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cc65/libsrc/runtime/lshl.s

45 lines
1.1 KiB
ArmAsm

;
; Ullrich von Bassewitz, 2004-06-30
;
; CC65 runtime: left shift support for long and unsigned long
;
; 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 tosasleax, tosshleax
.import popeax
.importzp sreg, tmp1
tosshleax:
tosasleax:
and #$1F ; Bring the shift count into a valid range
sta tmp1 ; Save it
jsr popeax ; Get the left hand operand
ldy tmp1 ; Get shift count
beq L9 ; Bail out if shift count zero
stx tmp1 ; Save byte 1
; Do the actual shift. Faster solutions are possible but need a lot more code.
L2: asl a
rol tmp1
rol sreg
rol sreg+1
dey
bne L2
; Shift done
ldx tmp1
L9: rts