diff --git a/libsrc/atari/siocall.s b/libsrc/atari/siocall.s
index db71e44ec..0a8f9acdd 100644
--- a/libsrc/atari/siocall.s
+++ b/libsrc/atari/siocall.s
@@ -1,10 +1,10 @@
 ;
 ; Christian Groessler, October 2000
 ;
-; this file provides the __sio_call function
-; the function does a SIO call, it's not completely
+; This file provides the __sio_call function
+; The function does a SIO call, it's not completely
 ; generic (e.g. transfer size is fixed), it's used
-; to save space with _dio_read and _dio_write functions
+; to save space with _dio_read and _dio_write functions.
 ;
 ; unsigned char __fastcall__ _sio_call(_driveid_t drive_id,
 ;				       _sectnum_t sect_num,
@@ -12,12 +12,13 @@
 ;				       unsigned int sio_val);
 ; _driveid_t - 8bit
 ; _sectnum_t - 16bit
-; sio_cal is (sio_command | sio_direction << 8)
+; sio_val is (sio_command | sio_direction << 8)
 ;
 
 	.export		__sio_call
 	.include	"atari.inc"
 	.import		popa,popax
+	.import		sectsizetab
 
 .proc	__sio_call
 
@@ -30,18 +31,21 @@
 	sta	DAUX1		; set sector #
 	stx	DAUX2
 	jsr	popa
-	cmp	#16
+	cmp	#NUMDRVS
 	bcs	_inv_dev	; invalid device #
 	adc	#1
 	sta	DUNIT		; unit number (d1,d2,d3,...)
+	sbc	#0
+	asl	a
+	tax
+	lda	sectsizetab,x
+	sta	DBYTLO		; low byte of bytes to transfer
+	lda	sectsizetab+1,x
+	sta	DBYTHI		; high byte of bytes to transfer
 	lda	#$31		; D1 (drive_id == 0) has id $31
 	sta	DDEVIC
-	lda	#0
-	sta	DBYTHI		; high byte of bytes to transfer
 	lda	#15
 	sta	DTIMLO		; value got from DOS source
-	lda	#128
-	sta	DBYTLO		; low byte of bytes to transfer	
 
 	jsr	SIOV		; execute