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https://github.com/akkartik/mu

Soul of a tiny new machine. More thorough tests → More comprehensible and rewrite-friendly software → More resilient society.
https://github.com/akkartik/mu

linux machine-code programming-literacy unit-tested white-box-testing x86

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Soul of a tiny new machine. More thorough tests → More comprehensible and rewrite-friendly software → More resilient society.

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# Mu: a human-scale computer



Mu is a minimal-dependency hobbyist computing stack (everything above the

processor).



Mu is not designed to operate in large clusters providing services for

millions of people. Mu is designed for _you_, to run one computer. (Or a few.)

Running the code you want to run, and nothing else.



Here's the Mu computer running [Conway's Game of Life](https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life).



```sh

git clone https://github.com/akkartik/mu

cd mu

./translate apps/life.mu  # emit a bootable code.img

qemu-system-i386 code.img

```



![screenshot of Game of Life running on the Mu computer](html/life.png)



([Colorized sources.](http://akkartik.github.io/mu/html/apps/life.mu.html)

This is memory-safe code, and most statements map to a single instruction of

machine code.)



Rather than start from some syntax and introduce layers of translation to

implement it, Mu starts from the processor's instruction set and tries to get

to _some_ safe and clear syntax with as few layers of translation as possible.

The emphasis is on internal consistency at any point in time rather than

compatibility with the past. ([More details.](http://akkartik.name/akkartik-convivial-20200607.pdf))



Tests are a key mechanism here for creating a computer that others can make

their own. I want to encourage a style of active and interactive reading with

Mu. If something doesn't make sense, try changing it and see what tests break.

Any breaking change should cause a failure in some well-named test somewhere.



Mu requires a 32-bit x86 processor. It supports a short list of generic

hardware. There's no networking support yet. Development has slowed, but I

still care about it. Feedback, bug reports and other forms of contribution

continue to be appreciated.



_Mu in the press_



* [Qemu Advent Calendar 2023](http://qemu-advent-calendar.org/2023/) (day 4)

* [Advent of v86](https://copy.sh/v86/advent/2023/?day=4)



## Goals



In priority order:



- [Reward curiosity.](http://akkartik.name/about)

  - Easy to build, easy to run. [Minimal dependencies](https://news.ycombinator.com/item?id=16882140#16882555),

    so that installation is always painless.

  - All design decisions comprehensible to a single individual. (On demand.)

  - All design decisions comprehensible without needing to talk to anyone.

    (I always love talking to you, but I try hard to make myself redundant.)

  - [A globally comprehensible _codebase_ rather than locally clean code.](http://akkartik.name/post/readable-bad)

  - Clear error messages over expressive syntax.

- Safe.

  - Thorough test coverage. If you break something you should immediately see

    an error message.

  - Memory leaks over memory corruption.

- Teach the computer bottom-up.



Thorough test coverage in particular deserves some elaboration. It implies

that any manual test should be easy to turn into a reproducible automated

test. Mu has some unconventional methods for providing this guarantee. It

exposes testable interfaces for hardware using dependency injection so that

tests can run on -- and make assertions against -- fake hardware. It also

performs [automated white-box testing](http://akkartik.name/post/tracing-tests)

which enables robust tests for performance, concurrency, fault-tolerance, etc.



## Non-goals



- Speed. Staying close to machine code should naturally keep Mu fast enough.

- Efficiency. Controlling the number of abstractions should naturally keep Mu

  using far less than the gigabytes of memory modern computers have.

- Portability. Mu will run on any computer as long as it's x86. I will

  enthusiastically contribute to support for other processors -- in separate

  forks. Readers shouldn't have to think about processors they don't have.

- Compatibility. The goal is to get off mainstream stacks, not to perpetuate

  them. Sometimes the right long-term solution is to [bump the major version number](http://akkartik.name/post/versioning).

- Syntax. Mu code is meant to be comprehended by [running, not just reading](http://akkartik.name/post/comprehension).

  It will always be just a thin memory-safe veneer over machine code.

  I don't know how to make higher-level notations both fast and

  comprehensible, so they are likely to remain slow and comprehensible, useful

  for prototyping but invariably needing to be rewritten in statements that

  map 1:1 with machine code. The goal of a prototype should be a risk-free

  rewrite, thanks to tests that capture all the details of lessons learned.



