# 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 ``` screenshot of Mu running on Qemu 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 ``` Finally, there's a whole mini-universe inside the `baremetal/` sub-directory, Mu programs that use no Linux services, and can control the screen and keyboard directly without an OS. You can make things like this with them: screenshot of a Mu program running without any intervening Operating System ## 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: ex2.mu To reproduce it: ```sh $ ./translate_mu_baremetal baremetal/ex2.mu # emit disk.img $ qemu-system-i386 disk.img ``` [More details on Mu syntax →](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 →](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: - [“Bootstrapping a compiler from nothing”](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). - [“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/master/cheatsheet.pdf)) - [Minimal Linux Live](http://minimal.linux-bg.org) for teaching how to create a bootable disk image. - [“Writing a simple operating system from scratch”](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).