69 lines
804 B
ArmAsm
69 lines
804 B
ArmAsm
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;==========================
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; Signed 16x16=32 multiply
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;==========================
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; multiplies NUM1H:NUM1L by NUM2H:NUM2L
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; result in RESULT3:RESULT2:RESULT1:RESULT0
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multiply_s16x16:
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; check if we need to invert result, save for later
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lda NUM1H
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eor NUM2H
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php
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; if NUM1 negative, invert it
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lda NUM1H
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bpl num1_positive
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sec
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lda #0
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sbc NUM1L
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sta NUM1L
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lda #0
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sbc NUM1H
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sta NUM1H
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num1_positive:
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; if NUM2 negative, invert it
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lda NUM2H
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bpl num2_positive
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sec
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lda #0
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sbc NUM2L
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sta NUM2L
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lda #0
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sbc NUM2H
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sta NUM2H
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num2_positive:
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jsr multiply_u16x16
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; done, see if we need to negate
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plp
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bpl done_s16x16
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sec
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lda #0
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sbc RESULT0
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sta RESULT0
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lda #0
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sbc RESULT1
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sta RESULT1
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lda #0
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sbc RESULT2
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sta RESULT2
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lda #0
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sbc RESULT3
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sta RESULT3
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done_s16x16:
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rts
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