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<h1>Xv6, a simple Unix-like teaching operating system</h1>
<br><br>
Xv6 is a teaching operating system developed
in the summer of 2006 for MIT's operating systems course,
&ldquo;6.828: Operating Systems Engineering.&rdquo;
We used it for 6.828 in Fall 2006 and Fall 2007
and are using it this semester (Fall 2008).
We hope that xv6 will be useful in other courses too.
This page collects resources to aid the use of xv6
in other courses.
<h2>History and Background</h2>
For many years, MIT had no operating systems course.
In the fall of 2002, Frans Kaashoek, Josh Cates, and Emil Sit
created a new, experimental course (6.097)
to teach operating systems engineering.
In the course lectures, the class worked through Sixth Edition Unix (aka V6)
using John Lions's famous commentary.
In the lab assignments, students wrote most of an exokernel operating
system, eventually named Jos, for the Intel x86.
Exposing students to multiple systems&ndash;V6 and Jos&ndash;helped
develop a sense of the spectrum of operating system designs.
In the fall of 2003, the experimental 6.097 became the
official course 6.828; the course has been offered each fall since then.
<br><br>
V6 presented pedagogic challenges from the start.
Students doubted the relevance of an obsolete 30-year-old operating system
written in an obsolete programming language (pre-K&R C)
running on obsolete hardware (the PDP-11).
Students also struggled to learn the low-level details of two different
architectures (the PDP-11 and the Intel x86) at the same time.
By the summer of 2006, we had decided to replace V6
with a new operating system, xv6, modeled on V6
but written in ANSI C and running on multiprocessor
Intel x86 machines.
Xv6's use of the x86 makes it more relevant to
students' experience than V6 was
and unifies the course around a single architecture.
Adding multiprocessor support also helps relevance
and makes it easier to discuss threads and concurrency.
(In a single processor operating system, concurrency&ndash;which only
happens because of interrupts&ndash;is too easy to view as a special case.
A multiprocessor operating system must attack the problem head on.)
Finally, writing a new system allowed us to write cleaner versions
of the rougher parts of V6, like the scheduler and file system.
<br><br>
6.828 substituted xv6 for V6 in the fall of 2006.
Based on that experience, we cleaned up rough patches
of xv6 for the course in the fall of 2007.
Since then, xv6 has stabilized, so we are making it
available in the hopes that others will find it useful too.
<br><br>
6.828 uses both xv6 and Jos.
Courses taught at UCLA, NYU, and Stanford have used
Jos without xv6; we believe other courses could use
xv6 without Jos, though we are not aware of any that have.
<h2>Xv6 sources</h2>
The latest xv6 is <a href="xv6-rev2.tar.gz">xv6-rev2.tar.gz</a>.
We distribute the sources in electronic form but also as
a printed booklet with line numbers that keep everyone
together during lectures. The booklet is available as
<a href="xv6-rev2.pdf">xv6-rev2.pdf</a>.
<br><br>
xv6 compiles using the GNU C compiler,
targeted at the x86 using ELF binaries.
On BSD and Linux systems, you can use the native compilers;
On OS X, which doesn't use ELF binaries,
you must use a cross-compiler.
Xv6 does boot on real hardware, but typically
we run it using the Bochs emulator.
Both the GCC cross compiler and Bochs
can be found on the <a href="../../2007/tools.html">6.828 tools page</a>.
<h2>Lectures</h2>
In 6.828, the lectures in the first half of the course
introduce the PC hardware, the Intel x86, and then xv6.
The lectures in the second half consider advanced topics
using research papers; for some, xv6 serves as a useful
base for making discussions concrete.
This section describe a typical 6.828 lecture schedule,
linking to lecture notes and homework.
A course using only xv6 (not Jos) will need to adapt
a few of the lectures, but we hope these are a useful
starting point.
<br><br><b><i>Lecture 1. Operating systems</i></b>
<br><br>
The first lecture introduces both the general topic of
operating systems and the specific approach of 6.828.
After defining &ldquo;operating system,&rdquo; the lecture
examines the implementation of a Unix shell
to look at the details the traditional Unix system call interface.
This is relevant to both xv6 and Jos: in the final
Jos labs, students implement a Unix-like interface
and culminating in a Unix shell.
<br><br>
<a href="l1.html">lecture notes</a>
<br><br><b><i>Lecture 2. PC hardware and x86 programming</i></b>
<br><br>
This lecture introduces the PC architecture, the 16- and 32-bit x86,
the stack, and the GCC x86 calling conventions.
It also introduces the pieces of a typical C tool chain&ndash;compiler,
assembler, linker, loader&ndash;and the Bochs emulator.
