path: root/README.md

# Mu: a human-scale computer

Mu is a minimal-dependency hobbyist computing stack (everything above the
processor and OS kernel).

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.

```sh
$ git clone https://github.com/akkartik/mu
$ cd mu
$ ./translate_mu apps/ex2.mu  # emit a.elf
$ ./a.elf  # add 3 and 4
$ echo $?
7
```

[![Build Status](https://api.travis-ci.org/akkartik/mu.svg?branch=master)](https://travis-ci.org/akkartik/mu)

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))

Currently Mu requires a 32-bit x86 processor. Generated programs require just
a Linux kernel and nothing else.

## 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. If you can manually test for something you should be
    able to write an automated test for it.
  - Memory leaks over memory corruption.
- Teach the computer bottom-up.

## 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).
  For now it's a thin veneer over machine code. I'm working on memory safety
  before expressive syntax.

## Toolchain

The Mu stack consists of:
- the Mu type-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
ELF binaries that run natively on Linux. 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's an emulator for running Mu binaries (more slowly) on other Unix-like
systems.

```sh
$ ./translate_mu_emulated apps/ex2.mu  # emit a.elf using the emulator
$ ./bootstrap run ./a.elf  # run in the emulator
$ echo $?
```

The emulator is also useful for [debugging](subx_debugging.md).

### incomplete tools

There's a prototype Mu shell, a postfix language with a dynamically updating
environment. It might turn into the initial experience when a Mu computer
boots.

Once generated, ELF binaries can be packaged up with a Linux kernel into a
bootable disk image. Here's how the Mu shell might look on startup:

```sh
$ ./translate_mu apps/tile/*.mu  # emit a.elf
# dependencies
$ sudo apt install build-essential flex bison wget libelf-dev libssl-dev xorriso
$ tools/iso/linux a.elf
$ qemu-system-x86_64 -m 256M -cdrom mu_linux.iso -boot d
```

<img alt='screenshot of Mu running on Qemu' src='html/20201114-qemu.png'>

The disk image also runs on [any cloud server that supports custom images](http://akkartik.name/post/iso-on-linode).

Mu also runs on the minimal hobbyist OS [Soso](https://github.com/ozkl/soso).
(Requires graphics and sudo access. Currently doesn't work on a cloud server.)

```sh
$ ./translate_mu apps/ex2.mu  # emit a.elf
# dependencies
$ sudo apt install build-essential util-linux nasm xorriso  # maybe also dosfstools and mtools
$ tools/iso/soso a.elf  # requires sudo
$ qemu-system-i386 -cdrom mu_soso.iso
```

## 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
say `a + b*c`, you have to break it up into two operations. Variables can live
in memory or in registers. Registers must be explicitly specified. There are
some shared lexical rules; comments always start with '#', and numbers are
always written in hex.

Here's an example program in Mu:

<img alt='ex2.mu' src='html/ex2.mu.png' width='400px'>

[More details on Mu syntax &rarr;](mu.md)

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 details on SubX syntax &rarr;](subx.md)

## Forks

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 your changes into this repo!

- [mu-normie](https://git.sr.ht/~akkartik/mu-normie): with a more standard
  build system that organizes the repo by header files and compilation units.
  Stays in sync with this repo.
- [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.
- [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.

## Desiderata

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

- The references on [Mu](mu.md) and [SubX](subx.md) syntax, and also [bare
  SubX](subx_bare.md) without any syntax sugar.

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

- [Some tips for debugging SubX programs.](subx_debugging.md)

- [Shared vocabulary of data types and functions shared by Mu programs.](vocabulary.md)
  Mu programs can transparently call low-level functions written in SubX.

- [A summary](mu_instructions) of how the Mu compiler translates instructions
  to SubX. ([colorized version](http://akkartik.github.io/mu/html/mu_instructions.html))

- [Some starter exercises for learning SubX](https://github.com/akkartik/mu/pulls)
  (labelled `hello`). Feel free to [ping me](mailto:ak@akkartik.com) with any questions.

- [Commandline reference for the bootstrap C++ program.](bootstrap.md)

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

- [Some details on the unconventional organization of this project.](http://akkartik.name/post/four-repos)

- 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:
- [&ldquo;Bootstrapping a compiler from nothing&rdquo;](http://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html) by Edmund Grumley-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).
- [&ldquo;Creating tiny ELF executables&rdquo;](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/master/cheatsheet.pdf))
- [Minimal Linux Live](http://minimal.linux-bg.org) for teaching how to create
  a bootable disk image.
- [&ldquo;Writing a simple operating system from scratch&rdquo;](https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf)
  by Nick Blundell. An incomplete draft more helpful to me than all the tomes
  of the internet on the subject.
- 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).