69 lines
1.2 KiB
ArmAsm
69 lines
1.2 KiB
ArmAsm
.define Mli2, Mlinp, Mul
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! The subroutine Mli2 multiplies two signed integers. The integers
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! are popped from the stack.
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! The subroutine Mlinp expects the two integer to be in zeropage.
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! While the subroutine Mul an unsigned multiply subroutine is.
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! For the algoritme see A. S. Tanenbaum
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! Structured Computer Organisation. 1976.
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Mli2:
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stx ARTH
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sta ARTH+1
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jsr Pop
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stx ARTH+2
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sta ARTH+3
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Mlinp: ldy #1
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sty UNSIGN ! it's signed
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lda ARTH+1
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bpl 3f ! multiplier negative so:
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ldx ARTH
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jsr Ngi2 ! negate multiplier
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stx ARTH
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sta ARTH+1
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ldx ARTH+2
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lda ARTH+3
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jsr Ngi2 ! negate multiplicand
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stx ARTH+2
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sta ARTH+3
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Mul:
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3: lda #0
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sta ARTH+4
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sta ARTH+5
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sta ARTH+6
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sta ARTH+7 ! clear accumulator
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ldy #16
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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+6
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adc ARTH+2
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sta ARTH+6
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lda ARTH+7
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adc ARTH+3
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sta ARTH+7
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2: lsr ARTH+1
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ror ARTH ! shift multiplier
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lsr ARTH+7
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ror ARTH+6
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ror ARTH+5
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ror ARTH+4 ! shift accumulator
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lda UNSIGN
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beq 3f ! unsigned multiply: so no shift in of signbit
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lda ARTH+3
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bpl 3f
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lda #40h
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bit ARTH+7
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beq 3f
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lda ARTH+7
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ora #80h
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sta ARTH+7
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3: dey
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bne 1b
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ldx ARTH+4
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lda ARTH+5
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rts
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