move fdtoiocb function into its own file -- idea by Daniel Serpell

git-svn-id: svn://svn.cc65.org/cc65/trunk@4478 b7a2c559-68d2-44c3-8de9-860c34a00d81

libsrc/atari/fdtoiocb.s

@@ -0,0 +1,41 @@
+;
+; Christian Groessler, May-2000
+; Moved from fdtable.s to it's own file by Daniel Serpell, 2009.
+;
+; Convert file descriptor to IOCB number
+;
+
+	.include "atari.inc"
+	.include "fd.inc"
+	.import fd_table,fd_index
+	.export	fdtoiocb
+
+	.code
+
+; gets fd in ax
+; return iocb index in A, fd_table index in X
+; return N bit set for invalid fd
+; all registers destroyed
+.proc	fdtoiocb
+
+	cpx	#0
+	bne	inval
+	cmp	#MAX_FD_INDEX
+	bcs	inval
+	tax
+	lda	fd_index,x
+	asl	a			; create index into fd table
+	asl	a
+	tax
+	lda	#$ff
+	cmp	fd_table+ft_iocb,x	; entry in use?
+	beq	inval			; no, return error
+	lda	fd_table+ft_usa,x	; get usage counter
+	beq	inval			; 0? should not happen
+	lda	fd_table+ft_iocb,x	; get iocb
+	rts
+
+inval:	ldx     #$ff			; sets N
+	rts
+
+.endproc	; fdtoiocb