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Mu: A minimal hobbyist computing stack (akkartik.name)
105 points by panic on Oct 18, 2019 | hide | past | favorite | 34 comments

> I'd like Mu to be a stack a single person can hold in their head all at once, and modify in radical ways.

This reminds me of Alan Kay's STEPS project - an attempt to reinvent personal computer in 20K lines of code [0].

Always happy to see efforts in this direction.

[0] http://www.vpri.org/pdf/tr2011004_steps11.pdf

This dudes stuff has been posted here recently and it's worth it go read what he's said about his work.


Thanks! I took the opportunity to try to clarify those questions in this version.

Interesting project.

So far the only desktop OS + compiler I know that can be reasonably understood in its entirety is http://www.projectoberon.com

Some interesting parallels to choices made in Pascal in the second level language, too.

I'm not convinced that this approach is the best - the first level language is a bit like "literal programming" machine code (not assembler) - but I think a level of simple substitution would help a bit (think m4 as an assembler) - allowing at least mnemonics. The insitence on numbers seem somewhat arbitrary as they still need to be translated from strings (letter sequences) to actual integers[e] - I don't think a table/variable look up would be too onerous.

On the whole this feels a bit more convoluted than building a Forth as level one - but no matter what I applaud the effort.

[e] assuming program source code is text, not binary?

If it's binary I suppose "filtering out" comments might be an approach as opposed to "translating" the source.

An interesting thought experiment that I've pondered is what I would do if I was given a system that I had to program without even an assembler.

I've programmed on systems where the only access was through direct memory access to the machines memory, and other systems that had a boot loading initiated by a button that started a paper tape reader. I've also used minicomputers that had to have instructions manually keyed in on binary panel switches directly into memory to start a card reader where the cards contained machine instructions to bootstrap the rest of the boot loader off of the remaining cards.

In every case though, somewhere I had an assembler to write the code I needed. What would I do if I didn't even have an assembler? I could write one, but I don't want to do it in machine code. Now days, the easy way would be to simply construct a cross-compiler or cross-assembler that was written in say python on a different machine.

One alternative is to build a hand assembled, really simple macro system. m4[0] or Calvin Mooers' TRAC[1] are possible prototypes, but these flexible macro systems would be difficult to hand compiler into machine code so one might get by with a really basic, more limited macro system. Then each machine instruction could be encoded as a macro and then a primitive assembler could be written as macros.

However, real assemblers do more work, they generally take two passes because they allow symbols to be used instead of literal addresses. So the next step would be to program a real assembler using the primitive assembly language.

After that I suppose a compiler for a simple language like Pascal, basic Lisp, or Per Brinch Hansen's Edison[2] language could be written in assembly language.

Per Brinch Hansen provides the basic directions for construction a very simple operating system from scratch, including the assembler and compiler in the book Programming a Personal Computer[3].

[0] https://www.gnu.org/software/m4/

[1] https://dl.acm.org/citation.cfm?id=365270

[2] http://brinch-hansen.net/papers/1981b.pdf

[3] https://www.amazon.com/Programming-personal-computer-Brinch-...

I seem to recall an old text file on programming with just debug.exe - I thought it was an early phrack magazine article - but I can't seem to find it now. The idea was similar to 5a here:


But the text I'm thinking of was more a how-to on bootstrapping development under win 3.11 if you had no outside resorces, and just a plain install with no other development tools.

This is a very acute critique, so it seems like the perfect place to elaborate on why I chose numbers.

We tend to think of Assembly programming as using mnemonics, but really the names of instructions in x86 are more than that: there are 14 different kinds of add instructions (https://c9x.me/x86/html/file_module_x86_id_5.html) If a single name can map to this many distinct cases, it's not a mnemonic anymore, it's a little more than that.