<br><br>
Reading: PC Assembly Language
<br><br>
Homework: familiarize with Bochs
<br><br>
<a href="l2.html">lecture notes</a>
<a href="x86-intro.html">homework</a>
<br><br><b><i>Lecture 3. Operating system organization</i></b>
<br><br>
This lecture continues Lecture 1's discussion of what
an operating system does.
An operating system provides a &ldquo;virtual computer&rdquo;
interface to user space programs.
At a high level, the main job of the operating system
is to implement that interface
using the physical computer it runs on.
<br><br>
The lecture discusses four approaches to that job:
monolithic operating systems, microkernels,
virtual machines, and exokernels.
Exokernels might not be worth mentioning
except that the Jos labs are built around one.
<br><br>
Reading: Engler et al., Exokernel: An Operating System Architecture
for Application-Level Resource Management
<br><br>
<a href="l3.html">lecture notes</a>
<br><br><b><i>Lecture 4. Address spaces using segmentation</i></b>
<br><br>
This is the first lecture that uses xv6.
It introduces the idea of address spaces and the
details of the x86 segmentation hardware.
It makes the discussion concrete by reading the xv6
source code and watching xv6 execute using the Bochs simulator.
<br><br>
Reading: x86 MMU handout,
xv6: bootasm.S, bootother.S, <a href="src/bootmain.c.html">bootmain.c</a>, <a href="src/main.c.html">main.c</a>, <a href="src/init.c.html">init.c</a>, and setupsegs in <a href="src/proc.c.html">proc.c</a>.
<br><br>
Homework: Bochs stack introduction
<br><br>
<a href="l4.html">lecture notes</a>
<a href="xv6-intro.html">homework</a>
<br><br><b><i>Lecture 5. Address spaces using page tables</i></b>
<br><br>
This lecture continues the discussion of address spaces,
examining the other x86 virtual memory mechanism: page tables.
Xv6 does not use page tables, so there is no xv6 here.
Instead, the lecture uses Jos as a concrete example.
An xv6-only course might skip or shorten this discussion.
<br><br>
Reading: x86 manual excerpts
<br><br>
Homework: stuff about gdt
XXX not appropriate; should be in Lecture 4
<br><br>
<a href="l5.html">lecture notes</a>
<br><br><b><i>Lecture 6. Interrupts and exceptions</i></b>
<br><br>
How does a user program invoke the operating system kernel?
How does the kernel return to the user program?
What happens when a hardware device needs attention?
This lecture explains the answer to these questions:
interrupt and exception handling.
<br><br>
It explains the x86 trap setup mechanisms and then
examines their use in xv6's SETGATE (<a href="src/mmu.h.html">mmu.h</a>),
tvinit (<a href="src/trap.c.html">trap.c</a>), idtinit (<a href="src/trap.c.html">trap.c</a>), <a href="src/vectors.pl.html">vectors.pl</a>, and vectors.S.
<br><br>
It then traces through a call to the system call open:
<a href="src/init.c.html">init.c</a>, usys.S, vector48 and alltraps (vectors.S), trap (<a href="src/trap.c.html">trap.c</a>),
syscall (<a href="src/syscall.c.html">syscall.c</a>),
sys_open (<a href="src/sysfile.c.html">sysfile.c</a>), fetcharg, fetchint, argint, argptr, argstr (<a href="src/syscall.c.html">syscall.c</a>),
<br><br>
The interrupt controller, briefly:
pic_init and pic_enable (<a href="src/picirq.c.html">picirq.c</a>).
The timer and keyboard, briefly:
timer_init (<a href="src/timer.c.html">timer.c</a>), console_init (<a href="src/console.c.html">console.c</a>).
Enabling and disabling of interrupts.
<br><br>
Reading: x86 manual excerpts,
xv6: trapasm.S, <a href="src/trap.c.html">trap.c</a>, <a href="src/syscall.c.html">syscall.c</a>, and usys.S.
Skim <a href="src/lapic.c.html">lapic.c</a>, <a href="src/ioapic.c.html">ioapic.c</a>, <a href="src/picirq.c.html">picirq.c</a>.
<br><br>
Homework: Explain the 35 words on the top of the
stack at first invocation of <code>syscall</code>.
<br><br>
<a href="l-interrupt.html">lecture notes</a>
<a href="x86-intr.html">homework</a>
<br><br><b><i>Lecture 7. Multiprocessors and locking</i></b>
<br><br>
This lecture introduces the problems of
coordination and synchronization on a
multiprocessor
and then the solution of mutual exclusion locks.
Atomic instructions, test-and-set locks,
lock granularity, (the mistake of) recursive locks.
<br><br>
Although xv6 user programs cannot share memory,
the xv6 kernel itself is a program with multiple threads
executing concurrently and sharing memory.
Illustration: the xv6 scheduler's proc_table_lock (<a href="src/proc.c.html">proc.c</a>)
and the spin lock implementation (<a href="src/spinlock.c.html">spinlock.c</a>).
