72 lines
1 KiB
ArmAsm
72 lines
1 KiB
ArmAsm
.define Mul4
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.sect .text
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.sect .rom
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.sect .data
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.sect .bss
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.sect .text
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! This subroutine multiplies two fourbyte signed integers.
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! For more details see mli.s
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! The only difference is that zeropage locations are twice as big.
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Mul4:
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3: lda #0
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sta ARTH+8
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sta ARTH+9
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sta ARTH+10
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sta ARTH+11
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sta ARTH+12
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sta ARTH+13
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sta ARTH+14
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sta ARTH+15 ! clear accumulator
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ldy #32
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1: lda #1h
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bit ARTH
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beq 2f ! multiplying by zero: no addition
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clc
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lda ARTH+12
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adc ARTH+4
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sta ARTH+12
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lda ARTH+13
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adc ARTH+5
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sta ARTH+13
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lda ARTH+14
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adc ARTH+6
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sta ARTH+14
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lda ARTH+15
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adc ARTH+7
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sta ARTH+15
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2: lsr ARTH+3
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ror ARTH+2
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ror ARTH+1
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ror ARTH ! shift multiplier
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lsr ARTH+15
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ror ARTH+14
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ror ARTH+13
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ror ARTH+12
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ror ARTH+11
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ror ARTH+10
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ror ARTH+9
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ror ARTH+8 ! shift accumulator
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lda UNSIGN
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beq 3f ! it's unsigned: so no shift in of signbit
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lda ARTH+7
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bpl 3f
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lda #40h
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bit ARTH+15
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beq 3f
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lda ARTH+15
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ora #80h
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sta ARTH+15
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3: dey
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bne 1b
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ldx ARTH+10
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lda ARTH+11
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jsr Push
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ldx ARTH+8
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lda ARTH+9
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jmp Push
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