When a single word can do so many different things, and when the underlying instruction set has gaps, it becomes hard to create good error messages. Oh you added two immediate values together. You can't do that. But what can you do? Any impedance mismatch between external notation and target format quickly adds to the amount of code needed to generate good, clear error messages. Since my goal was to build SubX in itself, it seemed like a good trade-off to force people to live closer to the target language. That makes the error messages easier to write. There's really only one case to address at any point.

(The alternative would be to give each opcode a distinct mnemonic. Does that seem useful? I didn't initially think it was, but I'm open to trying it.)

Maybe I skimmed, but I didn't note anything on this (anything this clear at any rate) in the text.

It certainly is a justification for exposing the different op codes.

I still think symbolic names for symbolic numbers make sense,though. And I'm not sure a table in one place looses out on clarity.

I do think it's good to re-think assembly notation (que gas/masm/nasm/x86/6800 notation wars).

I guess I feel at the level of level 1 - there is some symbolic manipulation and meta programming already (eg loops, labels for computed jumps) - and that "dumb" replacement might even turn out to me more clear.

Comments go away (replace with nothing), jump-to-labels become jump-to-offset, opcode mnemonic become binary numbers, numbers become binary numbers etc.

I confess I wonder at the choice of 32bit as I find amd64 to be both clearer, more sane (better? Calling convention) - and it should be more powerful (eg more registers) - faster and allow addressing more memory (if/when that becomes and issue - say mapping an entiere 6tb drive into contiguous memory?).

I went back and added a couple of paragraphs about the opcodes in OP (section 'instructions'). Thanks for the feedback!

You're absolutely right, this wasn't in OP. It seemed like a minor detail at the time.

What should the mnemonics be called? SubX is already quite verbose. Would you be willing to put up with long names like `add_immediate`?

32- vs 64-bit is certainly a subjective choice. I outlined some reasons for my choice a year ago: https://lobste.rs/s/jkmwer/what_are_you_working_on_this_week.... But 64-bit is certainly a valid processor to target.

Thanks for taking the time to reply. Is there really a 32 bit mode without segments ? I missed that last time I looked at assambly - I thought it was the other way around - you didn't escape them until you went 64bit.

As for needing needing 32bit in 64bit - I guess it would depend a bit on if you could settle for a "all 64bit all the time" subset or not (ie: no 32bit integers)?

Now, for mnemonics - I certainly don't know. I suppose I see why they're generally implied in most assemblers...

I think add_i is still a lot more readable than 0x83 or whatever. Is there a system to the numbers? Could one "mask" immediate etc (eg ADD|I MEDIATE, MUL|IMMEDIATE or some such? Where the pipe implies xor or something (forgive me, I haven't touched shifts etc in years)).

I generally prefer descriptive names to terse names - but for an assembler I don't think it fits to go overboard - not unless you expect to end up with a Forth-like quick ascent to abstraction (like building a macro/substitution that abstract away the add_immediate back to just add).

I guess I'm starting to see why primitive text macros a la m4 made its way into no just C (the C preprocessor), but also in various forms in various assemblers...

Anyway, as your system needs to to some parsing and expansion (eg: strings to binary numbers, loops, label computation) - would not exposing that as a macro/meta facility make sense?

Maybe it's more of a level 2 thing in your design?

Yeah, a simple token-based search/replace is certainly realistic to implement in level 1. The questions for me are:

a) Does it really make the code easier for newcomers to read? On the one hand the names are familiar. On the other hand it may make cross-referencing with the Intel manual harder. And you do still end up needing to do that to a certain extent. My online help refers to the standard names in parens: https://github.com/akkartik/mu/blob/3a2d36a93680f2e9eb2bd2a3...

b) Is it useful in enough places to be worth the lines of code it takes to implement?

In any case, this seems worth exploring. Perhaps we should continue on the project site? I've opened a ticket: https://github.com/akkartik/mu/issues/39. Perhaps the first step is to just hand-translate some function (say https://github.com/akkartik/mu/blob/master/apps/factorial.su...) and see how it looks.