## Toolchain



The Mu stack consists of:

- the Mu type-safe and memory-safe language;

- SubX, an unsafe notation for a subset of x86 machine code; and

- _bare_ SubX, a more rudimentary form of SubX without certain syntax sugar.



All Mu programs get translated through these layers into tiny zero-dependency

binaries that run natively. The translators for most levels are built out of

lower levels. The translator from Mu to SubX is written in SubX, and the

translator from SubX to bare SubX is built in bare SubX. There is also an

emulator for Mu's supported subset of x86, that's useful for [debugging SubX

programs](linux/subx_debugging.md).



Mu programs build natively either on Linux or on Windows using [WSL 2](https://docs.microsoft.com/en-us/windows/wsl/install-win10).

For Macs and other Unix-like systems, use the (much slower) emulator:



```sh

./translate_emulated apps/ex2.mu  # 2-5 minutes to emit code.img

```



Mu programs can be written for two very different environments:



* At the top-level, Mu programs emit a bootable image that runs without an OS

  (under emulation; I haven't tested on native hardware yet). There's rudimentary

  support for some core peripherals: a 1024x768 screen, a keyboard with some

  key-combinations, a PS/2 mouse that must be polled, a slow ATA disk drive.

  No hardware acceleration, no virtual memory, no process separation, no

  multi-tasking, no network. Boot always runs all tests, and only gets to

  `main` if all tests pass.



* The top-level is built using tools created under the `linux/` sub-directory.

  This sub-directory contains an entirely separate set of libraries intended

  for building programs that run with just a Linux kernel, reading from stdin

  and writing to stdout. The Mu compiler is such a program, at `linux/mu.subx`.

  Individual programs typically run tests if given a command-line argument

  called `test`.



The largest program built in Mu today is its prototyping environment for

writing slow, interpreted programs in a Lisp-based high-level language.



![screenshot of the Mu shell](html/20210624-shell.png)



(For more details, see [the `shell/` directory.](https://github.com/akkartik/mu/tree/main/shell#readme))



While I currently focus on programs without an OS, the `linux/` sub-directory

is fairly ergonomic. There's a couple of dozen example programs to try out

there. It is likely to be the option for a network stack in the foreseeable

future; I have no idea how to interact on the network without Linux.



## Syntax



The entire stack shares certain properties and conventions. Programs consist

of functions and functions consist of statements, each performing a single

operation. Operands to statements are always variables or constants. You can't

perform `a + b*c` in a single statement; you have to break it up into two.

Variables can live in memory or in registers. Registers must be explicitly

specified. There are some shared lexical rules. Comments always start with

'#'. Numbers are always written in hex. Many terms can have context-dependent

_metadata_ attached after '/'.



Here's an example program in Mu:



ex2.mu



More resources on Mu:



* [Mu Syntax reference](mu.md)



* [Library reference.](vocabulary.md) Mu programs can transparently call

  low-level functions written in SubX.



Here's an example program in SubX:



```sh

== code

Entry:

  # ebx = 1

  bb/copy-to-ebx  1/imm32

  # increment ebx

  43/increment-ebx

  # exit(ebx)

  e8/call  syscall_exit/disp32

```



More resources on SubX:



* [SubX syntax reference](subx.md)



* [Some starter exercises for learning SubX](https://github.com/akkartik/mu/pulls)

  (labelled `hello`). Feel free to [ping me](mailto:[email protected]) with any

  questions.



* The [list of x86 opcodes](subx_opcodes) supported in SubX: `linux/bootstrap/bootstrap help opcodes`.



* [Some tips for debugging SubX programs.](linux/subx_debugging.md)



## Mirrors and Forks



Updates to Mu can be downloaded from the following mirrors:

* https://github.com/akkartik/mu

* https://repo.or.cz/mu.git

* https://codeberg.org/akkartik/mu

* https://tildegit.org/akkartik/mu

* https://git.tilde.institute/akkartik/mu

* https://git.sr.ht/~akkartik/mu



Forks of Mu are encouraged. If you don't like something about this repo, feel

free to make a fork. If you show it to me, I'll link to it here. I might even

pull features upstream!



- [uCISC](https://github.com/grokthis/ucisc): a 16-bit processor being

  designed from scratch by [Robert Butler](https://www.youtube.com/channel/UCh4OpfF7T7UtezGejRTLxCw)

  and programmed with a SubX-like syntax.

- [subv](https://git.s-ol.nu/subv): experimental SubX-like syntax by [s-ol

  bekic](https://mmm.s-ol.nu) for the RISC-V instruction set.

- [mu-x86\_64](https://git.sr.ht/~akkartik/mu-x86_64): experimental fork for

  64-bit x86 in collaboration with [Max Bernstein](https://bernsteinbear.com).

  It's brought up a few concrete open problems that I don't have good solutions

  for yet.

- [mu-normie](https://git.sr.ht/~akkartik/mu-normie): with a more standard

  build system for the `linux/bootstrap/` directory that organizes the repo by

  header files and compilation units. Stays in sync with this repo.



## Desiderata



If you're still reading, here are some more things to check out:



- [A slow guided tour of Mu.](tutorial/index.md)



- [How to get your text editor set up for Mu and SubX programs.](editor/editor.md)



- [Some videos demonstrating Mu programs and features.](https://www.youtube.com/watch?v=SZBAXgXvzqE&list=PLxyWgjDVNN4qtMcoR7JRcYPbrwMdnGNsS)



- [A summary](mu_instructions) of how the Mu compiler translates statements

  to SubX. Most Mu statements map to a single x86 instruction.

  ([colorized version](http://akkartik.github.io/mu/html/mu_instructions.html))



- A prototype live-updating programming environment for a postfix language

  that I might work on again one day:



  ```sh

  cd linux

  ./translate tile/*.mu

  ./a.elf screen

  ```



- Previous prototypes: [mu0](https://github.com/akkartik/mu0), [mu1](https://github.com/akkartik/mu1).



## Credits



Mu builds on many ideas that have come before, especially:



- [Peter Naur](http://akkartik.name/naur.pdf) for articulating the paramount

  problem of programming: communicating a codebase to others;

- [Christopher Alexander](http://www.amazon.com/Notes-Synthesis-Form-Harvard-Paperbacks/dp/0674627512)

  and [Richard Gabriel](https://www.dreamsongs.com/Files/PatternsOfSoftware.pdf) for

  the intellectual tools for reasoning about the higher order design of a

  codebase;

- [David Parnas](http://www.cs.umd.edu/class/spring2003/cmsc838p/Design/criteria.pdf)

  and others for highlighting the value of separating concerns and stepwise

  refinement;

- The folklore of debugging by print and the trace facility in many Lisp

  systems;

- Automated tests for showing the value of developing programs inside an

  elaborate harness;



On a more tactical level, this project has made progress in a series of bursts

as I discovered the following resources. In autobiographical order, with no

claims of completeness:

- [“Bootstrapping a compiler from nothing”](http://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html) by Edmund Grimley-Evans.

- [StoneKnifeForth](https://github.com/kragen/stoneknifeforth) by [Kragen Sitaker](http://canonical.org/~kragen),

  including [a tiny sketch of an ELF loader](https://github.com/kragen/stoneknifeforth/blob/master/386.c).

- [“Creating tiny ELF executables”](https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html) by Brian Raiter.

- [Single-page cheatsheet for the x86 ISA](https://net.cs.uni-bonn.de/fileadmin/user_upload/plohmann/x86_opcode_structure_and_instruction_overview.pdf)

  by Daniel Plohmann ([cached local copy](https://github.com/akkartik/mu/blob/main/cheatsheet.pdf))

- [Minimal Linux Live](http://minimal.linux-bg.org) for teaching how to create

  a bootable disk image using the syslinux bootloader.

- [“Writing a bootloader from scratch”](https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf)

  by Nick Blundell.

- Wikipedia on BIOS interfaces: [Int 10h](https://en.wikipedia.org/wiki/INT_10H), [Int 13h](https://en.wikipedia.org/wiki/INT_13H).

- [Some tips on programming bootloaders](https://stackoverflow.com/questions/43786251/int-13h-42h-doesnt-load-anything-in-bochs/43787939#43787939)

  by Michael Petch.

- [xv6, the port of Unix Version 6 to x86 processors](https://github.com/mit-pdos/xv6-public)

- Some tips on handling keyboard interrupts by [Alex Dzyoba](https://alex.dzyoba.com/blog/os-interrupts)

  and [Michael Petch](https://stackoverflow.com/questions/37618111/keyboard-irq-within-an-x86-kernel).