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56 lines
1.3 KiB
ArmAsm
56 lines
1.3 KiB
ArmAsm
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; Ullrich von Bassewitz, 2004-06-30
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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
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.import popax
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.importzp tmp1
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tosshrax:
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and #$0F ; Bring the shift count into a valid range
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sta tmp1 ; Save it
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jsr popax ; Get the left hand operand
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ldy tmp1 ; Get shift count
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beq L9 ; Bail out if shift count zero
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cpy #8 ; Shift count 8 or greater?
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bcc L3 ; Jump if not
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; Shift count is greater 7. The carry is set when we enter here.
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tya
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sbc #8
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tay ; Adjust shift count
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txa
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ldx #$00 ; Shift by 8 bits
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beq L2 ; Branch always
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L1: lsr a
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L2: dey
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bpl L1
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rts
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; Shift count is less than 8. Do the actual shift.
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L3: stx tmp1 ; Save high byte of lhs
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L4: lsr tmp1
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ror a
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dey
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bne L4
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; Done with shift
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ldx tmp1
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L9: rts
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