Running a Unix-like OS on a home-built CPU with a home-built C compiler (2020)

https://fuel.edby.coffee/posts/how-we-ported-xv6-os-to-a-home-built-cpu-with-a-home-built-c-compiler/

by markus_zhang • 4 years ago

266 58 4 years ago

fuel.edby.coffee

IncRnd• 4 years ago

October 4, 2020, Running a Unix-like OS on a home-built CPU with a home-built C compiler

https://news.ycombinator.com/item?id=24680109

dj_mc_merlin3• 4 years ago

CPU experiment is a little famous exercise held in the winter of the junior year in my department, the Department of Information Science at the University of Tokyo. In the experiment, students are divided into groups of four or five students. Each group designs an own CPU architecture, implements it on an FPGA, builds an OCaml subset compiler for that CPU, and then runs a given ray-tracing program on the CPU. Typically, one or two people are responsible for each of the CPU, FPU, CPU simulator and compiler.

I have seen professional programmers fail projects much easier than this. How does the University of Tokyo manage to teach their students so well, or do they show up to it better trained/motivated than the average Western student?

trollied• 4 years ago

This same sort of thing was all taught on my degree course (Manchester, UK). Implement an ARM core (with a reduced instruction set), run on FPGA, write a compiler in the compilers course etc etc.

It set me up well - I've written a few emulators as a hobbyist.

neetfreak• 4 years ago

Got any materials on this? I wouldn’t mind learning and attempting something similar.

zmachinaz• 4 years ago

In order to get admitted to University of Tokyo, you have to be top on the common national university entrance examination, and then be top on the University of Tokyo entrance examination. So only the best of the best in Japan gets admitted. Highly competitive, as a degree from that University sets you for life.

lifthrasiir2• 4 years ago

I have been in a university with that much competition (except that I enrolled back when the competition was less fierce, and I observed this as a teaching assistant), and it is very frequent that the competition only lasts until the point of matriculation because university curricula are more lenient in those countries. I would not expect most CS students in the U of Tokyo would be able to do the same.

dingosity4• 4 years ago

Just 'cause you're smart, it doesn't mean you know how to design CPUs (or write C Compilers.) And if the national university entrance exam is testing for VLSI design domain knowledge, they're doing something wrong.

It sounds like UTokyo is like MIT. My first test there had material that wasn't in the book and wasn't covered in the lectures. When I asked my instructor about that, he said "Oh. You're supposed to know to do research outside of class," and when I asked my class-mates they said... "Oh. Yeah. My fraternity maintains a file of past tests for each instructor. You can get a good idea for what's going to be on various tests from reviewing the files. And if you don't understand it, you can get a frat brother to tutor you on it."

(As an aside... one of the frats had THE EXACT SAME TEST I had just taken. The prof didn't even change the problems. sigh)

So... my experience with MIT was that it's a place where bright kids go to get taught by upper-classmen and the classes are there only to prove you have some sort of mastery, or at least familiarity, with the material.

At Xmas that year I was bemoaning this fact to a friend from high school. His father was a physics prof at the local state university and overheard my complaints. He offered to give me a place in the lab if I xferred over. I took him up on his offer and wound up with a desk in the undergrad office, an account on the departmental VAX (this was a big thing back in the day) and a key to the physics building and the optics lab. I could drop in to my professor's offices virtually any time and most of them were excellent in explaining the finer points of quantum chromodynamics or math methods. I was recruited to be on the "let's build a super-cheap STM" team and landed a scholarship award for my work. After taking a hiatus to defend democracy I returned and landed a part-time gig at the Superconducting Super-Collider. That year I got my own MicroVAX. (Thank you, congress for all that SSC money.) But, of course, it didn't last. (Curse you, congress for taking all that SSC money away.)

My point may be that smart kids will do well at whatever university they attend. Also, I think I did MUCH better at the local state school than I would have at MIT. (Though I didn't actually graduate w/ a Physics B.S. as I planned. IBM hired me before I graduated and it was a PAIN IN THE ** to matriculate with even a B.A.)

Also. Lori Glaze (NASA director of Planetary Science) was a class-mate of mine, so... you know... it couldn't have been THAT bad of a school.

Now... as a software development manager, the thing that REALLY impressed me was the ability of a group of kids to effectively work together. The United States pumps out a lot of very bright CS grads, and our national mythos of "the rugged individual" helps in some ways, but I'm going to guess most CS students aren't getting a lot of experience in groups larger than 2 or 3 people. When I hire recent grads from US schools, the most important thing we have to teach them is how to play well with others. Sounds like the UTokyo team already learned this lesson.

lifthrasiir2• 4 years ago

When instructors were well motivated and designed the course appropriately, that can surely happen. I'm not sure if this particular course is mandatory though (see my sibling comment).

alfanick• 4 years ago

My experience is similar - Poznan, Poland - building your own CPU architecture is part of curriculum, most students hate it (and struggle to pass or don’t pass because of it). For people who like it and get it, it’s a perfect experience.

extinctpotato• 4 years ago

In case of Poznan, it's the lecturer who fails to teach this subject in a sane way.

iCurbix• 4 years ago

building your own CPU architecture is part of curriculum

I don't think it is any more. I did my BSc there and didn't have any course where you build CPU architecture of our own.

pjmlp2• 4 years ago

Most Portuguese universities have similar projects designing CPUs on digital circuits set of lectures.

When I did my degree, it was many years ago, we still had a 5 year degree for licentiate and this was spread between 2nd and 3rd year.

jcpst2• 4 years ago

A lot of professional programmers, including myself, did not study computer science at a university.

dj_mc_merlin3• 4 years ago

Could you explain? How does not studying CS relate to being able to build a CPU/compiler/ray tracer? You don't need advanced CS to do so. Obviously the people who developed the main methods/algorithms knew their shit, academically speaking, but you can basically just engineer your way through all of this.

Source: I also don't have a CS degree.

senkora• 4 years ago

These things are much easier for the average person when a course provides motivation, deadlines, requirements, and office hours.

I would expect any Western student at a top school who self-selects into taking OS to be capable of this with the right course structure.

dingosity4• 4 years ago

I think that's related to how commenters are reacting. The OP sort of implies the only contact he had with instructors was at the beginning of the year when they received their assignment.

I left a comment above about how I left MIT (which doesn't teach as much as it provides an infrastructure for taking tests students use to demonstrate mastery) for a state school (that actually taught the courses offered and provided students with ample lab opportunities.)

I'm now thinking one of the great benefits from being taught by an actual professor (instead of an upper-classman fraternity brother) is that you get a context that is formed over the lifetime of a research career. I learned equations by reading the textbook. But how they were developed, why they were developed, what equations were used before and why they needed better models? I don't think you're going to get that from your fraternity brother.

So... yes... hopefully the UTokyo team had access to the faculty to direct them towards the most fruitful areas of the academic garden. And yes... I think there are some very good CS students out there in the states that could do the same thing. But as I mentioned above, it's great to hear a tale about smart kids working together as a team.

shakow• 4 years ago

We did something similar in my engineering school in France too.

Design a simple SPARC-like CPU, make it work up to VGA/UART(and PS/2 IIRC?) I/O on an FPGA board, write a compiler for a tiny subset of Java for it, and make your OS work.

srvmshr• 4 years ago

Alumni here.

This is not a course everyone enrolls in. Generally the top 10% who are competitive enough enroll on it. But kids have done exceptional projects in this course. The creme de la creme students of Japan are in Tokyo U or Kyoto U, so the intellectual pool is generally very good (Tokyo University generally ranks within 40 consistently in CS/EE internationally)

RMarcus• 4 years ago

Thanks for the first-hand information! I was also curious about this.

This is not a course everyone enrolls in.

This is the ticket -- at both the University of Arizona and MIT, I've seen a group of folks graduate with a CS degree after taking OS, compilers, databases, and abstract algebra. Another another group of folks graduated after taking HCI, software engineering, design, and psychology courses. The two groups had some baseline skills (all knew the basic data structures and algorithms), but otherwise appeared quite distinct.

I don't know how to phrase this formally, but I think some statement like the following is true: within-university variance is higher than between-university variance.

(When I was younger, I had strong opinions on which one of these groups were "real" computer scientists. This was a very unfortunate way of thinking that prevented me from talking to folks who I later realized were some of the smartest around. I wish someone had corrected me sooner -- solving a problem with inputs/outputs well-defined enough to apply "rigorous" techniques doesn't make those problems inherently valuable or "harder" than others. God gave all the easy problems to the physicists.)

hnthrowaway03152• 4 years ago

Thanks for sharing. Can you please share some short stories about those 10%? I'm curious what kind of education they received before they managed to pull that off. Judging from the article posted by OP, one gentleman was able to create a mini-curse "in 4 hours", which is impressive. I have a feeling that they are "Carmack-grade" programmers.

matthewmacleod• 4 years ago

My CS course at Edinburgh had similar levels of projects, as did many others I’ve heard of - so I don’t think it’s anything particularly out of the ordinary for a good teaching university.

The thing is, “professional programming” is a very different challenge from a task like this one, and it’s hard to describe either as objectively “easier”. The provision of a clear outcome, resources, guidance, teaching time etc. can go a long way to helping deliver a good outcome - often things professional developers don’t have as much access to.

I’d bet the majority of developers I know (and respect) put on to this project would fail at it without support - similarly, an early-career student would likewise crash and burn if put on to, say, developing a front-end architecture for a modern web app. Horses for courses.

tails4e• 4 years ago

This is an impressive project, and great example of teamwork. Yeas ago I'd used uclinux on an fpga, which was designed at the time to run on simple processors without mmu support, had you looked at that option? That said it's a lot more fun for the cpu guys to build those features and see them working. Any stats on the final HW design, mhz, resources, etc?

neverartful• 4 years ago

It's fantastic to read about experiences like this. What a tremendous level of accomplishment and satisfaction they must have felt! The closest thing that I've done to this was in taking 'Nand to Tetris' which is brilliant (https://www.nand2tetris.org).

gigatexal2• 4 years ago

The ability to create almost everything I need for say a laptop or a desktop would be amazing. A custom built chip that could be 3D printed maybe — maybe some derivative of RiscV will get us there sometime in my lifetime (next 50 years).

digitallyfree2• 4 years ago

You already can get part of the way there by desigining your own FPGA board (I believe there are some Lattice chips with an open-source toolchain, otherwise the toolchain is closed). On top of that you can run your RISC-V CPU and the other components you need to create a working SoC, with all the RTL under your control. Custom silicon in our hands would be quite a stretch though.

robinsonb52• 4 years ago

Yes indeed - the open source toolchain (yosys, nextpnr and GHDL for VHDL support) isn't 100% "there" yet, but it is good enough to use for real projects as long as you keep the limitations in mind. It supports several different FPGA families now, with Lattice ICE40 (project icestorm) and Lattice ECP5 (project trellis) being pretty mature, and (somewhat more experimentally) project XRay for Xilinx series 7 and project Mistral for Cyclone V - and others too.

My own CPU (EightThirtyTwo) and SoC project (SDRAM, video, sound, interrupts, uart) can be built for ECP5 using Yosys and friends, as well as for a number Altera/Intel FPGAs using the proprietary Quartus.

robinsonb52• 4 years ago

Or maybe somewhat sooner? https://fosstodon.org/@mntmn@mastodon.social/109089458151612...

VMtest• 4 years ago

since this is a university project, related to a comment on ryujinx emulator from a day ago https://news.ycombinator.com/item?id=33750368

hardware2win• 4 years ago

Thats some high quality school

Holy shit

Im envy

markus_zhang2• 4 years ago

I'm thinking about getting nand2tetris done and go from there. None of the parts (build CPU using FPGA, bootstrapping compiler and porting an OS) seems to be particularly unreachable theoretically but of course they are all very difficult for me as a non CS guy.

bo10242• 4 years ago

Am I right that this is all simulated in software? I mean for the "implemented on FPGA" part, is there a physical FPGA running this OS? If so, how does it talk to the rest of the computer (i.e. drivers, etc)?

aewens• 4 years ago

A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturing – hence the term field-programmable.
FPGAs contain an array of programmable logic blocks, and a hierarchy of reconfigurable interconnects allowing blocks to be wired together. Logic blocks can be configured to perform complex combinational functions, or act as simple logic gates like AND and XOR. In most FPGAs, logic blocks also include memory elements, which may be simple flip-flops or more complete blocks of memory.[1] Many FPGAs can be reprogrammed to implement different logic functions, allowing flexible reconfigurable computing as performed in computer software.

The above being from the FPGA Wikipedia page[0].

So yes, it is done “in software”, but FPGAs is fancy hardware that can reprogrammed to act as new hardware on the fly using software. Many are implemented as PCI devices, so they could interact with the rest of the system over the PCI bus.

[0] https://en.m.wikipedia.org/wiki/Field-programmable_gate_arra...

bo10242• 4 years ago

Sorry, my question wasn't clear. What I am asking is whether the FPGA was being simulated in a VM, or whether the OS they built was running on a physical FPGA. And if the latter, how did the OS handle drivers for other components.

magicalhippo• 4 years ago

The original Xv6 code uses the x86 IO ports for example[1][2][3], it would be easy to implement something similar for the soft CPU I imagine. Some good info about IO ports in the answers to this[4] SO question.

[1] https://github.com/mit-pdos/xv6-public/blob/eeb7b415dbcb12cc...

[2] https://github.com/mit-pdos/xv6-public/blob/eeb7b415dbcb12cc...

[3] https://www.felixcloutier.com/x86/out

[4] https://stackoverflow.com/questions/3215878/what-are-in-out-...

LoganDark2• 4 years ago

Their software simulator was built to allow the porting process, but as far as I can tell, they did implement their design on a physical FPGA, yes.

dingosity4• 4 years ago

I used to joke that I was a "Full-Stack Engineer w/ VLSI Design Experience." But... I never wrote my own C Compiler from scratch (or an OCaml implementation.)

Tepix• 4 years ago

This would be a fantastic project to build a capture the flag challenge upon. Having to reverse engineer the platform to exploit it...

userbinator• 4 years ago

The instructor only asks the students to “take this ray-tracing program written in OCaml and run it on your CPU implemented on an FPGA”

However, some teams put more energy into doing fun such as running games or playing music by connecting a speaker with their CPU.

That reminded me of this Linus Akesson demo, where he himself makes an entire SoC, including a GPU, and the software for it: https://www.linusakesson.net/scene/parallelogram/index.php

LoganDark2• 4 years ago

I've always wanted to play with an FPGA in this way, but I know too little to pick one that could be used to implement a CPU. They're all expensive ($50 or more) and have limits that I don't know how to interpret. For example, would this[0] be suitable for implementing a CPU soft-core? I've done some electronics before but never used an HDL and never dealt with FPGA dev boards.

[0] https://www.ebay.com/itm/114058082998

hnthrowaway03152• 4 years ago

I'm thinking the same. I'd like to use FPGA to create some circuits, maybe start from simple ones such as ALU and build up to CPU. Not sure if it's good idea though.

justinlloyd• 4 years ago

  Design and build my own CPU: Check
  Write my own C compiler: Check
  Create a UNIX-like OS for the CPU that compiles with the C compiler: Now I have something to bookend my career with.

airswimmer• 4 years ago

it's cool to know that students are doing this for fun and learning.

There are many mips cpus out there. It's very popular in China.

Running a Unix-like OS on a home-built CPU with a home-built C compiler (2020)

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