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<b>Gavare's eXperimental Emulator:</b></font><br> |
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<font color="#000000" size="6"><b>Introduction</b> |
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</font></td></tr></table></td></tr></table><p> |
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$Id: intro.html,v 1.87 2006/06/23 10:00:41 debug Exp $ |
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Copyright (C) 2003-2006 Anders Gavare. All rights reserved. |
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Redistribution and use in source and binary forms, with or without |
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modification, are permitted provided that the following conditions are met: |
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1. Redistributions of source code must retain the above copyright |
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notice, this list of conditions and the following disclaimer. |
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notice, this list of conditions and the following disclaimer in the |
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documentation and/or other materials provided with the distribution. |
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3. The name of the author may not be used to endorse or promote products |
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derived from this software without specific prior written permission. |
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THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND |
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--> |
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|
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<a href="./">Back to the index</a> |
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|
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<p><br> |
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<h2>Introduction</h2> |
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|
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<p> |
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<table border="0" width="99%"><tr><td valign="top" align="left"> |
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<ul> |
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<li><a href="#overview">Overview</a> |
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<li><a href="#free">Is GXemul Free software?</a> |
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<li><a href="#build">How to compile/build the emulator</a> |
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<li><a href="#run">How to run the emulator</a> |
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<li><a href="#cpus">Which processor architectures does GXemul emulate?</a> |
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<li><a href="#hosts">Which host architectures are supported?</a> |
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<li><a href="#translation">What kind of translation does GXemul use?</a> |
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<li><a href="#accuracy">Emulation accuracy</a> |
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<li><a href="#emulmodes">Which machines does GXemul emulate?</a> |
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</ul> |
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</td><td valign="center" align="center"> |
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<a href="20050317-example.png"><img src="20050317-example_small.png"></a> |
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<p>NetBSD/pmax 1.6.2 with X11<br>running in GXemul</td></tr></table> |
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|
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|
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|
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|
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<p><br> |
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<a name="overview"></a> |
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<h3>Overview:</h3> |
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|
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GXemul is an experimental instruction-level machine emulator. Several |
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emulation modes are available. In some modes, processors and surrounding |
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hardware components are emulated well enough to let unmodified operating |
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systems (e.g. NetBSD) run as if they were running on a real machine. |
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|
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<p>Devices and processors (ARM, MIPS, PowerPC) are not simulated with 100% |
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accuracy. They are only ``faked'' well enough to allow guest operating |
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systems run without complaining too much. Still, the emulator could be of |
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interest for academic research and experiments, such as when learning how |
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to write operating system code. |
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|
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<p>The emulator is written in C, does not depend on third-party libraries, |
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and should compile and run on most 64-bit and 32-bit Unix-like systems. |
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|
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<p>The emulator contains code which tries to emulate the workings of CPUs |
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and surrounding hardware found in real machines, but it does not contain |
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any ROM code. You will need some form of program (in binary form) to run |
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in the emulator. For many emulation modes, PROM calls are handled by the |
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emulator itself, so you do not need to use any ROM image at all. |
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|
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<p>You can use pre-compiled kernels (for example NetBSD kernels, or |
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Linux), or other programs that are in binary format, and in some cases |
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even actual ROM images. A couple of different file formats are supported |
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(ELF, a.out, ECOFF, SREC, and raw binaries). |
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|
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<p>If you do not have a kernel as a separate file, but you have a bootable |
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disk image, then it is sometimes possible to boot directly from that |
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image. (This works for example with DECstation emulation, or when booting |
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from ISO9660 CDROM images.) |
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|
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|
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|
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|
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|
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|
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|
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|
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<p><br> |
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<a name="free"></a> |
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<h3>Is GXemul Free software?</h3> |
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|
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Yes. I have released GXemul under a Free license. The code in GXemul is |
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Copyrighted software, it is <i>not</i> public domain. (If this is |
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confusing to you, you might want to read up on the definitions of the |
