diff --git a/src/dos_m65/dos_memchk.s b/src/dos_m65/dos_memchk.s
index f646228b..cde237bb 100644
--- a/src/dos_m65/dos_memchk.s
+++ b/src/dos_m65/dos_memchk.s
@@ -6,6 +6,9 @@
 
 dos_MEMCHK:
 
+	pha
+	phx
+
 	; Check for magic string
 
 	ldx #$04
@@ -34,10 +37,14 @@ dos_MEMCHK:
 	cmp code_RTS_03
 	bne @fail
 
+	plx
+	pla
 	clc 
 	rts
 
 @fail:
 
+	plx
+	pla
 	sec
 	rts
diff --git a/src/dos_m65/dos_unitchk.s b/src/dos_m65/dos_unitchk.s
index 16317d41..610f617d 100644
--- a/src/dos_m65/dos_unitchk.s
+++ b/src/dos_m65/dos_unitchk.s
@@ -6,13 +6,13 @@
 
 dos_UNITCHK:
 
-	jsr dos_ENTER
-
-	; Check if memory content is not damaged
+	; Before the start make sure memory content is not damaged
 
 	jsr dos_MEMCHK
-	bcs dos_UNITCHK_fail
-	lda REG_A
+	bcc @ok
+	rts
+@ok:
+	jsr dos_ENTER
 
 	; Check for unit
 
diff --git a/src/dos_m65/dos_unitnum.s b/src/dos_m65/dos_unitnum.s
index dbdc3c99..3acd1a4e 100644
--- a/src/dos_m65/dos_unitnum.s
+++ b/src/dos_m65/dos_unitnum.s
@@ -6,13 +6,14 @@
 
 dos_UNITNUM:
 
-	jsr dos_ENTER
-
-	; Check if memory content is not damaged
+	; Before the start make sure memory content is not damaged
 
 	jsr dos_MEMCHK
-	bcs dos_UNITNUM_fail
-	lda REG_A
+	bcc @ok
+	lda #$00 ; failure
+	rts
+@ok:
+	jsr dos_ENTER
 
 	; Check what kind of device was requested
 
diff --git a/src/dos_m65/dos_unitset.s b/src/dos_m65/dos_unitset.s
index 2de4eebe..e57c29eb 100644
--- a/src/dos_m65/dos_unitset.s
+++ b/src/dos_m65/dos_unitset.s
@@ -6,14 +6,14 @@
 
 dos_UNITSET:
 
-	jsr dos_ENTER
-
-	; Check if memory content is not damaged
+	; Before the start make sure memory content is not damaged
 
 	jsr dos_MEMCHK
-	bcc @1
+	bcc @ok
 	rts
-@1:
+@ok:
+	jsr dos_ENTER
+
 	; XXX do not allow illegal device numbers... above 30 or below 8 ?
 
 	; Check what kind of device was requested