 ; Branch always
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@L0: tya ; Add current multiplicand
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add ptr3
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tay
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txa
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adc ptr3+1
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tax
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@L1: asl ptr3
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rol ptr3+1
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@L2: lsr ptr1 ; Get next bit of Right-Hand Side into carry
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bcs @L0
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bnz @L1 ; Loop if more one-bits in multiplier
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tya ; Put result into cc65's accumulator
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rts
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; The multiplier is negative.
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; Therefore, make it positive; and, subtract when multiplying.
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NegMult:
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eor #%11111111
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sta ptr1
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inc ptr1
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bnz @L2 ; Branch always
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@L0: tya ; Subtract current multiplicand
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sub ptr3
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tay
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txa
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sbc ptr3+1
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tax
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@L1: asl ptr3
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rol ptr3+1
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@L2: lsr ptr1 ; Get next bit of Right-Hand Side into carry
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bcs @L0
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bnz @L1 ; Loop if more one-bits in multiplier
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tya ; Put result into cc65's accumulator
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rts
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