354 lines
14 KiB
HTML
354 lines
14 KiB
HTML
<!-- AUTOMATICALLY GENERATED: EDIT the .txt version, not the .html version -->
|
|
<html>
|
|
<head>
|
|
<title>Xv6, a simple Unix-like teaching operating system</title>
|
|
<style type="text/css"><!--
|
|
body {
|
|
background-color: white;
|
|
color: black;
|
|
font-size: medium;
|
|
line-height: 1.2em;
|
|
margin-left: 0.5in;
|
|
margin-right: 0.5in;
|
|
margin-top: 0;
|
|
margin-bottom: 0;
|
|
}
|
|
|
|
h1 {
|
|
text-indent: 0in;
|
|
text-align: left;
|
|
margin-top: 2em;
|
|
font-weight: bold;
|
|
font-size: 1.4em;
|
|
}
|
|
|
|
h2 {
|
|
text-indent: 0in;
|
|
text-align: left;
|
|
margin-top: 2em;
|
|
font-weight: bold;
|
|
font-size: 1.2em;
|
|
}
|
|
--></style>
|
|
</head>
|
|
<body bgcolor=#ffffff>
|
|
<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,
|
|
“6.828: Operating Systems Engineering.”
|
|
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–V6 and Jos–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–which only
|
|
happens because of interrupts–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 “operating system,” 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–compiler,
|
|
assembler, linker, loader–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 “virtual computer”
|
|
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 “soft updates” 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>
|
|
</body>
|
|
</html>
|