<br><br>
Reading: xv6: <a href="src/spinlock.c.html">spinlock.c</a>. Skim <a href="src/mp.c.html">mp.c</a>.
<br><br>
Homework: Interaction between locking and interrupts.
Try not disabling interrupts in the disk driver and watch xv6 break.
<br><br>
<a href="l-lock.html">lecture notes</a>
<a href="xv6-lock.html">homework</a>
<br><br><b><i>Lecture 8. Threads, processes and context switching</i></b>
<br><br>
The last lecture introduced some of the issues
in writing threaded programs, using xv6's processes
as an example.
This lecture introduces the issues in implementing
threads, continuing to use xv6 as the example.
<br><br>
The lecture defines a thread of computation as a register
set and a stack. A process is an address space plus one
or more threads of computation sharing that address space.
Thus the xv6 kernel can be viewed as a single process
with many threads (each user process) executing concurrently.
<br><br>
Illustrations: thread switching (swtch.S), scheduler (<a href="src/proc.c.html">proc.c</a>), sys_fork (<a href="src/sysproc.c.html">sysproc.c</a>)
<br><br>
Reading: <a href="src/proc.c.html">proc.c</a>, swtch.S, sys_fork (<a href="src/sysproc.c.html">sysproc.c</a>)
<br><br>
Homework: trace through stack switching.
<br><br>
<a href="l-threads.html">lecture notes (need to be updated to use swtch)</a>
<a href="xv6-sched.html">homework</a>
<br><br><b><i>Lecture 9. Processes and coordination</i></b>
<br><br>
This lecture introduces the idea of sequence coordination
and then examines the particular solution illustrated by
sleep and wakeup (<a href="src/proc.c.html">proc.c</a>).
It introduces and refines a simple
producer/consumer queue to illustrate the
need for sleep and wakeup
and then the sleep and wakeup
implementations themselves.
<br><br>
Reading: <a href="src/proc.c.html">proc.c</a>, sys_exec, sys_sbrk, sys_wait, sys_exec, sys_kill (<a href="src/sysproc.c.html">sysproc.c</a>).
<br><br>
Homework: Explain how sleep and wakeup would break
without proc_table_lock. Explain how devices would break
without second lock argument to sleep.
<br><br>
<a href="l-coordination.html">lecture notes</a>
<a href="xv6-sleep.html">homework</a>
<br><br><b><i>Lecture 10. Files and disk I/O</i></b>
<br><br>
This is the first of three file system lectures.
This lecture introduces the basic file system interface
and then considers the on-disk layout of individual files
and the free block bitmap.
<br><br>
Reading: iread, iwrite, fileread, filewrite, wdir, mknod1, and
code related to these calls in <a href="src/fs.c.html">fs.c</a>, <a href="src/bio.c.html">bio.c</a>, <a href="src/ide.c.html">ide.c</a>, and <a href="src/file.c.html">file.c</a>.
<br><br>
Homework: Add print to bwrite to trace every disk write.
Explain the disk writes caused by some simple shell commands.
<br><br>
<a href="l-fs.html">lecture notes</a>
<a href="xv6-disk.html">homework</a>
<br><br><b><i>Lecture 11. Naming</i></b>
<br><br>
The last lecture discussed on-disk file system representation.
This lecture covers the implementation of
file system paths (namei in <a href="src/fs.c.html">fs.c</a>)
and also discusses the security problems of a shared /tmp
and symbolic links.
<br><br>
Understanding exec (<a href="src/exec.c.html">exec.c</a>) is left as an exercise.
<br><br>
Reading: namei in <a href="src/fs.c.html">fs.c</a>, <a href="src/sysfile.c.html">sysfile.c</a>, <a href="src/file.c.html">file.c</a>.
<br><br>
Homework: Explain how to implement symbolic links in xv6.
<br><br>
<a href="l-name.html">lecture notes</a>
<a href="xv6-names.html">homework</a>
<br><br><b><i>Lecture 12. High-performance file systems</i></b>
<br><br>
This lecture is the first of the research paper-based lectures.
It discusses the &ldquo;soft updates&rdquo; paper,
using xv6 as a concrete example.
<h2>Feedback</h2>
If you are interested in using xv6 or have used xv6 in a course,
we would love to hear from you.
If there's anything that we can do to make xv6 easier
to adopt, we'd like to hear about it.
We'd also be interested to hear what worked well and what didn't.
<br><br>
Russ Cox (rsc@swtch.com)<br>
Frans Kaashoek (kaashoek@mit.edu)<br>
Robert Morris (rtm@mit.edu)
<br><br>
You can reach all of us at 6.828-staff@pdos.csail.mit.edu.
<br><br>
<br><br>
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