gzip on PDP-11: not so simple

Robin Birch robin at falstaf.demon.co.uk
Thu Mar 26 07:56:05 AEST 1998


In message <199803252033.HAA03043 at henry.cs.adfa.oz.au>, Warren Toomey
<wkt at henry.cs.adfa.oz.au> writes
>In article by Allison J Parent:
>> I find this situation funny as in the 8080/z80 (8 bit data 64kbyte address 
>> space) world there is LZH, Crunch, ARK, ARC, LBR... compressors and 
>> decompressors.  Atleast a handful are written in C.
>> 
>> Also PDP11 address space (no I&D) is 32kW... Instructions are always 
>> words so code can eat up a fair portion of the 64k bytes.
>
>Well, I've got uncompress working, but I thought having gunzip would
>be good as it gives better compression results.
>
>       Warren
I looked at this several years ago and gave up at the save point as
Warren.  I looked at compress using 16 bits and hit the same sort of
constructs.  After a bit of thinking I believe there may be a way round
it but at the time I didn't know the algorithms used in compress or gzip
so didn't try playing.

The problem is that the compression algorithm needs a 64k space to do
all of its sums in, don't ask me why, if someone could tell us the
algorithm them I would understand a lot better.

These are defined as 64k address spaces which the data page isn't
holding cos they don't fit.  If you write a virtual mem system then this
will work.  This causes problems in the standalone world obviously but
steve wrote a vm lookalike for 2.11 that uses files, yes a lump of real
mem aka the partition concept with movable windows in RSX would be nice
but we can't have everything, but compress and maybe gip should be able
to be cooked into using such a system for vm.  This would be slow but
what are we after?, an all singing all dancing system or something that
would work in the background whilst we get a beer and wait for the
system to install?.

Cheers

Robin
Robin Birch     robin at falstaf.demon.co.uk

M1ASU/2E0ARJ    Old computers and radios always welcome

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