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|
;
; Ullrich von Bassewitz, 2004-06-30
;
; CC65 runtime: right shift support for ints
;
; Note: The standard declares a shift count that is negative or >= the
; bitcount of the shifted type for undefined behaviour.
;
; Note^2: The compiler knowns about the register/zero page usage of this
; function, so you need to change the compiler source if you change it!
;
.export tosasrax, asraxy
.import popax
.importzp tmp1
tosasrax:
sta tmp1 ; Save shift count
jsr popax ; Get the left hand operand
ldy tmp1 ; Get shift count
; Run into asraxy
asraxy:
pha
tya
and #$0F
beq L2 ; Nothing to shift
sec
sbc #8 ; Shift count 8 or greater?
beq L3 ; Jump if exactly 8
bcc L6 ; 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: cmp #$80 ; Sign bit into carry
ror a ; Carry into A
dey
bne L1
beq L4 ; Sign extend and return
; 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 a
dey
bne L7
; Done with shift
ldx tmp1
rts
|
