De-obfuscated JSLinux

I wanted to understand how the amazing JsLinux worked.

I have no idea if he passed it through a minifier or if the code was generated algorithmically from stuff in the QEMU codebase. In any case, it's hard to follow the action as presented originally, let alone extend it to do new tricks.

I hand de-obfuscated the codebase (primarily the core cpu-emulation routines and a bit of the rest as well) while studying it over a few days' time.

In the off-chance someone else might be interested in this code as a basis for further weird in-browser x86 hacking I'm posting this redacted version of the code here.

There is a much more complete, ground-up project to build a 386-style emulator in javascript called jslm32.

Status

It's still a dense code base, it's an emulator of a rather complicated architecture, after all. However this version is nowhere near so incomprehensible as the original. Nearly all of the global variables and function names have been named somewhat sensibly. Many comments have been added.

The core opcode execution loop has been autocommented to indicate what instruction operation the opcode refers to.

One mystery is, why does CPUID(1) return 8 << 8 in EBX? EBX[15:8] is now used to indicate CLFLUSH line size, but that field must have been used for something else in the past.

ETC

I highly recommend, by the way, the excellent JSShaper library for transforming large javascript code bases. The hacks I made from it are in this repo: a little symbol-name-transformer node.js script and an emacs function for doing this in live buffers.

Caveat Coder

This is a pedagogical/aesthetic reinterpretation of the original JSLinux code Copyright (c) 2011 Fabrice Bellard. It seems to run identically to the original.

References

Some other helpful references for understanding what's going on:

x86

• http://pdos.csail.mit.edu/6.828/2005/readings/i386/
• http://pdos.csail.mit.edu/6.828/2010/readings/i386.pdf (PDF of above)
• http://ref.x86asm.net/coder32.html
• http://www.sandpile.org/
• http://en.wikibooks.org/wiki/X86_Assembly/X86_Architecture
• http://en.wikipedia.org/wiki/X86
• http://en.wikipedia.org/wiki/Control_register
• http://en.wikipedia.org/wiki/X86_assembly_language
• http://en.wikipedia.org/wiki/Translation_lookaside_buffer

Bit Hacking

• http://graphics.stanford.edu/~seander/bithacks.html

Other devices

• http://en.wikibooks.org/wiki/Serial_Programming/8250_UART_Programming