mirror of https://github.com/cc65/cc65.git
89 lines
2.0 KiB
ArmAsm
89 lines
2.0 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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; 2019-11-05, Piotr Fusik
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;
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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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; Extend sign of Left-Hand Side
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lda #$7f
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cmp ptr3
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adc #$80
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sta ptr3+1
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; Clear .XY accumulator
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ldy #<$0000
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ldx #>$0000
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; Check the multiplier sign.
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lda ptr1
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bpl PosStart
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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 NegStart ; Branch always
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NegAdd:
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tya ; Subtract current multiplicand
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; sec
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sbc 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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NegShift:
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asl ptr3
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rol ptr3+1
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NegStart:
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lsr ptr1 ; Get next bit of Right-Hand Side into carry
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bcs NegAdd
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bnz NegShift ; 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 positive.
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PosAdd:
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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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PosShift:
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asl ptr3
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rol ptr3+1
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PosStart:
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lsr ptr1 ; Get next bit of Right-Hand Side into carry
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bcs PosAdd
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bnz PosShift ; 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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