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<h1>Xv6, a simple Unix-like teaching operating system</h1>
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<br><br>
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Xv6 is a teaching operating system developed
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in the summer of 2006 for MIT's operating systems course,
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“6.828: Operating Systems Engineering.”
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We used it for 6.828 in Fall 2006 and Fall 2007
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and are using it this semester (Fall 2008).
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We hope that xv6 will be useful in other courses too.
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This page collects resources to aid the use of xv6
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in other courses.
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<h2>History and Background</h2>
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For many years, MIT had no operating systems course.
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In the fall of 2002, Frans Kaashoek, Josh Cates, and Emil Sit
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created a new, experimental course (6.097)
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to teach operating systems engineering.
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In the course lectures, the class worked through Sixth Edition Unix (aka V6)
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using John Lions's famous commentary.
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In the lab assignments, students wrote most of an exokernel operating
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system, eventually named Jos, for the Intel x86.
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Exposing students to multiple systems–V6 and Jos–helped
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develop a sense of the spectrum of operating system designs.
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In the fall of 2003, the experimental 6.097 became the
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official course 6.828; the course has been offered each fall since then.
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<br><br>
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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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<br><br>
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6.828 substituted xv6 for V6 in the fall of 2006.
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Based on that experience, we cleaned up rough patches
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of xv6 for the course in the fall of 2007.
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Since then, xv6 has stabilized, so we are making it
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available in the hopes that others will find it useful too.
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<br><br>
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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="xv6-rev2.tar.gz">xv6-rev2.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="xv6-rev2.pdf">xv6-rev2.pdf</a>.
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<br><br>
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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="../../2007/tools.html">6.828 tools page</a>.
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<h2>Lectures</h2>
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In 6.828, the lectures in the first half of the course
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introduce the PC hardware, the Intel x86, and then xv6.
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The lectures in the second half consider advanced topics
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using research papers; for some, xv6 serves as a useful
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base for making discussions concrete.
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This section describe a typical 6.828 lecture schedule,
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linking to lecture notes and homework.
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A course using only xv6 (not Jos) will need to adapt
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a few of the lectures, but we hope these are a useful
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starting point.
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<br><br><b><i>Lecture 1. Operating systems</i></b>
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<br><br>
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The first lecture introduces both the general topic of
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operating systems and the specific approach of 6.828.
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After defining “operating system,” the lecture
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examines the implementation of a Unix shell
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to look at the details the traditional Unix system call interface.
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This is relevant to both xv6 and Jos: in the final
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Jos labs, students implement a Unix-like interface
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and culminating in a Unix shell.
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<br><br>
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<a href="l1.html">lecture notes</a>
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<a href="os-lab-1.pdf">OS abstractions slides</a>
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<br><br><b><i>Lecture 2. PC hardware and x86 programming</i></b>
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<br><br>
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This lecture introduces the PC architecture, the 16- and 32-bit x86,
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the stack, and the GCC x86 calling conventions.
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It also introduces the pieces of a typical C tool chain–compiler,
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assembler, linker, loader–and the Bochs emulator.
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<br><br>
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Reading: PC Assembly Language
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<br><br>
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Homework: familiarize with Bochs
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<br><br>
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<a href="l2.html">lecture notes</a>
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<a href="os-lab-2.pdf">x86 intro slides</a>
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<a href="x86-intro.html">homework</a>
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<br><br><b><i>Lecture 3. Operating system organization</i></b>
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<br><br>
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This lecture continues Lecture 1's discussion of what
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an operating system does.
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An operating system provides a “virtual computer”
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interface to user space programs.
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At a high level, the main job of the operating system
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is to implement that interface
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using the physical computer it runs on.
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<br><br>
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The lecture discusses four approaches to that job:
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monolithic operating systems, microkernels,
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virtual machines, and exokernels.
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Exokernels might not be worth mentioning
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except that the Jos labs are built around one.
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<br><br>
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Reading: Engler et al., Exokernel: An Operating System Architecture
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for Application-Level Resource Management
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<br><br>
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<a href="l3.html">lecture notes</a>
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<br><br><b><i>Lecture 4. Address spaces using segmentation</i></b>
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<br><br>
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This is the first lecture that uses xv6.
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It introduces the idea of address spaces and the
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details of the x86 segmentation hardware.
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It makes the discussion concrete by reading the xv6
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source code and watching xv6 execute using the Bochs simulator.
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<br><br>
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Reading: x86 MMU handout,
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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>.
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<br><br>
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Homework: Bochs stack introduction
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<br><br>
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<a href="l4.html">lecture notes</a>
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<a href="os-lab-3.pdf">x86 virtual memory slides</a>
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<a href="xv6-intro.html">homework</a>
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<br><br><b><i>Lecture 5. Address spaces using page tables</i></b>
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<br><br>
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This lecture continues the discussion of address spaces,
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examining the other x86 virtual memory mechanism: page tables.
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Xv6 does not use page tables, so there is no xv6 here.
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Instead, the lecture uses Jos as a concrete example.
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An xv6-only course might skip or shorten this discussion.
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<br><br>
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Reading: x86 manual excerpts
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<br><br>
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Homework: stuff about gdt
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XXX not appropriate; should be in Lecture 4
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<br><br>
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<a href="l5.html">lecture notes</a>
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<br><br><b><i>Lecture 6. Interrupts and exceptions</i></b>
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<br><br>
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How does a user program invoke the operating system kernel?
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How does the kernel return to the user program?
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What happens when a hardware device needs attention?
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This lecture explains the answer to these questions:
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interrupt and exception handling.
