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cc65/libsrc/common/strlen.s

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;
; Ullrich von Bassewitz, 31.05.1998
;
; int strlen (const char* s);
;
.export _strlen
.importzp ptr1
_strlen:
sta ptr1 ; Save s
stx ptr1+1
ldx #0 ; YX used as counter
ldy #0
L1: lda (ptr1),y
beq L9
iny
bne L1
inc ptr1+1
inx
bne L1
L9: tya ; get low byte of counter, hi's all set
rts