1
0
mirror of https://github.com/cc65/cc65.git synced 2024-10-31 04:04:49 +00:00
cc65/libsrc/runtime/asr.s

61 lines
1.5 KiB
ArmAsm
Raw Normal View History

;
; 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
.import popax
.importzp tmp1
tosasrax:
and #$0F ; Bring the shift count into a valid range
sta tmp1 ; Save it
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 L1 ; Jump if not
; Shift count is greater 8. The carry is set when we enter here.
tya
sbc #8
tay ; Adjust shift count
txa
ldx #$00 ; Shift by 8 bits
cmp #$00 ; Test sign bit
bpl L1
dex ; Make X the correct sign extended value
; Save the high byte so we can shift it
L1: stx tmp1 ; Save high byte
jmp L3
; Do the actual shift
L2: cpx #$80 ; Copy bit 15 into the carry
ror tmp1
ror a
L3: dey
bpl L2
; Done with shift
ldx tmp1
L9: rts