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four freedoms associated with Free software, <a |
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href="http://www.gnu.org/philosophy/free-sw.html">http://www.gnu.org/philosophy/free-sw.html</a>.) |
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|
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<p>The code I have written is released under a 3-clause BSD-style license |
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(or "revised BSD-style" if one wants to use <a |
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href="http://www.gnu.org/philosophy/bsd.html">GNU jargon</a>). Apart from |
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the code I have written, some files are copied from other sources such as |
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NetBSD, for example header files containing symbolic names of bitfields in |
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device registers. They are also covered by similar licenses, but with some |
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additional clauses. The main point, however, is that the licenses require |
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that the original Copyright and license terms are included when you make a |
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copy or modification. |
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|
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<p>If you plan to redistribute GXemul <i>without</i> supplying the source |
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code, then you need to comply with each individual source file some other |
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way, for example by writing additional documentation containing copyright |
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notes. I have not done this, since I do not plan on making distributions |
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without source code. You need to check all individual files for details. |
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The "easiest way out" if you plan to redistribute code from GXemul is, of |
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course, to let it remain open source and simply supply the source code. |
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|
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<p>In case you want to reuse parts of GXemul, but you need to do that |
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under a different license (e.g. the GPL), then contact me and I might |
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re-license/dual-license files on a case-by-case basis. |
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|
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|
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|
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|
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|
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<p><br> |
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<a name="build"></a> |
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<h3>How to compile/build the emulator:</h3> |
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|
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Uncompress the .tar.gz distribution file, and run |
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<pre> |
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$ <b>./configure</b> |
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$ <b>make</b> |
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</pre> |
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|
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<p>This should work on most Unix-like systems. GXemul does not require any |
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specific libraries to build, however, if you build on a system which does |
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not have X11 libraries installed, some functionality will be lost. |
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|
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<p>The emulator's performance is highly dependent on both runtime settings |
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and on compiler settings, so you might want to experiment with different |
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CC and CFLAGS environment variable values. For example, on an AMD Athlon |
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host, you might want to try setting <tt>CFLAGS</tt> to <tt>-march=athlon</tt> |
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before running <tt>configure</tt>. |
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|
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|
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|
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|
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|
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|
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|
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<p><br> |
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<a name="run"></a> |
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<h3>How to run the emulator:</h3> |
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|
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Once you have built GXemul, running it should be rather straight-forward. |
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Running <tt><b>gxemul</b></tt> without arguments (or with the |
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<b><tt>-h</tt></b> or <b><tt>-H</tt></b> command line options) will |
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display a help message. |
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|
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<p> |
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To get some ideas about what is possible to run in the emulator, please |
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read the section about <a href="guestoses.html">installing "guest" |
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operating systems</a>. If you are interested in using the emulator to |
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develop code on your own, then you should also read the section about |
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<a href="experiments.html#hello">Hello World</a>. |
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|
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<p> |
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To exit the emulator, type CTRL-C to enter the |
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single-step debugger, and then type <tt><b>quit</b></tt>. |
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|
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<p> |
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If you are starting an emulation by entering settings directly on the |
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command line, and you are not using the <tt><b>-x</b></tt> option, then all |
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terminal input and output will go to the main controlling terminal. |
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CTRL-C is used to break into the debugger, so in order to send CTRL-C to |
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the running (emulated) program, you may use CTRL-B. |
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(This should be a reasonable compromise to allow the emulator to be usable |
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even on systems without X Windows.) |
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|
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<p> |
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There is no way to send an actual CTRL-B to the emulated program, when |
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typing in the main controlling terminal window. The solution is to either |
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use <a href="configfiles.html">configuration files</a>, or use |
