--- trunk/doc/misc.html	2007/10/08 16:21:17	34
+++ trunk/doc/misc.html	2007/10/08 16:21:53	38
@@ -10,7 +10,7 @@
 
 <!--
 
-$Id: misc.html,v 1.66 2006/12/30 13:30:51 debug Exp $
+$Id: misc.html,v 1.67 2007/03/24 06:39:29 debug Exp $
 
 Copyright (C) 2003-2007  Anders Gavare.  All rights reserved.
 
@@ -51,6 +51,8 @@
 	developing firmware, using GXemul</a>
   <li><a href="#compilercontruct">Using GXemul in compiler contruction courses</a>
   <li><a href="#disk">How to start the emulator with a disk image</a>
+  <li><a href="#tape_images">How to start the emulator with tape images</a>
+  <li><a href="#disk_overlays">How to use disk image overlays</a>
   <li><a href="#filexfer">Transfering files to/from the guest OS</a>
   <li><a href="#largeimages">How to extract large gzipped disk images</a>
   <li><a href="#userland">Running userland binaries</a>
@@ -197,7 +199,16 @@
 </pre>
 It is probably possible to use harddisks as well this way, but I would not
 recommend it.
-<p>
+
+
+
+
+
+
+<p><br>
+<a name="tape_images"></a>
+<h3>How to start the emulator with tape images:</h3>
+
 Using emulated tape drives is a bit more complicated than disks, because a
 tape can be made up of several "files" with space in between. The solution
 I have choosen is to have one file in the host's file system space for each
@@ -247,6 +258,71 @@
 on the command line. By changing what the symbolic name points to,
 you can "switch tapes" without quiting and restarting the emulator.
 
+<p>
+<font color="#ff0000">Note: Tape support is most likely very buggy, 
+because it has not been tested much, and has probably also suffered 
+from bit-rot by now.</font>
+
+
+
+
+
+
+<p><br>
+<a name="disk_overlays"></a>
+<h3>How to use disk image overlays:</h3>
+
+This is most likely best understood by an example:
+
+<p><ul>
+  <li>Install e.g. <a href="guestoses.html#netbsdcatsinstall">NetBSD/cats</a>.
+	You will end up with a disk image called
+	<tt>nbsd_cats.img</tt>.
+  <p>
+  <li>Running the following command will boot straight from the disk
+	image, with no overlay images:<pre>
+	<b>gxemul -XEcats -d nbsd_cats.img netbsd.aout-GENERIC.gz</b>
+
+</pre>
+  <li>You may now create an overlay file, a corresponding map file,
+	and start the emulator with the overlay image connected to
+	the same (explicit) ID as the base disk image:<pre>
+	<b>touch overlay.img overlay.img.map
+	gxemul -XEcats -d 0:nbsd_cats.img -d V0:overlay.img netbsd.aout-GENERIC.gz</b>
+
+</pre>
+  <li>Any changes to the filesystem you perform when using the overlay
+	will only be written to that overlay. For example, to perform
+	a "roll back", you can do the following:<pre>
+	<b>rm -f overlay.img overlay.img.map
+	touch overlay.img overlay.img.map</b>
+
+</pre>
+	and then simply start the emulator again, with the newly created
+	overlay image.
+</ul>
+
+<p>It is also possible to add multiple overlays. In that case, writes 
+always go the the <i>last</i> added overlay.
+
+<p>GXemul uses Unix' way of supporting files with "holes",
+so even if <tt>ls -l overlay.img</tt> says that the overlay is several 
+gigabytes large, <tt>du overlay.img</tt> should reveal that only the
+blocks that have actually been written to have been stored in the
+overlay, e.g.:<pre>
+	<b>$ ls -l
+	..
+	-rw-r--r--  1 debug  wheel  3072319488 Mar 24 11:59 nbsd_cats.img
+	-rw-r--r--  1 debug  wheel     2465354 Mar 24 11:44 netbsd.aout-GENERIC.gz
+	-rw-r--r--  1 debug  wheel  2930841600 Mar 24 14:02 overlay.img
+	-rw-r--r--  1 debug  wheel      715538 Mar 24 14:02 overlay.img.map
+	$ du overlay.img
+	864     overlay.img
+</b>
+</pre>
+
+<p>The .map file is simply a raw bitmap telling which blocks of the 
+overlay file that are in use.