diff --git a/libsrc/atari/diopncls.s b/libsrc/atari/diopncls.s
index 8995e79e6..09398b9ac 100644
--- a/libsrc/atari/diopncls.s
+++ b/libsrc/atari/diopncls.s
@@ -16,7 +16,7 @@
 	.export		sectsizetab
 	.import		__oserror, __sio_call, _dio_read
 	.import		pushax, addysp, subysp
-	.importzp	ptr2, tmp1, sp
+	.importzp	ptr2, sp
 	.include	"atari.inc"
 
 
@@ -27,34 +27,7 @@ sectsizetab:
 
 .code
 
-
-.proc	_dio_open
-
-	cmp	#NUMDRVS	; valid drive id?
-	bcs	_inv_drive
-	tay			; drive #
-	asl	a		; make index from drive id
-	asl	a
-	tax
-	lda	#128		; preset sectsize, will be overridden by query_sectorsize
-	sta	sectsizetab+sst_sectsize,x
-	sta	sectsizetab+sst_flag,x		; set flag that drive is "open"
-	lda	#0
-	sta	sectsizetab+sst_sectsize+1,x
-	sta	__oserror			; success
-	tya
-	sta	sectsizetab+sst_driveno,x
-	stx	tmp1
-	lda	#<sectsizetab
-	clc
-	adc	tmp1
-	sta	tmp1
-	lda	#>sectsizetab
-	adc	#0
-	tax
-	lda	tmp1
-	jmp	query_sectorsize		; query drive for current sector size
-;	rts
+; code for _dio_open
 
 _inv_drive:
 	lda	#NONDEV		; non-existent device
@@ -63,80 +36,80 @@ _inv_drive:
 	tax
 	rts			; return NULL
 
-.endproc
+_dio_open:
 
-.proc	_dio_close
-
-	sta	ptr2
-	stx	ptr2+1
-	lda	#0
-	ldy	#sst_flag
-	sta	(ptr2),y
-	sta	__oserror	; success
+	cmp	#NUMDRVS	; valid drive id?
+	bcs	_inv_drive
+	tay			; drive #
+	asl	a		; make index from drive id
+	asl	a
 	tax
-	rts			; return no error
-
-.endproc
+	lda	#128				; preset sectsize
+	sta	sectsizetab+sst_sectsize,x
+	sta	sectsizetab+sst_flag,x		; set flag that drive is "open"
+	lda	#0
+	sta	sectsizetab+sst_sectsize+1,x
+	sta	__oserror			; success
+	tya
+	sta	sectsizetab+sst_driveno,x
+	stx	ptr2
+	lda	#<sectsizetab
+	clc
+	adc	ptr2
+	sta	ptr2
+	lda	#>sectsizetab
+	adc	#0
+	tax
+	stx	ptr2+1		; ptr2: pointer to sectsizetab entry
 
 ; query drive for current sector size
 ; procedure:
-;   - read sector #4 (SIO command $54) to update drive status
-;     read length is 128 bytes, buffer is below the stack pointer,
-;	   sector data is ignored
-;     command status is ignored, we will get an error with a DD disk
-;	   anyway (read size 128 vs. sector size 256)
-;   - issue SIO command $53 (get status) to retrieve the sector size
-;     if the command returns with an error, we set sector size to 128
-;	   bytes
+;   - read sector #4 (SIO command $54) to update drive status;
+;     read length is 128 bytes, buffer is allocated on the stack,
+;	   sector data is ignored;
+;     returned command status is ignored, we will get an error with
+;	   a DD disk anyway (read size 128 vs. sector size 256);
+;   - issue SIO command $53 (get status) to retrieve the sector size;
+;     use the DVSTAT system area as return buffer;
+;     if the command returns with an error, set sector size to 128
+;	   bytes;
 ;
-; AX - handle
-;
-.proc	query_sectorsize
-
-	sta	ptr2
-	stx	ptr2+1		; remember pointer to sectsizetab entry
-
-;	jsr	pushax		; handle for subsequent __sio_call
 
 	ldy	#128
-	jsr	subysp		; use buffer on the stack
+	jsr	subysp		; allocate buffer on the stack
 
 	lda	sp
 	pha
 	lda	sp+1
-	pha
+	pha			; save sp (buffer address) on processor stack
 
 	lda	ptr2
 	ldx	ptr2+1
-
 	jsr	pushax		; handle
+
 	ldx	#0
 	lda	#4
 	jsr	pushax		; sect_num
 
-;	ldy	#128
-;	jsr	subysp		; use buffer on the stack
-;	lda	sp
-;	ldx	sp+1
 	pla
 	tax
-	pla
-;	jsr	pushax		; buffer address (not pushed, _dio_read is __fastcall__)
+	pla			; AX - buffer address
 
+				; sst_sectsize currently 128
 	jsr	_dio_read	; read sector to update status
 
 	ldy	#128
 	jsr	addysp		; discard stack buffer
 
-
 	lda	ptr2
 	ldx	ptr2+1
-	jsr	pushax		; handle for subsequent __sio_call
-
+	jsr	pushax		; handle
 
 	ldx	#0
 	lda	#4
-	jsr	pushax		; dummy sector #
+	jsr	pushax		; dummy sector #, ignored by this SIO command,
+				; but set to circumvent the special 1-3 sector
+				; handling in __sio_call
 
 	ldx	#>DVSTAT
 	lda	#<DVSTAT
@@ -158,12 +131,11 @@ _inv_drive:
 	and	#%100000
 	beq	s128
 
+;s256
 	lda	#0
 	sta	(ptr2),y
 	iny
 	lda	#1
-;	sta	(ptr2),y
-;	bne	fini2
 
 finish:	sta	(ptr2),y	; set default sector size	
 fini2:	lda	ptr2
@@ -174,4 +146,18 @@ error:	ldy	#sst_sectsize
 s128:	lda	#128
 	bne	finish
 
+; end of _dio_open
+
+
+.proc	_dio_close
+
+	sta	ptr2
+	stx	ptr2+1
+	lda	#0
+	ldy	#sst_flag
+	sta	(ptr2),y
+	sta	__oserror	; success
+	tax
+	rts			; return no error
+
 .endproc