Atari: fix lookup of default device on XDOS.

Stefan Dorndorf, author of XDOS, pointed out that retrieving the
default device by looking at an undocumented memory location won't
work in future XDOS versions.
He also showed a way to get the default device in a compatible
manner.

This change implements his method and adds a version check (XDOS
versions below 2.4 don't support this -- for them the behaviour
will be the same as, for example, AtariDOS: no notion of a default
drive).

libsrc/atari/getdefdev.s

@@ -77,16 +77,33 @@ finish: lda     #<__defdev
         ldx     #>__defdev
         rts
 
-; XDOS version
+; XDOS default device retrieval
 
-xdos:   lda     XDEFDEV
+xdos:
+
+; check XDOS version (we need >= 2.4)
+
+        lda     XGLIN
+        cmp     #$4C            ; there needs to be a 'JMP' opcode here
+        bne     finish          ; older version, use DEFAULT_DEVICE or D1:
+        lda     XVER            ; get BCD encoded version ($24 for 2.4)
+        cmp     #$24
+        bcc     finish          ; too old, below 2.4
+
+; good XDOS version, get default drive
+
+        lda     #ATEOL
+        sta     XLINE           ; simulate empty command line
+        ldy     #0
+        jsr     XMOVE           ; create an FMS filename (which in this case only contains the drive)
+        lda     XFILE+1
         bne     done
 
         .data
 
 crvec:  jmp     $FFFF           ; target address will be set to crunch vector
 
-; Default device
+; Default device string
 
 __defdev:
 .ifdef  DEFAULT_DEVICE
@@ -94,4 +111,3 @@ __defdev:
 .else
         .byte   "D1:", 0
 .endif
-