One final word on 32-bit vs 64-bit. I didn't realize when I wrote the comment from a year ago how easy it is to run 32-bit code on 64-bit machines. The reverse is obviously impossible.

I'd add TempleOS and its HolyC to that list.

TempleOS may be understandable, but trying to actually make and do anything with it's a completely different story.

Author here; I just found this thread. Feel free to ask me anything.

What's old is new again.

Your thoughts on the benefits of language slightly higher level than assembler are interesting. Others have trod this path before.

Bliss32 was the systems language for vax/vms back in the 1980s through 2000s. Most of vms was written in it. It was heavily influenced by PL/360 from the days when IBM 360 or 370 mainframes were mostly programmed in assembler.

Both languages emitted very predictable assembler from their higher level constructs. They had no optimizations. But their objectives seemed to be focused on readability and predictable output, like your system.

The source code for 99% of vms was published on microfiche and distributed with the binary tapes. Worth finding a copy and the accompanying text on vms internals and data structures. An excellent intro to nonTorvalds operating system architecture.

Fascinating. Glad to have this on my radar.

Would you recommend this for someone who hasn't yet officially learned operating systems/compilers, but is looking to break into the realm?

I hope so! It's new so you may find breakage, but I'll try to be very responsive answering questions.

Mu doesn't have its own OS at the moment. It runs on Linux and on Soso (https://github.com/ozkl/soso). I don't know either well, but hopefully I'll at least be a good study partner. And you can definitely ask me questions about the language side and the interface with the OS.

Mu doesn't have every feature of modern compilers and OSs, but it should hopefully give you a view of something simple working from end to end that you can then carry with you when making sense of more complex systems.

"Mu doesn't have every feature of modern compilers and OSs, but it should hopefully give you a view of something simple working from end to end that you can then carry with you when making sense of more complex systems."

Awesome, that's what I was hoping for: something that will lower the entry bar but still provide useful education for the greater world.

I know you would like to make Mu open source, which is to say, free software. What are the problems standing in your way, and how can we help to solve them?

This reminds me very much of 1970s Unix. Unix was also an research exercise into OS and userland minimalism and a new, streamlined language (yes, C) that created a new niche in the "like asm but better" space.

In other words, it had nothing in common with today's Linux. (-:

Indeed, much of Mu's design was motivated by the question, "if we imitated the codesign of OS and language that led to Unix and C, given what we know now, what would we create?" This question doesn't seem to be often asked in recent decades, with languages and OSs evolving separately.

akkartik, I see the following 3 syntaxes for 'MOV ESP, EBP':

    89/<-                       ebp  4/r32/esp  # copy esp to ebp    
    89/<-                5/rm32/ebp  4/r32/esp  # copy esp to ebp    
    89/<-  3/mod/direct  5/rm32/ebp  4/r32/esp  # copy esp to ebp
Only the last one contains all information needed to encode the instruction. And the first one is used in the factorial example. Can you explain why all the 3 syntaxes are in use?

Yeah, the last one is the 'ground truth' that both translators understand. sigils.subx adds some syntax sugar so you can say

    89/<- %ebp 4/r32/esp
By my self-imposed rules I can't use this sugar until I build it, so there's little code that uses it at the moment. But any future Subx code will use it, such as the implementation of Level 2. You can also see it used in calls.subx, which is another later layer of syntax sugar.

Could you point me at where you see your first two versions?

In http://akkartik.name/post/mu-2019-1 if you search for the pattern '/89' you'll find the first two versions.

The 3rd one is here: https://github.com/akkartik/mu/blob/master/apps/factorial.su...

Can you explain in a sentence what rules are used to complete the encoding for the variant:

> 89/<- %ebp 4/r32/esp

Ah, I see. Yes, I mentioned in the post that I was dropping some magic numbers. Mu is intended to be read in a text editor, and I didn't want to scare people too much while reading the post in a browser. Slightly evil, I know.

The concrete rules are illustrated at https://github.com/akkartik/mu/blob/469a1a9ace9b290658beb9d0.... In this case `%<reg>` always translates to `3/mod <reg-code>/rm32`.

Note to self (as the author of the "Mu" editor, https://codewith.mu/) - two letter project names are just asking for name collisions. ;-)

And also https://www.djcbsoftware.nl/code/mu/mu4e.html the mail client for emacs :)

I find this really inspiring! I've got a similar hobby project (that I've been neglecting recently...) but with some different design choices. I don't want to hijack the thread but I thought it might be interesting to compare and contrast.

Instead of targeting x86 it uses Prof. Wirth's RISC CPU for Project Oberon. This is a really simple chip, there is a model function for it written in Oberon that is less than a page of code. It's also very well documented, and emulators exist in Java, C, Python, and Javascript, and there is a VHDL (or is it Verilog?) description which people have used to make FPGA-based workstations.

For the basis notation I'm using a dialect of Joy, an extremely simple and elegant purely functional, stack-based, concatinative language. It's like a combination of the best parts of Forth and Lisp. https://en.wikipedia.org/wiki/Joy_(programming_language)

(I'm pretty sure there's no simpler useful language than Joy.)

To bridge Joy to the CPU I took the high road: a compiler written in Prolog. It would be possible to use Forth lore to bootstrap from the metal up, but I was gobsmacked by David Warren's "Logic Programming and Compiler Writing" ( https://news.ycombinator.com/item?id=17674859 ) and have spent the last year "moulting" from Python to Prolog programmer.

Anyhow, the compiler so far is concise but the model it implements for Joy on the CPU is still very crude. Between life, work, and learning more Prolog I haven't had much time to improve it.

The thing is, Prolog is a very simple language, the compiler is very simple and small, and Joy is also very very simple and small (especially if it's implemented in Prolog), and the underlying CPU is very simple and small. The tricky bit would be to, say, implement a Prolog interpreter in Joy. But if you do that you've closed the loop for self-hosting: Prolog-in-Joy, running on Joy-on-the-metal, implementing a compiler for Joy to the metal, bootstrapped through Prolog on the host.

(It would be simple to convert the Joy compiler in Prolog to a Joy compiler in Joy using the Prolog interpreter in Joy and partial evaluation. FWIW.)

For the OS I'm going to build a simple clone of OberonOS but using Joy rather than Oberon language. I made a model of this OS in Python and Tkinter to guide the reimplementation in Joy. This will also serve as a stress test for Joy: can you implement a whole (albeit simple) "OS" in a purely functional, stack-based, concatinative language? Maybe it sucks?

To sum up, a simple text-based UI/OS inspired by and loosely imitating OberonOS, targeting a simple but capable 32-bit RISC chip, with Joy as the shell and glue language and Prolog as the under-the-hood powerhouse systems language. And the whole thing should fit in under a hundred pages of code.

This sounds really interesting. I'm very happy you shared it. I'd love to see it when you publish it. Based on your description it seems like a lot of code, so I'm curious to see how tight we can keep the dependency chain.

Cheers! The project is here: http://joypy.osdn.io/ FWIW.

The Prolog code is under the ./thun subdir. I apologize for the state of the repo & docs. I need to go through and update everything.

If you look at the Python code the vast majority of it is the type inference module and the library of functions and combinators. The parser, main loop, and most of the library are trivial. Most of the joy.utils.types module is a crude re-implementation of a subset of Prolog and can be discarded. The Prolog version of Joy is even shorter than the Python version.

The Joy compiler (which is only about half done) is really tight and simple (Joy doesn't use variable so no need for a symbol table; Joy doesn't use subroutines so no need for a call stack) and the last half of the functionality won't require double the code.

Meta-programming in both Joy and Prolog is really easy and powerful, and Joy in particular lends itself to a kind of Huffman encoding of your graph (Joy code forms a simple DAG.)

- - - -

I want to repeat, Mu is really inspiring. I'm looking forward to watching your progress. :-)

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