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Rewrite code for arithmetic right shift.

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git-svn-id: svn://svn.cc65.org/cc65/trunk@5779 b7a2c559-68d2-44c3-8de9-860c34a00d81
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uz 2012-07-11 20:57:02 +00:00
parent 40a3084553
commit 6cdd8e7281
1 changed files with 46 additions and 28 deletions
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72
libsrc/runtime/asr.s
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@ -11,50 +11,68 @@
; ;
.export tosasrax .export tosasrax, asraxy
.import popax .import popax
.importzp tmp1 .importzp tmp1
tosasrax: tosasrax:
and #$0F ; Bring the shift count into a valid range sta tmp1 ; Save shift count
sta tmp1 ; Save it
jsr popax ; Get the left hand operand jsr popax ; Get the left hand operand
ldy tmp1 ; Get shift count ldy tmp1 ; Get shift count
beq L9 ; Bail out if shift count zero
cpy #8 ; Shift count 8 or greater? ; Run into asraxy
bcc L1 ; Jump if not
; Shift count is greater 8. The carry is set when we enter here.
asraxy:
pha
tya tya
sbc #8 and #$0F
tay ; Adjust shift count beq L2 ; Nothing to shift
txa sec
ldx #$00 ; Shift by 8 bits sbc #8 ; Shift count 8 or greater?
cmp #$00 ; Test sign bit beq L3 ; Jump if exactly 8
bpl L1 bcc L6 ; Jump if less than 8
dex ; Make X the correct sign extended value
; Save the high byte so we can shift it ; Shift count is greater than 8.
L1: stx tmp1 ; Save high byte tay ; Shift count into Y
jmp L3 pla ; Discard low byte
txa ; Get high byte
; Do the actual shift L1: cmp #$80 ; Sign bit into carry
ror a ; Carry into A
dey
bne L1
beq L4 ; Sign extend and return
L2: cpx #$80 ; Copy bit 15 into the carry ; Shift count is zero
L2: pla
rts
; Shift count is exactly 8
L3: pla ; Drop low byte from stack ...
txa ; Move high byte to low
L4: ldx #$00 ; Clear high byte
cmp #$80 ; Check sign bit
bcc L5
dex
L5: rts
; Shift count is less than 8
L6: adc #8 ; Correct counter
tay ; Shift count into Y
pla ; Restore low byte
stx tmp1 ; Save high byte of lhs
L7: cpx #$80 ; Sign bit into carry
ror tmp1 ror tmp1
ror a ror a
L3: dey dey
bpl L2 bne L7
; Done with shift ; Done with shift
ldx tmp1 ldx tmp1
L9: rts rts