cc65/libsrc/common/_fdesc.s

;
; Ullrich von Bassewitz, 17.06.1998
;
; int _fdesc (void);
; /* Find a free descriptor slot */


       	.export		__fdesc
	.import		return0

        .include        "stdio.inc"
        .include        "_file.inc"

.proc   __fdesc

       	ldy	#0
        lda     #_FOPEN
Loop:  	and    	__filetab + _FILE::f_flags,y    ; load flags
       	beq    	Found		                ; jump if closed
.repeat .sizeof(_FILE)
      	iny
.endrepeat
       	cpy    	#(FOPEN_MAX * .sizeof(_FILE))   ; Done?
      	bne	Loop

; File table is full

      	jmp	return0

; Free slot found, get address

Found:  tya                     ; Offset
        clc
        adc     #<__filetab
        ldx     #>__filetab     ; High byte
        bcc     @L1             ; Jump if no overflow
        inx                     ; Bump high byte
@L1:    rts

.endproc