diff --git a/asminc/atari.inc b/asminc/atari.inc
index 7b16b528f..6b8c0dc30 100644
--- a/asminc/atari.inc
+++ b/asminc/atari.inc
@@ -1003,15 +1003,15 @@ diopp_size   = 5        ; size of structure
 ;-------------------------------------------------------------------------
 
 SPARTADOS    = 0
-OSADOS       = 1        ; OS/A+
-XDOS         = 2
-REALDOS      = 3
+REALDOS      = 1
+OSADOS       = 2        ; OS/A+
+XDOS         = 3
 ATARIDOS     = 4
 MYDOS        = 5
 NODOS        = 255
 ; The DOSes with dos_type below or equal MAX_DOS_WITH_CMDLINE do support
 ; command line arguments.
-MAX_DOS_WITH_CMDLINE = REALDOS
+MAX_DOS_WITH_CMDLINE = XDOS
 
 ;-------------------------------------------------------------------------
 ; XDOS defines (version 2.4, taken from xdos24.pdf)
diff --git a/libsrc/atari/getdefdev.s b/libsrc/atari/getdefdev.s
index 480639b4a..cad768783 100644
--- a/libsrc/atari/getdefdev.s
+++ b/libsrc/atari/getdefdev.s
@@ -3,7 +3,11 @@
 ;
 ; function to get default device: char *_getdefdev(void);
 ;
-; SpartaDOS:
+; AtariDOS/MyDOS:
+; Default device number is derived from DUNIT. Therefore "default
+; device" is the one the program was loaded from.
+;
+; SpartaDOS/RealDOS:
 ; the ZCRNAME routine is only used to get the default drive because
 ; ZCRNAME has two disadvantages:
 ; 1. It will convert D: into D1: instead of Dn: (n = default drive)
@@ -30,7 +34,7 @@ __getdefdev:
         cmp     #XDOS
         beq     xdos            ; only supported on XDOS ...
 ;       cmp     #OSADOS+1       ; (redundant: #OSADOS+1 = #XDOS)
-        bcs     finish          ; ... and on OS/A+ and SpartaDOS
+        bcs     use_DUNIT       ; ... and on OS/A+ and SpartaDOS
 
         ldy     #BUFOFF
         lda     #0
@@ -77,6 +81,17 @@ finish: lda     #<__defdev
         ldx     #>__defdev
         rts
 
+; On AtariDOS or MyDOS, use the DUNIT variable to setuo the default
+; device. The default device will then be the one the program was
+; loaded from.
+
+use_DUNIT:
+        lda     DUNIT
+        clc
+        adc     #'0'
+        sta     __defdev + 1
+        bne     finish          ; jump always
+
 ; XDOS default device retrieval
 
 xdos: