Re: [Etherboot-developers] upx compressor/decompressor...

[Eric W B. <ebi...@ln...>]

Markus Gutschke <ma...@gu...> writes:

> Eric W Biederman wrote:
> > In it's final form the overhead for the nrv2b decompressor is just
> > 180 bytes and should handle any size of file.  The 32bit infrastructure
> > is now an advantage instead of a liability size wise.
> 
> Sweet! That's very impressive. Now, if only there was some documentation on
> these algorithms. I've been browsing the project pages for UPX, but they are
> extremely sparse on documentation.

The simplied version is now checked in to etherboot cvs.

Conceptually it is based on copying bytes.  But instead of using
a hufman tree to encode the bits, it uses an encoding based upon stop
bits.

Check out the decompressor in nrv2b.c it is only about a screenful so
it isn't to hard to comprehend.  Watch out for the GETBIT code though
how it uses stop bits instead of a count to remember how far to go is
interesting.

Then look at the unnrv2b.S the optimized assembly version, and notice
how a lot of the awkward cases in C, become straight easy to
implement in the assembly.

Eric