--- trunk/doc/misc.html	2007/10/08 16:19:56	24
+++ trunk/doc/misc.html	2007/10/08 16:22:11	40
@@ -10,9 +10,9 @@
 
 <!--
 
-$Id: misc.html,v 1.63 2006/05/22 04:53:52 debug Exp $
+$Id: misc.html,v 1.68 2007/04/28 09:18:34 debug Exp $
 
-Copyright (C) 2003-2006  Anders Gavare.  All rights reserved.
+Copyright (C) 2003-2007  Anders Gavare.  All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:
@@ -47,11 +47,12 @@
 
 <p>
 <ul>
-  <li><a href="#networking">Networking</a>
   <li><a href="#devel">Writing operating system code, or
 	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>
@@ -64,47 +65,6 @@
 
 
 
-<p><br>
-<a name="networking"></a>
-<h3>Networking:</h3>
-
-It is possible to let the guest OS running inside the emulator get access to
-the Internet. If you are interested in the technical details, and the 
-reasons why networking is implemented in the emulator the way it currently 
-is implemented, you might want to read the <a href="technical.html#net">
-networking section in the technical documentation</a>.
-
-<p><font color="#ff0000">This is still experimental, hackish, and 
-rather buggy. With NetBSD running as guest operating system, it mostly 
-works.</font>
-
-<p>When only one machine is being emulated, the following default values 
-apply:<pre>
-	IPv4 address:			10.0.0.1
-	Netmask:			255.0.0.0
-	Gateway / default route:	10.0.0.254
-	Nameserver:			10.0.0.254
-</pre>
-
-<p>The emulated machine must of course have a NIC which is emulated 
-correctly. At the moment, the following NICs should work:
-<ul>
-  <li><tt><b>ether</b></tt>, the "fake" experimental ethernet device
-	(documented <a href="experiments.html#expdevices_ether">here</a>)
-  <li><tt><b>le</b></tt>, Turbochannel Lance Ethernet, as used in
-	DECstation 5000/200 ("3max")
-  <li><tt><b>mec</b></tt>, the SGI O2's ethernet controller
-  <li><tt><b>dec21143</b></tt>, Digital's 21143 NIC (known as <tt>dc</tt>
-	in OpenBSD, or <tt>tlp</tt> in NetBSD)
-</ul>
-
-<p>The emulator acts as a NAT-like gateway/firewall; to the outside world
-it will seem like it is the host's OS that connects to other machines on
-the internet, not the guest OS.
-
-
-
-
 
 
 <p><br>
@@ -145,8 +105,8 @@
 	<li>CPU details in GXemul are usually wrong. If your code depends
 	on, say, R10000 or MIPS64 specifics, chances are that GXemul will
 	not be sufficient. One example is different revisions of ISAs;
-	64-bit MIPS instructions which should trigger an exception on a
-	real 32-bit MIPS processor usually execute anyway in GXemul. Another
+	some instructions which should trigger an exception on a
+	real MIPS processor usually execute anyway in GXemul. Another
 	example is if userland code tries to access kernel memory; in some
 	cases there is protection against this, but not in all cases (to get
 	higher performance).
@@ -239,7 +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
@@ -289,6 +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.
 
 
 
@@ -298,9 +332,9 @@
 <a name="filexfer"></a>
 <h3>Transfering files to/from the guest OS:</h3>
 
-If the emulated machine supports networking (see
-<a href="#networking">above</a>), then transfering files via FTP is
-probably easiest.
+If the emulated machine supports networking (see <a 
+href="networking.html#intro">this section</a> for more info), then the easiest
+way to transfer files is probably to use FTP or similar methods.
 
 <p>There is another way of transfering files which works for any kind of 
 emulated machine which supports disks (either SCSI or IDE). Any file can