added protection against DOS memory damage

src/dos_m65/dos_init.s

@@ -5,6 +5,20 @@
 
 dos_INIT:
 
+	// Set the magic string 'CBDOS'
+
+	phx
+	ldx #$04
+!:
+	lda dos_magicstr, x
+	sta MAGICSTR, x
+	dex
+	bpl !-
+
+	plx
+
+	// Set default device numbers
+
 	lda #CONFIG_UNIT_SDCARD
 	sta UNIT_SDCARD
 	lda #CONFIG_UNIT_FLOPPY
@@ -15,3 +29,7 @@ dos_INIT:
 	// XXX initialize drivers, etc.
 
 	rts
+
+dos_magicstr: // 'CBDOS'
+
+	.byte $43, $42, $44, $4F, $53