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A description of positions [left, right] is appropriate more for C code than for Assembly code. (A description of timing [first argument, second argument] is more appropriate for the way that Assembly code is written.)
36 lines
965 B
ArmAsm
36 lines
965 B
ArmAsm
;
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; Ullrich von Bassewitz, 2010-11-02
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; CC65 runtime: 8x8 => 16 unsigned multiplication
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;
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.export umul8x8r16, umul8x8r16m
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.importzp ptr1, ptr3
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;---------------------------------------------------------------------------
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; 8x8 => 16 unsigned multiplication routine.
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;
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; LHS RHS result result in also
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; -------------------------------------------------------------
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; .A (ptr3-low) ptr1-low .XA ptr1
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;
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umul8x8r16:
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sta ptr3
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umul8x8r16m:
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lda #0 ; Clear byte 1
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ldy #8 ; Number of bits
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lsr ptr1 ; Get first bit of RHS into carry
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@L0: bcc @L1
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clc
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adc ptr3
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@L1: ror
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ror ptr1
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dey
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bne @L0
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tax
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stx ptr1+1 ; Result in .XA and ptr1
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lda ptr1 ; Load the result
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rts ; Done
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