Re: [Etherboot-developers] Seperating out the x86 BIOS calls...

[Markus G. <ma...@gu...>]

Ken Yap wrote:
> There are some limits of the de/compression code, Markus can explain
> those.

The decompressor is based on the code in contrib/compressor/lzhuf.c, but has 
been aggressively optimized for size. If you want to make any changes to the 
assembly code, you should probably first read the C source and try to understand it.

The decompressor currently runs in real mode only. As such, it uses segment 
registers and cannot extract more than at most 64kB (there probably is a limit 
even before 64kB is reached, but I don't remember all the details). As we know 
that we are never going to deal with more than 64kB of data, we can also tell 
that the pattern frequency can never get bigger than 2^16 (in fact, it'll stay 
considerably smaller than that). Thus, I stripped out the reconst() function 
which would never get called. If you want to deal with larger images, you 
probably have to implement it, though.

Also, I managed to shrink the code size, by compressing all the preinitialized 
tables (using some kind of run length encoding); and I stripped off a few more 
bytes by ordering these tables in a way so that once I have initialized one 
table my registers already have the right values that I will need for 
initializing the next one.

Due to the small register file, there always is considerable register pressure 
when writing code for the x86. I managed to keep almost all data in registers, 
but this required me to split some of the registers into 4bit nybbles. You might 
need to keep this in mind if you have to make changes to the code.


Markus

-- 
Markus Gutschke
3637 Fillmore Street #106
San Francisco, CA 94123-1600
+1-415-567-8449
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