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linuxrc in i386 assembly

Introduction

I was originally disappointed by how large initrd was primarily because it used libc. I knew a way around this was to use assembly language. I realise a statically linked C program also does not require libc, but the job that initrd does is so simple that I assumed an assembly language version would not be taxing. Indeed I was right, so long as I didn't want compression and decompression of the rootfs file.

Basics

If we ignore the gunzipping part of linuxrc for now, the basic outline of linuxrc involves making a few mounts and unmounts, opening and reading a few files and copying some stuff. Pretty simple even in assembly. I made it easier by using the asmutils framework from www.linuxassembly.org. It didn't take too long to throw together a program that did it all, but just dd a 4mb copy of the rootfs directly off the CD. This I noticed was quite slow compared to using a gzipped copy, so I set about converting gunzip to assembly ... and soon gave up.

Then I looked for some simpler compression/ decompression algortihms and came across a thing called the 624. It was designed for the 4k intro 'scene'. I think the basic goal of it was to crunch a 6k assembly program down to about 4k and attach a very small decruncher to it. The author had the compressor part written in C and the decruncher part written in 386 asm. Perfect. I just mangled the code so that it wasn't compressing such small files and it wasn't decompressing as part of an executable. That all worked eventually, though the compressor didn't seem to compress as well as gzip. My 4mb rootfs went down to 1.1mb with gzip, but only 1.6mb with the 624 based compressor. Not a major problem really.

The source

The relevant source code is shown below. pcomp can be compiled as gcc -o pcomp pcomp.c and will compress whatever you pass to it as the 1st argument (might take a while). linuxrc.asm was assembled using nasm as part of asmutils (ie. it uses asmutil's include files).

May 06,2001