54 lines
758 B
ArmAsm
54 lines
758 B
ArmAsm
;=================================================
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; M1 * M2
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multiply_s8x8:
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lda M2
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eor M1 ; calc if we need to adjust at end
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; (++ vs +- vs -+ vs --)
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php ; save status on stack
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; if M1 negative, negate it
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lda M1
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bpl m1_positive
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eor #$ff
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clc
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adc #0
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m1_positive:
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sta M1
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; if M2 negative, naegate it
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lda M2
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bpl m2_positive
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eor #$ff
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clc
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adc #0
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m2_positive:
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sta M2
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;==================
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; unsigned multiply
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jsr multiply_u8x8
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; done, high result in factor2, low result in factor1
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; adjust to be signed
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; if m1 and m2 positive, good
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; if m1 and m2 negative, good
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; otherwise, negate result
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plp ; restore saved pos/neg value
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bpl done_result
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negate_result:
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sec
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lda #0
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sbc M1
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lda #0
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sbc M2
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done_result:
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sta M2
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rts
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