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<tt><b>-x</b></tt>. Both these solutions cause new xterms to be opened for |
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each emulated serial port that is written to. CTRL-B and CTRL-C both have |
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their original meaning in those xterm windows. |
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|
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|
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|
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|
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|
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<p><br> |
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<a name="cpus"></a> |
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<h3>Which processor architectures does GXemul emulate?</h3> |
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|
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The architectures that are emulated well enough to let at least one |
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guest operating system run (per architecture) are ARM, MIPS, and |
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PowerPC. |
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|
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|
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|
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|
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|
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<p><br> |
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<a name="hosts"></a> |
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<h3>Which host architectures are supported?</h3> |
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|
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As of release 0.4.0 of GXemul, the old binary translation subsystem, which |
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was used for emulation of MIPS processors on Alpha and i386 hosts, has |
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been removed. The current dynamic translation subsystem should work on any |
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host. |
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|
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|
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|
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|
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|
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<p><br> |
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<a name="translation"></a> |
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<h3>What kind of translation does GXemul use?</h3> |
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|
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<b>Static vs. dynamic:</b> |
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|
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<p>In order to support guest operating systems, which can overwrite old |
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code pages in memory with new code, it is necessary to translate code |
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dynamically. It is not possible to do a "one-pass" (static) translation. |
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Self-modifying code and Just-in-Time compilers running inside |
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the emulator are other things that would not work with a static |
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translator. GXemul is a dynamic translator. However, it does not |
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necessarily translate into native code, like many other emulators. |
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|
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<p><b>"Runnable" Intermediate Representation:</b> |
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|
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<p>Dynamic translators usually translate from the emulated architecture |
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(e.g. MIPS) into a kind of <i>intermediate representation</i> (IR), and then |
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to native code (e.g. AMD64 or x86 code). Since one of my main goals for |
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GXemul is to keep everything as portable as possible, I have tried to make |
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sure that the IR is something which can be executed regardless of whether |
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the final step (translation from IR to native code) has been implemented |
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or not. |
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|
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<p>The IR in GXemul consists of arrays of pointers to functions, and a few |
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arguments which are passed along to those functions. The functions are |
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implemented in either manually hand-coded C, or automatically generated C. |
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In any case, this is all statically linked into the GXemul binary at link |
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time. |
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|
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<p>Here is a simplified diagram of how these arrays work. |
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|
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<p><center><img src="simplified_dyntrans.png"></center> |
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|
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<p>There is one instruction call slot for every possible program counter |
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location. In the MIPS case, instruction words are 32 bits in length, |
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and pages are (usually) 4 KB large, resulting in 1024 instruction call |
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slots. After the last of these instruction calls, there is an additional |
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call to a special "end of page" function (which doesn't count as an executed |
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instruction). This function switches to the first instruction |
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on the next virtual page (which might cause exceptions, etc). |
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|
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<p>The complexity of individual instructions vary. A simple example of |
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what an instruction can look like is the MIPS <tt>addiu</tt> instruction: |
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<pre> |
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X(addiu) |
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{ |
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reg(ic->arg[1]) = (int32_t) |
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((int32_t)reg(ic->arg[0]) + (int32_t)ic->arg[2]); |
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} |
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</pre> |
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|
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<p>It stores the result of a 32-bit addition of the register at arg[0] |
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with the immediate value arg[2] (treating both as signed 32-bit |
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integers) into register arg[1]. If the emulated CPU is a 64-bit CPU, |
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then this will store a correctly sign-extended value into arg[1]. |
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If it is a 32-bit CPU, then only the lowest 32 bits will be stored, |
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and the high part ignored. <tt>X(addiu)</tt> is expanded to |
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<tt>mips_instr_addiu</tt> in the 64-bit case, and <tt>mips32_instr_addiu</tt> |
