added protection against DOS memory damage

src/dos_m65/,aliases_dos.s

@@ -6,6 +6,9 @@
 	// Page 0
 	//
 
-	.label UNIT_SDCARD  = $00
-	.label UNIT_FLOPPY  = $01
-	.label UNIT_RAMDISK = $02
+	.label MAGICSTR     = $00 // magic string; if not matching, DOS considered non-functional
+
+	.label UNIT_SDCARD  = $05
+	.label UNIT_FLOPPY  = $06
+	.label UNIT_RAMDISK = $07
+

src/dos_m65/dos_chkunit.s

@@ -6,6 +6,24 @@
 
 dos_CHKUNIT:
 
+	// Check for magic string
+
+	pha
+	phx
+
+	ldx #$04
+!:
+	lda MAGICSTR, x
+	cmp dos_magicstr, x
+	bne dos_CHKUNIT_mem_fail
+	dex
+	bpl !-
+
+	plx
+	pla
+
+	// Check for unit
+
 	cmp UNIT_SDCARD
 	beq dos_CHKUNIT_sdcard
 	cmp UNIT_FLOPPY
@@ -36,3 +54,10 @@ dos_CHKUNIT_ramdisk:
 	
 	clc
 	rts
+
+dos_CHKUNIT_mem_fail:
+
+	plx
+	pla
+	sec
+	rts

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