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61 lines
1.5 KiB
ArmAsm
61 lines
1.5 KiB
ArmAsm
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; Ullrich von Bassewitz, 2004-06-30
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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
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.import popax
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.importzp tmp1
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tosasrax:
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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 L1 ; Jump if not
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; Shift count is greater 8. 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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cmp #$00 ; Test sign bit
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bpl L1
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dex ; Make X the correct sign extended value
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; Save the high byte so we can shift it
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L1: stx tmp1 ; Save high byte
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jmp L3
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; Do the actual shift
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L2: cpx #$80 ; Copy bit 15 into the carry
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ror tmp1
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ror a
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L3: dey
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bpl L2
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; Done with shift
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ldx tmp1
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L9: rts
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