I wanted to understand how the amazing JsLinux worked.

However the original was passed through a minifier and was completely incomprehensible in that form. (Mr Bellard's standards for the code that he open sources is very high.) I couldn't wait for the proper release of the opus, so in a fit of mania 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, with permission of Mr. Bellard.

Note that there is a much more readable, ground-up project to build an open-source 386-style emulator in javascript called jslm32.

The current codebase won't run on recent webkit browsers due to a breaking change in the way Synchronous XHR requests are handled. The binary loading routines need to be rewritten to be asynchronous, not terribly hard but annoying enough that I haven't just done it. (Fabrice's original online version has been patched and runs fine.)

jslinux-deobfuscated is 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.

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.

This is a pedagogical/aesthetic derivative of the original JSLinux code Copyright (c) 2011-2013 Fabrice Bellard. It is posted here with permission of the original author subject to his original constraints : Redistribution or commercial use is prohibited without the (original) author's permission.

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

• 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

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

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