diff --git a/libsrc/atari/dio_cts.s b/libsrc/atari/dio_cts.s
new file mode 100644
index 000000000..75a65a670
--- /dev/null
+++ b/libsrc/atari/dio_cts.s
@@ -0,0 +1,32 @@
+;
+; Christian Groessler, October 2000
+;
+; this file provides the _dio_chs_to_snum function
+;
+; on the Atari this function is a dummy, it ignores
+; cylinder and head 0 and returns as sector number the
+; sector number it got
+;
+; _sectnum_t __fastcall__ _dio_chs_to_snum(unsigned int cyl, unsigned int head, unsigned int sector);
+;
+; _driveid_t - 8bit
+; _sectnum_t - 16bit
+;
+
+ 	.export		__dio_chs_to_snum
+	.import		addsp4
+	.include	"atari.inc"
+
+.proc	__dio_chs_to_snum
+
+	pha			; save sector value
+	txa
+	pha
+	jsr	addsp4		; ignore other parameters
+	pla
+	tay
+	pla
+	rts
+
+.endproc
+