83 lines
1.5 KiB
ArmAsm
83 lines
1.5 KiB
ArmAsm
.define Dvi2, Div, Duv
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! The subroutine Dvi2 performs a signed division.
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! Its operands are on the stack.
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! The subroutine Div performs also a signed division, ecxept that
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! its operand are already in zeropage.
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! The subroutine Duv performs a n unsigned division.
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! For an explanation of the algoritm used see
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! A. S. Tanenbaum's Structered Computer Organisation. 1976
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Dvi2:
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stx ARTH
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sta ARTH+1 ! store divisor
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jsr Pop
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stx ARTH+2
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sta ARTH+3 ! store dividend
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ldy #1
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sty UNSIGN ! used for result sign
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Div:
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ldy #0
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sty SIGN
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lda ARTH+1
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bpl 1f ! if divisor is negative
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ldx ARTH ! make it positive
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jsr Ngi2
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ldy #1
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sty SIGN
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stx ARTH
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sta ARTH+1
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1: lda ARTH+3
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bpl 1f ! if dividend is negative
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ldx ARTH+2 ! make it positive
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jsr Ngi2
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pha
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lda SIGN
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eor #1 ! excusive or with sign of divisor
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sta SIGN
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lda #1
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sta NBYTES
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pla
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stx ARTH+2
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sta ARTH+3
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Duv:
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1: ldy #0
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sty ARTH+4
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sty ARTH+5
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ldy #17
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4: lda ARTH+5
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cmp ARTH+1
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bcc 1f ! no subtraction
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bne 2f ! divisor goes into dividend
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lda ARTH+4
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cmp ARTH
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bcc 1f ! no subtraction
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2: sec ! divisor goes into dividend
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lda ARTH+4
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sbc ARTH
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sta ARTH+4
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lda ARTH+5
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sbc ARTH+1
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sta ARTH+5 ! subtract divisor from dividend
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sec
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rol ARTH+2 ! a subtraction so shift in a 1
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bne 3f
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1: asl ARTH+2 ! no subtraction so shift in a 0
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3: rol ARTH+3
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rol ARTH+4
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rol ARTH+5 ! shift dividend
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dey
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bne 4b
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ldx ARTH+2
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lda ARTH+3
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ldy UNSIGN ! is it an unsigned division
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beq 1f
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ldy SIGN ! is the result negative
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beq 1f
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jsr Ngi2
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1: rts
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