Xv6, a simple Unix-like teaching operating system

[bane]

Really cool project. Older, simpler, unixes are surprisingly relevant for undergrad-level learning.

It reminds me of my OS class in my undergrad days. We skipped a few bits by using the JVM to get a few things for free, but we still had to write (as groups of 3-5 people) an entire unix-like, complete with user-space programs, messaging systems, etc.

Definitely one of the most valuable classes I took for my degree.

[btian]

Also xv6 book that's meant to accompany the source code. http://pdos.csail.mit.edu/6.828/2012/xv6/book-rev7.pdf

[csense]

The line numbers in the book reference a 92-page printout of the source code here: http://pdos.csail.mit.edu/6.828/2012/xv6/xv6-rev7.pdf

[kirbyk]

Purdue has a similar OS, Xinu, that was created for teaching. It is actually used in industry around the world. - http://www.xinu.cs.purdue.edu/

[luckydude]

+1 I learned a boatload by reading the xinu books.

Also met Doug, he's a great guy, old school C hacker.

[n1cked]

The OS classes at several universities revolve around reimplementing Xinu and really getting to know a linksys router.

[adamnemecek]

That reminds me, I stumbled upon the website of the Unix Heritage Society (http://www.tuhs.org/) which has the source code for the early versions of Unix.

Here is the first version

http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1

I did not really spend enough time reading the source to understand what's going on but it's interesting regardless.

[derekp7]

There's also a port of V7 Unix to x86, by http://www.nordier.com/v7x86/

I only wish that there was an add-on tcp/ip stack for that version, then one could really have fun with it. Yes, I know, the ip stack add on is called BSD.

[swetland]

Because splatting object files and intermediates all over the same directories as the source drives me nuts, I shuffled things so that it's a bit easier to find things (splitting source into kernel/, user/, ulib/, tools/, and include/ directories) and generated files are collected into kobj/, uobj/, out/, and fs/ directories.

https://github.com/swetland/xv6

I also tweaked the scheduler to halt if there's nothing to schedule, per tankenmate's suggestion.

A side-effect of this change is the generate-source-code-printout targets were rather severely broken, so they're simply removed for the moment.

[tankenmate]

If you are running this on your laptop and want to save your battery then use this patch...

http://pastebin.com/BxXDDtTh

[userbinator]

I remember in the pre-Pentium II days people cared rather little about power consumption (TDPs were only a few watts or maybe a dozen at most), and the idle process was basically a very tight infinite loop.

Around the time of the PII people (especially overclockers) started noticing that the CPU would stay cooler when halted, and thus started a little industry of "CPU cooling software"... there's an interesting historical page on that here:

http://estu.nit.ac.jp/~e982457/other/cpuidle/idle.htm

A modern processor, on the other hand, takes a few watts halted and a hundred or more when active, and transitions between these states many many times per second. How much things have changed...

[swetland]

Nice catch. Actually speeds everything up a bunch. I did it a bit more tersely with:

if (p == &ptable.proc[NPROC]) hlt();

after the sti() and initializing p to 0;

[akkartik]

Interesting. How does the processor restart after hlt?

[oscargrouch]

>Russ Cox ([email protected])

>Frans Kaashoek ([email protected])

>Robert Morris ([email protected])

sh$%t, this got to be good!

Cloning the repo and surfing at the source code, just for fun, right now :)

[Jonanin]

xv6 is a pleasure to hack on and learn from! It's got everything you'd expect from a unix-like OS (multi-processor support, simple scheduler, unix-like FS, many of the standard syscalls, fork/exec based shell, etc).

Linux and *BSD may be the modern standard for unix-like OSes, but they're often too complex and opaque for beginners to learn from. It's super easy to read and toy with xv6's code if you want to learn more about OS internals, though. For example, I was able to implement a simple log structured file system: https://github.com/Jonanin/xv6-lfs

Note: if you're looking for companion reading material, check out this OS book: http://pages.cs.wisc.edu/~remzi/OSTEP/ (IMO it's super approachable and understandable for undergrad-level learning)

[kps]

I just did a quick grep through the code, and apparently students are expected to understand this.

[bodyfour]

Shame this is getting downvoted, probably by people who don't understand the reference. See the second section ("You are not expected to understand this") in: http://cm.bell-labs.com/cm/cs/who/dmr/odd.html

[capkutay]

I wonder how this compares to pintos (http://www.stanford.edu/class/cs140/projects/pintos/pintos_1...) which is an i386 unix like OS. In my under grad class we had to build thread scheduling, sys calls, virtual memory, and some file system functionality on top of pintos. It was the most challenging and most interesting set of projects I've ever completed.

[merkury7]

Adding to the list of alternatives:

MINIX (http://www.minix3.org/)

[mwcampbell]

At the risk of reigniting an old flame war, it seems to me that a microkernel in which core OS functions such as process management and the virtual file system are in separate servers would be more complicated than a monolithic kernel. Having device drivers and specific file systems in user space may be useful, though.

[blt]

Awesome! I really hope MIT releases video lectures for 6.828 so I can do the full course on my own. I'm listening to Berkeley's OS lectures right now but I would love a course built around xv6.

[ash]

This page has links to 6.828 lecture videos (2011):

http://pdos.csail.mit.edu/6.828/2011/schedule.html

[__mp]

Alternatively there's also Barrelfish: http://www.barrelfish.org/ but there the focus lies on multiple cores (48ish+++).

[singular]

For anyone else who is playing with this in OS X Mavericks, I've written a blog post describing how to get things working on it (there are a couple steps that need to be tweaked.) - http://blog.ljs.io/post/71424794630/xv6

Hopefully this is useful to somebody and isn't too blatantly link whorey :)

[mjg59]

Someone's going to have the fun job of porting this to UEFI at some point in the not-too distant future.

[swetland]

Doesn't seem like it'd be a big ordeal. Just adapt an existing bootloader to load the kernel.

Also it looks like they mostly run it in qemu.

[geofft]

It's multiboot, so you can just boot it from grub.efi. See sheet 10, xv6/entry.S.

The more annoying thing is likely to be that it's 32-bit only. Are there any common machines that do x86-32 UEFI boot? Alternatively, does a 64-bit grub.efi know how to exit long mode and go into protected mode, to multiboot a 32-bit kernel?

(I was actually talking the other day with some folks about whether a 6.828 project involving a UEFI port of JOS, the other teaching OS, would make sense, but the 64-bit thing seemed to make it less interesting unless you also plan to do a 64-bit port of JOS. Which has been done before in a final project, admittedly...)

[userbinator]

As long as there are things like SeaBIOS, there is no need for UEFI.

[chris_wot]

The link to the schedule points to:

http://pdos.csail.mit.edu/6.828/schedule.html

It should be:

http://pdos.csail.mit.edu/6.828/2012/schedule.html

[Moral_]

Were using the Xv6 kernel for the OS class here at the university of Utah, for the first time. In my opinion an OS class that makes you hack on low level code is significantly more beneficial tha just a theroy class.

[contingencies]

Alternatively, historical codebases of various unices at http://www.tuhs.org/archive_sites.html

[csense]

If "Xv6's use of the x86 [is intended to make] it more relevant to students' experience," then why use AT&T assembly language syntax rather than Intel?

[rsc]

Because the GNU toolchain uses AT&T syntax and most students have the GNU toolchain readily available?

[samograd]

I thought this was pretty awesome when I found it a couple of years ago, especially since it's multicore.