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in the 32-bit case. Both are compiled into the GXemul executable; no code |
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is created during run-time. |
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|
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<p>Here are examples of what the <tt>addiu</tt> instruction actually |
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looks like when it is compiled, on various host architectures: |
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|
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<p><center><table border="0"> |
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<tr><td><b>GCC 4.0.1 on Alpha:</b></td> |
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<td width="35"></td><td></td> |
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<tr> |
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<td valign="top"> |
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<pre>mips_instr_addiu: |
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ldq t1,8(a1) |
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ldq t2,24(a1) |
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ldq t3,16(a1) |
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ldq t0,0(t1) |
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addl t0,t2,t0 |
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stq t0,0(t3) |
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ret</pre> |
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</td> |
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<td></td> |
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<td valign="top"> |
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<pre>mips32_instr_addiu: |
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ldq t2,8(a1) |
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ldq t0,24(a1) |
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ldq t3,16(a1) |
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ldl t1,0(t2) |
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addq t0,t1,t0 |
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stl t0,0(t3) |
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ret</pre> |
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</td> |
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</tr> |
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|
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<tr><td><b><br>GCC 3.4.4 on AMD64:</b></td> |
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<tr> |
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<td valign="top"> |
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<pre>mips_instr_addiu: |
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mov 0x8(%rsi),%rdx |
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mov 0x18(%rsi),%rax |
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mov 0x10(%rsi),%rcx |
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add (%rdx),%eax |
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cltq |
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mov %rax,(%rcx) |
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retq</pre> |
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</td> |
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<td></td> |
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<td valign="top"> |
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<pre>mips32_instr_addiu: |
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mov 0x8(%rsi),%rcx |
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mov 0x10(%rsi),%rdx |
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mov (%rcx),%eax |
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add 0x18(%rsi),%eax |
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mov %eax,(%rdx) |
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retq</pre> |
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</td> |
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</tr> |
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|
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<tr><td><b><br>GCC 4.0.1 on i386:</b></td> |
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<tr> |
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<td valign="top"> |
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<pre>mips_instr_addiu: |
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mov 0x8(%esp),%eax |
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mov 0x8(%eax),%ecx |
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mov 0x4(%eax),%edx |
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mov 0xc(%eax),%eax |
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add (%edx),%eax |
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mov %eax,(%ecx) |
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cltd |
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mov %edx,0x4(%ecx) |
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ret</pre> |
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</td> |
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<td></td> |
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<td valign="top"> |
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<pre>mips32_instr_addiu: |
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mov 0x8(%esp),%eax |
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mov 0x8(%eax),%ecx |
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mov 0x4(%eax),%edx |
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mov 0xc(%eax),%eax |
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add (%edx),%eax |
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mov %eax,(%ecx) |
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ret</pre> |
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</td> |
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</tr> |
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</table></center> |
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|
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<p>On 64-bit hosts, there is not much difference, but on 32-bit hosts (and |
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to some extent on AMD64), the difference is enough to make it worthwhile. |
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|
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|
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<p><b>Performance:</b> |
385 |
|
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<p>The performance of using this kind of runnable IR is obviously lower |
387 |
than what can be achieved by emulators using native code generation, but |
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can be significantly higher than using a naive fetch-decode-execute |
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interpretation loop. In my opinion, using a runnable IR is an interesting |
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compromise. |
391 |
|
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<p>The overhead per emulated instruction is usually around or below |
393 |
approximately 10 host instructions. This is very much dependent on your |
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host architecture and what compiler and compiler switches you are using. |
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Added to this instruction count is (of course) also the C code used to |
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implement each specific instruction. |
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|
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<p><b>Instruction Combinations:</b> |
399 |
|
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<p>Short, common instruction sequences can sometimes be replaced by a |
401 |
"compound" instruction. An example could be a compare instruction followed |
402 |
by a conditional branch instruction. The advantages of instruction |
403 |
combinations are that |
404 |
<ul> |
405 |
<li>the amortized overhead per instruction is slightly reduced, and |
406 |
<p> |
407 |
<li>the host's compiler can make a good job at optimizing the common |
408 |
instruction sequence. |
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</ul> |
410 |
|
411 |
<p>The special cases where instruction combinations give the most gain |
412 |
