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-** Xv6, a simple Unix-like teaching operating system
-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.
-
-* History and Background
-
-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.
-
-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 requires handling concurrency head on with
-locks and threads (instead of using special-case solutions for
-uniprocessors such as
-enabling/disabling interrupts) and helps relevance.
-Finally, writing a new system allowed us to write cleaner versions
-of the rougher parts of V6, like the scheduler and file system.
-
-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.
-
-6.828 uses both xv6 and Jos.  
-Courses taught at UCLA, NYU, Peking University, Stanford, Tsinghua,
-and University Texas (Austin) have used
-Jos without xv6; we believe other courses could use
-xv6 without Jos, though we are not aware of any that have.
-
-
-* Xv6 sources
-
-The latest xv6 is [xv6-rev4.tar.gz].
-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
-[xv6-rev4.pdf].
-The xv6 source code is licensed under the traditional <a href="http://www.opensource.org/licenses/mit-license.php">MIT license</a>;
-see the LICENSE file in the source distribution.
-
-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 [../../2007/tools.html | 6.828 tools page].
-
-
-* Lectures
-
-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.
-
-
-Lecture 1. Operating systems
-
-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.
-
-[l1.html | lecture notes]
-[os-lab-1.pdf | OS abstractions slides]
-
-
-Lecture 2.  PC hardware and x86 programming
-
-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.
-
-Reading: PC Assembly Language
-
-Homework: familiarize with Bochs
-
-[l2.html | lecture notes]
-[os-lab-2.pdf | x86 intro slides]
-[x86-intro.html | homework]
-
-
-Lecture 3.  Operating system organization
-
-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.
-
-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.
-
-Reading: Engler et al., Exokernel: An Operating System Architecture
-for Application-Level Resource Management
-
-[l3.html | lecture notes]
-
-
-Lecture 4.  Address spaces using segmentation
-
-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.
-
-Reading: x86 MMU handout,
-xv6: bootasm.S, bootother.S, bootmain.c, main.c, init.c, and setupsegs in proc.c.
-
-Homework: Bochs stack introduction
-
-[l4.html | lecture notes]
-[os-lab-3.pdf | x86 virtual memory slides]
-[xv6-intro.html | homework]
- 
-
-Lecture 5.  Address spaces using page tables
-
-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.
-
-Reading: x86 manual excerpts
-
-Homework: stuff about gdt
-XXX not appropriate; should be in Lecture 4
-
-[l5.html | lecture notes]
-
-
-Lecture 6.  Interrupts and exceptions
-
-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.
-
-It explains the x86 trap setup mechanisms and then
-examines their use in xv6's SETGATE (mmu.h),
-tvinit (trap.c), idtinit (trap.c), vectors.pl, and vectors.S.
-
-It then traces through a call to the system call open:
-init.c, usys.S, vector48 and alltraps (vectors.S), trap (trap.c),
-syscall (syscall.c),
-sys_open (sysfile.c), fetcharg, fetchint, argint, argptr, argstr (syscall.c),
-
-The interrupt controller, briefly:
-pic_init and pic_enable (picirq.c).
-The timer and keyboard, briefly:
-timer_init (timer.c), console_init (console.c).
-Enabling and disabling of interrupts.
-
-Reading: x86 manual excerpts,
-xv6: trapasm.S, trap.c, syscall.c, and usys.S.
-Skim lapic.c, ioapic.c, picirq.c.
-
-Homework: Explain the 35 words on the top of the
-stack at first invocation of <code>syscall</code>.
-
-[l-interrupt.html | lecture notes]
-[x86-intr.html | homework]
-
-
-Lecture 7.  Multiprocessors and locking
-
-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.
-
-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 (proc.c)
-and the spin lock implementation (spinlock.c).
-
-Reading: xv6: spinlock.c.  Skim mp.c.
-
-Homework: Interaction between locking and interrupts.
-Try not disabling interrupts in the disk driver and watch xv6 break.
-
-[l-lock.html | lecture notes]
-[xv6-lock.html | homework]
-
-
-Lecture 8.  Threads, processes and context switching
-
-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.
-
-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.
-
-Illustrations: thread switching (swtch.S), scheduler (proc.c), sys_fork (sysproc.c)
-
-Reading: proc.c, swtch.S, sys_fork (sysproc.c)
-
-Homework: trace through stack switching.
-
-[l-threads.html | lecture notes (need to be updated to use swtch)]
-[xv6-sched.html | homework]
-
-
-Lecture 9.  Processes and coordination
-
-This lecture introduces the idea of sequence coordination
-and then examines the particular solution illustrated by
-sleep and wakeup (proc.c).
-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.
-
-Reading: proc.c, sys_exec, sys_sbrk, sys_wait, sys_exec, sys_kill (sysproc.c).
-
-Homework: Explain how sleep and wakeup would break
-without proc_table_lock.  Explain how devices would break
-without second lock argument to sleep.
-
-[l-coordination.html | lecture notes]
-[xv6-sleep.html | homework]
-
-
-Lecture 10.  Files and disk I/O
-
-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.
-
-Reading: iread, iwrite, fileread, filewrite, wdir, mknod1, and 
-	code related to these calls in fs.c, bio.c, ide.c, and file.c.
-
-Homework: Add print to bwrite to trace every disk write.
-Explain the disk writes caused by some simple shell commands.
-
-[l-fs.html | lecture notes]
-[xv6-disk.html | homework]
-
-
-Lecture 11.  Naming
-
-The last lecture discussed on-disk file system representation.
-This lecture covers the implementation of
-file system paths (namei in fs.c)
-and also discusses the security problems of a shared /tmp
-and symbolic links.
-
-Understanding exec (exec.c) is left as an exercise.
-
-Reading: namei in fs.c, sysfile.c, file.c.
-
-Homework: Explain how to implement symbolic links in xv6.
-
-[l-name.html | lecture notes]
-[xv6-names.html | homework]
-
-
-Lecture 12.  High-performance file systems
-
-This lecture is the first of the research paper-based lectures.
-It discusses the ``soft updates'' paper,
-using xv6 as a concrete example.
-
-
-* Feedback
-
-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.
-
-Russ Cox ([email protected])<br>
-Frans Kaashoek ([email protected])<br>
-Robert Morris ([email protected])
-
-You can reach all of us at [email protected].
-