## What is this? 

A suite of tools for directly programming in (32-bit x86) machine code without
a compiler. The generated ELF binaries require just a Unix-like kernel to run.
(It isn't self-hosted yet, so generating the binaries requires a C compiler
and libc.)

## Why in the world?

1. It seems wrong-headed that our computers look polished but are plagued by
   foundational problems of security and reliability. I'd like to learn to
   walk before I try to run. The plan: start out using the computer only to
   check my program for errors rather than to hide low-level details. Force
   myself to think about security by living with raw machine code for a while.
   Reintroduce high level languages (HLLs) only after confidence is regained
   in the foundations (and when the foundations are ergonomic enough to
   support developing a compiler in them). Delegate only when I can verify
   with confidence.

2. The software in our computers has grown incomprehensible. Nobody
   understands it all, not even experts. Even simple programs written by a
   single author require lots of time for others to comprehend. Compilers are
   a prime example, growing so complex that programmers have to choose to
   either program them or use them. I think they may also contribute to the
   incomprehensibility of the stack above them. I'd like to explore how much
   of a HLL I can build without a monolithic optimizing compiler, and see if
   deconstructing the work of the compiler can make the stack as a whole more
   comprehensible to others.

3. I want to learn about the internals of the infrastructure we all rely on in
   our lives.

## Running

```
$ git clone https://github.com/akkartik/mu
$ cd mu/subx
$ ./subx
```

Running `subx` will transparently compile it as necessary.

## Usage

`subx` currently has the following sub-commands:

* `subx test`: runs all automated tests.

* `subx translate <input file> <output ELF binary>`: translates a text file
  containing hex bytes and macros into an executable ELF binary.

* `subx run <ELF binary>`: simulates running the ELF binaries emitted by `subx
  translate`. Useful for debugging, and also enables more thorough testing of
  `translate`.

Putting them together, build and run one of the example programs:

<img alt='ex1.1.subx' src='html/ex1.png'>

```
$ ./subx translate ex1.1.subx ex1
$ ./subx run ex1
```

If you're running on Linux, `ex1` will also be runnable directly:
```
$ chmod +x ex1
$ ./ex1
```

I'm not building general infrastructure here for all of the x86 ISA and ELF
format. SubX is about programming with a small, regular subset of 32-bit x86:

* Only instructions that operate on the 32-bit E\*X registers. (No
  floating-point yet.)
* Only instructions that assume a flat address space; no instructions that use
  segment registers.
* No instructions that check the carry or parity flags; arithmetic operations
  always operate on signed integers (while bitwise operations always operate
  on unsigned integers)
* Only relative jump instructions (with 8-bit or 16-bit offsets).

## Resources

* [Single-page cheatsheet for the x86 ISA](https://net.cs.uni-bonn.de/fileadmin/user_upload/plohmann/x86_opcode_structure_and_instruction_overview.pdf)
  (pdf; [cached local copy](https://github.com/akkartik/mu/blob/master/subx/cheatsheet.pdf))
* [Concise reference for the x86 ISA](https://c9x.me/x86)
* [Intel programming manual](http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf) (pdf)

## Inspirations

* [&ldquo;Creating tiny ELF executables&rdquo;](https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html)
* [&ldquo;Bootstrapping a compiler from nothing&rdquo;](http://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html)
* Forth implementations like [StoneKnifeForth](https://github.com/kragen/stoneknifeforth)