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45 lines
1.1 KiB
ArmAsm
45 lines
1.1 KiB
ArmAsm
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; Ullrich von Bassewitz, 2004-06-30
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; CC65 runtime: right shift support for longs
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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 tosasreax
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.import popeax
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.importzp sreg, tmp1
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tosasreax:
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and #$1F ; Bring the shift count into a valid range
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sta tmp1 ; Save it
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jsr popeax ; 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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stx tmp1 ; Save byte 1
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ldx sreg+1 ; Load byte 3
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; Do the actual shift. Faster solutions are possible but need a lot more code.
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L2: cpx #$80 ; Copy bit 31 into the carry
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ror sreg+1
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ror sreg
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ror tmp1
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ror a
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dey
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bne L2
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; Shift done
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ldx tmp1
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L9: rts
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