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Rewrite to add an entry point for shraxy that doesn't need to pass values over

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the stack.


git-svn-id: svn://svn.cc65.org/cc65/trunk@5761 b7a2c559-68d2-44c3-8de9-860c34a00d81
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uz 2012-07-06 19:57:24 +00:00
parent db8cac9501
commit c6db670be5
1 changed files with 42 additions and 22 deletions
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64
libsrc/runtime/shr.s
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@ -11,45 +11,65 @@
;
.export tosshrax
.export tosshrax, shraxy
.import popax
.importzp tmp1
tosshrax:
and #$0F ; Bring the shift count into a valid range
sta tmp1 ; Save it
sta tmp1 ; Save shift count
jsr popax ; Get the left hand operand
ldy tmp1 ; Get shift count
beq L9 ; Bail out if shift count zero
cpy #8 ; Shift count 8 or greater?
bcc L3 ; Jump if not
; Shift count is greater 7. The carry is set when we enter here.
; Run into shraxy
shraxy:
pha
tya
sbc #8
tay ; Adjust shift count
txa
ldx #$00 ; Shift by 8 bits
beq L2 ; Branch always
and #$0F
beq L2 ; Nothing to shift
sec
sbc #8 ; Shift count 8 or greater?
beq L3 ; Jump if exactly 8
bcc L4 ; Jump if less than 8
; Shift count is greater than 8.
tay ; Shift count into Y
pla ; Discard low byte
txa ; Get high byte
L1: lsr a
L2: dey
bpl L1
dey
bne L1
ldx #$00 ; High byte is zero
rts
; Shift count is less than 8. Do the actual shift.
; Shift count is zero
L3: stx tmp1 ; Save high byte of lhs
L4: lsr tmp1
L2: pla
rts
; Shift count is exactly 8
L3: pla ; Drop low byte from stack ...
txa ; Move high byte to low
ldx #$00 ; Clear high byte
rts
; Shift count is less than 8
L4: adc #8 ; Correct counter
tay ; Shift count into Y
pla ; Restore low byte
stx tmp1 ; Save high byte of lhs
L5: lsr tmp1
ror a
dey
bne L4
bne L5
; Done with shift
ldx tmp1
L9: rts
rts