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79 lines
2.0 KiB
ArmAsm
79 lines
2.0 KiB
ArmAsm
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; Ullrich von Bassewitz, 2004-06-30
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;
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; CC65 runtime: right shift support for ints
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;
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; Note: The standard declares a shift count that is negative or >= the
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; bitcount of the shifted type for undefined behaviour.
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;
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; Note^2: The compiler knowns about the register/zero page usage of this
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; function, so you need to change the compiler source if you change it!
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;
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.export tosasrax, asraxy
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.import popax
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.importzp tmp1
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tosasrax:
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sta tmp1 ; Save shift count
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jsr popax ; Get the left hand operand
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ldy tmp1 ; Get shift count
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; Run into asraxy
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asraxy:
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pha
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tya
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and #$0F
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beq L2 ; Nothing to shift
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sec
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sbc #8 ; Shift count 8 or greater?
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beq L3 ; Jump if exactly 8
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bcc L6 ; Jump if less than 8
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; Shift count is greater than 8.
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tay ; Shift count into Y
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pla ; Discard low byte
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txa ; Get high byte
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L1: cmp #$80 ; Sign bit into carry
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ror a ; Carry into A
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dey
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bne L1
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beq L4 ; Sign extend and return
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; Shift count is zero
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L2: pla
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rts
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; Shift count is exactly 8
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L3: pla ; Drop low byte from stack ...
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txa ; Move high byte to low
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L4: ldx #$00 ; Clear high byte
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cmp #$80 ; Check sign bit
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bcc L5
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dex
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L5: rts
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; Shift count is less than 8
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L6: adc #8 ; Correct counter
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tay ; Shift count into Y
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pla ; Restore low byte
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stx tmp1 ; Save high byte of lhs
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L7: cpx #$80 ; Sign bit into carry
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ror tmp1
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ror a
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dey
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bne L7
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; Done with shift
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ldx tmp1
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rts
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