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diff --git a/web/index.html b/web/index.html new file mode 100644 index 0000000..d5f940c --- /dev/null +++ b/web/index.html @@ -0,0 +1,353 @@ +<!-- 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 ([email protected])<br> +Frans Kaashoek ([email protected])<br> +Robert Morris ([email protected]) +<br><br> +You can reach all of us at [email protected]. +<br><br> +<br><br> +</body> +</html> |