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<title>Xv6, a simple Unix-like teaching operating system</title>
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2011-02-19 16:36:14 +01:00
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2008-09-03 06:50:04 +02:00
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<h1>Xv6, a simple Unix-like teaching operating system</h1>
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2011-02-19 16:36:14 +01:00
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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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<p><font color="red">Status: The xv6 code is in pretty good shape, but
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the commentary is rough.</font>
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2008-09-03 06:50:04 +02:00
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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
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of 2002, Frans Kaashoek, Josh Cates, and Emil Sit created a new,
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experimental course (6.097) to teach operating systems engineering.
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In the course lectures, the class worked through <a href="#v6">Sixth
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Edition Unix (aka V6)</a> using John Lions's famous commentary. In
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the lab assignments, students wrote most of an exokernel operating
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system, eventually named Jos, for the Intel x86. Exposing students to
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multiple systems–V6 and Jos–helped develop a sense of the
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spectrum of operating system designs. In the fall of 2003, the
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experimental 6.097 became the official course 6.828; the course has
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been offered each fall since then.
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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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<p> 6.828 substituted xv6 for V6 in the fall of 2006. Based on
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that experience, we cleaned up rough patches of xv6. Since then, xv6
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has stabilized, so we are making it available in the hopes that others
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will find it useful too.
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<p>
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6.828 uses both xv6 and Jos.
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Courses taught at UCLA, NYU, Peking University, Stanford, Tsinghua,
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and University Texas (Austin) have used
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Jos without xv6; we believe other courses could use
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xv6 without Jos, though we are not aware of any that have.
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<h2>Xv6 sources</h2>
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The latest xv6 is <a href="../src/xv6-rev4.tar.gz">xv6-rev4.tar.gz</a>.
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We distribute the sources in electronic form but also as
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a printed booklet with line numbers that keep everyone
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together during lectures. The booklet is available as
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<a href="../src/xv6-rev4.pdf">xv6-rev4.pdf</a>.
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The xv6 source code is licensed under the traditional <a href="http://www.opensource.org/licenses/mit-license.php">MIT license</a>;
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see the LICENSE file in the source distribution.
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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 introduce the
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PC hardware, the Intel x86, and then xv6. The lectures in the second
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half consider advanced topics using research papers; for some, xv6
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serves as a useful base for making discussions concrete. The lecture
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notes are available from the 6.828 schedule page, and the chapters of
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the commentary are below.
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<h2>Xv6 commentary (rough)</h2>
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<p>The chapters are rough drafts.
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<p>Introduction yet to be written.<br>
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<ul>
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<li>read with the code side by side
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<li>code references look like (xxxx) or (xxxx-yyyy) in small text.
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<li><a href="../src/xv6-rev4.pdf">this pdf</a> is the one with matching line numbers.
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<li>each chapter starts with an introduction to the topic,
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spends most of the text on code,
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and then wraps up talking about how xv6
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compares to real-world operating systems.
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</ul>
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<a href="unix.pdf">Chapter 0: Operating system interfaces</a>
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<blockquote>
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The Unix system call interface. (rev 4)
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</blockquote>
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<a href="boot.pdf">Chapter 1: Bootstrap</a>
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<blockquote>
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From power on to kernel start. (rev 4)
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</blockquote>
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<a href="mem.pdf">Chapter 2: Processes</a>
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<blockquote>
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Memory and process allocation, segments, the first user process. (rev 4)
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</blockquote>
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<a href="trap.pdf">Chapter 3: Traps</a>
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<blockquote>
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Low-level trap mechanism, trap handler, system call arguments, sbrk, fork.
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</blockquote>
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<a href="lock.pdf">Chapter 4: Locks</a>
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<blockquote>
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Locks and interrupts.
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</blockquote>
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<a href="sched.pdf">Chapter 5: Scheduling and coordination</a>
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<blockquote>
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Scheduling, sleep and wakeup, pipes, wait and exit.
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</blockquote>
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<a href="disk.pdf">Chapter 6: Buffer cache</a>
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<blockquote>
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Buffer cache and IDE disk driver.
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</blockquote>
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<a href="fsdata.pdf">Chapter 7: File system data</a>
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<blockquote>
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Block in use bitmap, block allocation, inode structure, inode contents,
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directories, path names.
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</blockquote>
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<a href="fscall.pdf">Chapter 8: File system calls</a>
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<blockquote>
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FIle descriptors, open, close, dup, read, write.
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</blockquote>
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<a href="exec.pdf">Chapter 9: Exec</a>
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<blockquote>
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Exec
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</blockquote>
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Appendix A: Low-level C and inline assembly
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<blockquote>
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Intro to C and inline assembly for people who only know Java (say).
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Examples drawn entirely from xv6 source.
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</blockquote>
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Appendix B: Additional drivers.
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<blockquote>
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Keyboard, screen, probably MP hardware.
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</blockquote>
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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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2008-09-03 06:50:04 +02:00
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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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