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76 lines
1.8 KiB
ArmAsm
76 lines
1.8 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 unsigneds
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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 tosshrax, shraxy
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.import popax
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.importzp tmp1
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tosshrax:
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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 shraxy
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shraxy:
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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 L4 ; 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: lsr a
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dey
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bne L1
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ldx #$00 ; High byte is zero
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rts
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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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ldx #$00 ; Clear high byte
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rts
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; Shift count is less than 8
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L4: 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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L5: lsr tmp1
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ror a
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dey
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bne L5
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; Done with shift
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ldx tmp1
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rts
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