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cc65/libsrc/common/strlower.s
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ArmAsm

;
; Ullrich von Bassewitz, 02.06.1998
;
; char* strlower (char* s);
; char* strlwr (char* s);
;
; Non-ANSI
;
.export _strlower, _strlwr
.import popax
.import __ctype, __cdiff
.importzp ptr1, ptr2
_strlower:
_strlwr:
sta ptr1 ; Save s (working copy)
stx ptr1+1
sta ptr2
sta ptr2+2 ; save function result
ldy #0
loop: lda (ptr1),y ; get character
beq L9 ; jump if done
tax
lda __ctype,x ; get character classification
and #$02 ; upper case char?
beq L1 ; jump if no
txa ; get character back into accu
sec
sbc __cdiff ; make lower case char
sta (ptr1),y ; store back
L1: iny ; next char
bne loop
inc ptr1+1 ; handle offset overflow
bne loop ; branch always
; Done, return the argument string
L9: lda ptr2
ldx ptr2+1
rts