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<br><br>
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It explains the x86 trap setup mechanisms and then
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examines their use in xv6's SETGATE (<a href="src/mmu.h.html">mmu.h</a>),
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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.
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<br><br>
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It then traces through a call to the system call open:
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<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>),
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syscall (<a href="src/syscall.c.html">syscall.c</a>),
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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>),
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<br><br>
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The interrupt controller, briefly:
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pic_init and pic_enable (<a href="src/picirq.c.html">picirq.c</a>).
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The timer and keyboard, briefly:
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timer_init (<a href="src/timer.c.html">timer.c</a>), console_init (<a href="src/console.c.html">console.c</a>).
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Enabling and disabling of interrupts.
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<br><br>
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Reading: x86 manual excerpts,
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xv6: trapasm.S, <a href="src/trap.c.html">trap.c</a>, <a href="src/syscall.c.html">syscall.c</a>, and usys.S.
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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>.
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<br><br>
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Homework: Explain the 35 words on the top of the
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stack at first invocation of <code>syscall</code>.
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<br><br>
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<a href="l-interrupt.html">lecture notes</a>
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<a href="x86-intr.html">homework</a>
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<br><br><b><i>Lecture 7. Multiprocessors and locking</i></b>
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<br><br>
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This lecture introduces the problems of
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coordination and synchronization on a
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multiprocessor
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and then the solution of mutual exclusion locks.
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Atomic instructions, test-and-set locks,
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lock granularity, (the mistake of) recursive locks.
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<br><br>
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Although xv6 user programs cannot share memory,
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the xv6 kernel itself is a program with multiple threads
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executing concurrently and sharing memory.
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Illustration: the xv6 scheduler's proc_table_lock (<a href="src/proc.c.html">proc.c</a>)
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and the spin lock implementation (<a href="src/spinlock.c.html">spinlock.c</a>).
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<br><br>
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Reading: xv6: <a href="src/spinlock.c.html">spinlock.c</a>. Skim <a href="src/mp.c.html">mp.c</a>.
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<br><br>
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Homework: Interaction between locking and interrupts.
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Try not disabling interrupts in the disk driver and watch xv6 break.
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<br><br>
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<a href="l-lock.html">lecture notes</a>
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<a href="xv6-lock.html">homework</a>
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<br><br><b><i>Lecture 8. Threads, processes and context switching</i></b>
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<br><br>
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The last lecture introduced some of the issues
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in writing threaded programs, using xv6's processes
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as an example.
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This lecture introduces the issues in implementing
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threads, continuing to use xv6 as the example.
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<br><br>
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The lecture defines a thread of computation as a register
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set and a stack. A process is an address space plus one
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or more threads of computation sharing that address space.
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Thus the xv6 kernel can be viewed as a single process
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with many threads (each user process) executing concurrently.
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<br><br>
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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>)
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<br><br>
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Reading: <a href="src/proc.c.html">proc.c</a>, swtch.S, sys_fork (<a href="src/sysproc.c.html">sysproc.c</a>)
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<br><br>
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Homework: trace through stack switching.
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<br><br>
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<a href="l-threads.html">lecture notes (need to be updated to use swtch)</a>
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<a href="xv6-sched.html">homework</a>
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<br><br><b><i>Lecture 9. Processes and coordination</i></b>
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<br><br>
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This lecture introduces the idea of sequence coordination
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and then examines the particular solution illustrated by
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sleep and wakeup (<a href="src/proc.c.html">proc.c</a>).
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It introduces and refines a simple
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producer/consumer queue to illustrate the
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need for sleep and wakeup
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and then the sleep and wakeup
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implementations themselves.
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<br><br>
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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>).
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<br><br>
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Homework: Explain how sleep and wakeup would break
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without proc_table_lock. Explain how devices would break
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without second lock argument to sleep.
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<br><br>
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<a href="l-coordination.html">lecture notes</a>
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<a href="xv6-sleep.html">homework</a>
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<br><br><b><i>Lecture 10. Files and disk I/O</i></b>
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<br><br>
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This is the first of three file system lectures.
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This lecture introduces the basic file system interface
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and then considers the on-disk layout of individual files
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and the free block bitmap.
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<br><br>
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Reading: iread, iwrite, fileread, filewrite, wdir, mknod1, and
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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>.
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<br><br>
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Homework: Add print to bwrite to trace every disk write.
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Explain the disk writes caused by some simple shell commands.
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<br><br>
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<a href="l-fs.html">lecture notes</a>
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<a href="xv6-disk.html">homework</a>
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<br><br><b><i>Lecture 11. Naming</i></b>
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<br><br>
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The last lecture discussed on-disk file system representation.
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This lecture covers the implementation of
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file system paths (namei in <a href="src/fs.c.html">fs.c</a>)
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and also discusses the security problems of a shared /tmp
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and symbolic links.
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<br><br>
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Understanding exec (<a href="src/exec.c.html">exec.c</a>) is left as an exercise.
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<br><br>
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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>.
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<br><br>
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Homework: Explain how to implement symbolic links in xv6.
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<br><br>
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<a href="l-name.html">lecture notes</a>
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<a href="xv6-names.html">homework</a>
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<br><br><b><i>Lecture 12. High-performance file systems</i></b>
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<br><br>
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This lecture is the first of the research paper-based lectures.
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It discusses the “soft updates” paper,
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using xv6 as a concrete example.
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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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<br><br>
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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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<br><br>
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You can reach all of us at 6.828-staff@pdos.csail.mit.edu.
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<br><br>
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<br><br>
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