158 lines
5.3 KiB
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158 lines
5.3 KiB
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<h1>Xv6, a simple Unix-like teaching operating system</h1>
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<h2>Introduction</h2>
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Xv6 is a teaching operating system developed in the summer of 2006 for
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MIT's operating systems
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course, <a href="http://pdos.csail.mit.edu/6.828">6.828: operating
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systems Engineering</a>. We hope that xv6 will be useful in other
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courses too. This page collects resources to aid the use of xv6 in
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other courses, including a commentary on the source code itself.
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<h2>History and Background</h2>
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<p>For many years, MIT had no operating systems course. In the fall of 2002,
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one was created to teach operating systems engineering. In the course lectures,
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the class worked through <a href="#v6">Sixth Edition Unix (aka V6)</a> using
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John Lions's famous commentary. In the lab assignments, students wrote most of
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an exokernel operating system, eventually named Jos, for the Intel x86.
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Exposing students to multiple systems–V6 and Jos–helped develop a
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sense of the spectrum of operating system designs.
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<p>
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V6 presented pedagogic challenges from the start.
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Students doubted the relevance of an obsolete 30-year-old operating system
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written in an obsolete programming language (pre-K&R C)
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running on obsolete hardware (the PDP-11).
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Students also struggled to learn the low-level details of two different
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architectures (the PDP-11 and the Intel x86) at the same time.
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By the summer of 2006, we had decided to replace V6
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with a new operating system, xv6, modeled on V6
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but written in ANSI C and running on multiprocessor
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Intel x86 machines.
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Xv6's use of the x86 makes it more relevant to
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students' experience than V6 was
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and unifies the course around a single architecture.
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Adding multiprocessor support requires handling concurrency head on with
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locks and threads (instead of using special-case solutions for
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uniprocessors such as
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enabling/disabling interrupts) and helps relevance.
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Finally, writing a new system allowed us to write cleaner versions
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of the rougher parts of V6, like the scheduler and file system.
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6.828 substituted xv6 for V6 in the fall of 2006.
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<h2>Xv6 sources and text</h2>
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The latest xv6 is <a href="xv6-rev5.tar.gz">xv6-rev5.tar.gz</a>. We distribute
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the sources in electronic form but also as a printed booklet with line numbers
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that keep everyone together during lectures. The booklet is available as <a
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href="xv6-rev5.pdf">xv6-rev5.pdf</a>. The xv6 source code is licensed under
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the traditional <a href="http://www.opensource.org/licenses/mit-license.php">MIT
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license</a>; see the LICENSE file in the source distribution. To help students
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read through xv6 and learn about the main ideas in operating systems we also
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distribute a <a href="book-rev6.pdf">textbook/commentary</a> for the latest xv6.
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<p>
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xv6 compiles using the GNU C compiler,
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targeted at the x86 using ELF binaries.
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On BSD and Linux systems, you can use the native compilers;
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On OS X, which doesn't use ELF binaries,
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you must use a cross-compiler.
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Xv6 does boot on real hardware, but typically
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we run it using the Bochs emulator.
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Both the GCC cross compiler and Bochs
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can be found on the <a href="../2010/tools.html">6.828 tools page</a>.
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<h2>Xv6 lecture material</h2>
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In 6.828, the lectures in the first half of the course cover the xv6 sources and
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text. The lectures in the second half consider advanced topics using research
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papers; for some, xv6 serves as a useful base for making discussions concrete.
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The lecture notes are available from the 6.828 schedule page.
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<a name="v6"></a>
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<h2>Unix Version 6</h2>
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<p>6.828's xv6 is inspired by Unix V6 and by:
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<ul>
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<li>Lions' <i>Commentary on UNIX' 6th Edition</i>, John Lions, Peer to
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Peer Communications; ISBN: 1-57398-013-7; 1st edition (June 14, 2000).
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<ul>
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<li>An on-line version of the <a
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href="http://www.lemis.com/grog/Documentation/Lions/">Lions
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commentary</a>, and <a href="http://v6.cuzuco.com/">the source code</a>.
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<li>The v6 source code is also available <a
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href="http://minnie.tuhs.org/UnixTree/V6/usr/sys/">online</a>
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through <a
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href="http://minnie.tuhs.org/PUPS/">the PDP Unix Preservation
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Society</a>.
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</ul>
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</ul>
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The following are useful to read the original code:
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<ul>
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<li><i>
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The PDP11/40 Processor Handbook</i>, Digital Equipment Corporation, 1972.
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<ul>
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<li>A <a href="http://pdos.csail.mit.edu/6.828/2005/readings/pdp11-40.pdf">PDF</a> (made from scanned images,
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and not text-searchable)
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<li>A <a href="http://pdos.csail.mit.edu/6.828/2005/pdp11/">web-based
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version</a> that is indexed by instruction name.
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</ul>
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</ul>
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<h2>Feedback</h2>
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If you are interested in using xv6 or have used xv6 in a course,
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we would love to hear from you.
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If there's anything that we can do to make xv6 easier
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to adopt, we'd like to hear about it.
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We'd also be interested to hear what worked well and what didn't.
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<p>
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Russ Cox (rsc@swtch.com)<br>
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Frans Kaashoek (kaashoek@mit.edu)<br>
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Robert Morris (rtm@mit.edu)
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<p>
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You can reach all of us at 6.828-staff@pdos.csail.mit.edu.
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