cc65/libsrc/common/tolower.s

;
; Ullrich von Bassewitz, 02.06.1998
;
; int tolower (int c);
;

        .export         _tolower
        .import         __ctype

_tolower:
        cpx     #$00            ; Outside valid range?
        bne     L9              ; If so, return the argument unchanged
        tay                     ; Get C into Y
        lda     __ctype,y       ; Get character classification
        lsr     a
        lsr     a               ; Get bit 1 (upper case char) into carry
        tya                     ; Get char back into A
        bcc     L9              ; Jump if no upper case char
        sbc     #<('A'-'a')     ; Make lower case char (carry already set)
L9:     rts