are in the cores of string/memory manipulation functions such as |
413 |
<tt>memset()</tt> or <tt>strlen()</tt>. The core loop can then (at least |
414 |
to some extent) be replaced by a native call to the equivalent function. |
415 |
|
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<p>The implementations of compound instructions still keep track of the |
417 |
number of executed instructions, etc. When single-stepping, these |
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translations are invalidated, and replaced by normal instruction calls |
419 |
(one per emulated instruction). |
420 |
|
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<p><b>Native Code Back-ends: (not in this release)</b> |
422 |
|
423 |
<p>In theory, it will be possible to implement native code generation |
424 |
(similar to what is used in high-performance emulators such as QEMU), |
425 |
as long as that generated code abides to the C ABI on the host, but |
426 |
for now I wanted to make sure that GXemul works without such native |
427 |
code back-ends. For this reason, as of release 0.4.0, GXemul is |
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completely free of native code back-ends. |
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|
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|
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|
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|
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|
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|
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<p><br> |
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<a name="accuracy"></a> |
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<h3>Emulation accuracy:</h3> |
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|
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GXemul is an instruction-level emulator; things that would happen in |
440 |
several steps within a real CPU are not taken into account (e.g. pipe-line |
441 |
stalls or out-of-order execution). Still, instruction-level accuracy seems |
442 |
to be enough to be able to run complete guest operating systems inside the |
443 |
emulator. |
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|
445 |
<p>The existance of instruction and data caches is "faked" to let |
446 |
operating systems think that they are there, but for all practical |
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purposes, these caches are non-working. |
448 |
|
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<p>The emulator is <i>not</i> timing-accurate. It can be run in a |
450 |
"deterministic" mode, <tt><b>-D</b></tt>. The meaning of deterministic is |
451 |
simply that running two emulations with the same settings will result in |
452 |
identical runs. Obviously, this requires that no user interaction is |
453 |
taking place, and that clock speeds are fixed with the <tt><b>-I</b></tt> |
454 |
option. (Deterministic in this case does <i>not</i> mean that the |
455 |
emulation will be identical to some actual real-world machine.) |
456 |
|
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<p>(Note that user interaction means <i>both</i> input to the emulated |
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program/OS, and interaction with the emulator's debugger. Breaking into the |
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debugger and then continuing execution may affect when/how interrupts |
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occur.) |
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|
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|
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|
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|
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|
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|
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<p><br> |
468 |
<a name="emulmodes"></a> |
469 |
<h3>Which machines does GXemul emulate?</h3> |
470 |
|
471 |
A few different machine types are emulated. The following machine types |
472 |
are emulated well enough to run at least one "guest OS": |
473 |
|
474 |
<p> |
475 |
<ul> |
476 |
<li><b><u>ARM</u></b> |
477 |
<ul> |
478 |
<li><b>CATS</b> (NetBSD/cats, OpenBSD/cats) |
479 |
<li><b>IQ80321</b> (NetBSD/evbarm) |
480 |
</ul> |
481 |
<p> |
482 |
<li><b><u>MIPS</u></b> |
483 |
<ul> |
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<li><b>DECstation 5000/200</b> (NetBSD/pmax, OpenBSD/pmax, Ultrix, |
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Linux/DECstation, Sprite) |
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<li><b>Acer Pica-61</b> (NetBSD/arc) |
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<li><b>NEC MobilePro 770, 780, 800, and 880</b> (NetBSD/hpcmips) |
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<li><b>Cobalt</b> (NetBSD/cobalt) |
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<li><b>Malta</b> (NetBSD/evbmips) |
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<li><b>SGI O2 (aka IP32)</b> <font color="#0000e0">(<super>*</super>)</font> |
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(NetBSD/sgi) |
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</ul> |
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<p> |
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<li><b><u>PowerPC</u></b> |
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<ul> |
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<li><b>IBM 6050/6070 (PReP, PowerPC Reference Platform)</b> (NetBSD/prep) |
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</ul> |
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</ul> |
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|
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<p><small><font color="#0000e0">(<super>*</super>)</font> = |
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Enough for root-on-nfs, but not for disk boot.)</small> |
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|
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<p>There is code in GXemul for emulation of many other machine types; the |
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degree to which these work range from almost being able to run a complete |
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OS, to almost completely unsupported (perhaps just enough support to |
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output a few boot messages via serial console). |
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|
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<p>In addition to emulating real machines, there is also a "test-machine". |
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A test-machine consists of one or more CPUs and a few experimental devices |
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such as: |
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|
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<p> |
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<ul> |
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<li>a console I/O device (putchar() and getchar()...) |
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<li>an inter-processor communication device, for SMP experiments |
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<li>a very simple linear framebuffer device (for graphics output) |
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<li>a simple SCSI disk controller |
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<li>a simple ethernet controller |
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</ul> |
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|
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<p>This mode is useful if you wish to run experimental code, but do not |
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wish to target any specific real-world machine type, for example for |
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educational purposes. |
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|
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<p>You can read more about these experimental devices <a |
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href="experiments.html#expdevices">here</a>. |
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|
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</body> |
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</html> |