Path: utzoo!mnetor!uunet!husc6!bbn!uwmcsd1!ig!jade!ucbvax!decvax!tektronix!dadla!amadeus!jamesa
From: jamesa@amadeus.TEK.COM (James Akiyama)
Newsgroups: comp.sources.d
Subject: Re: help with compress on XENIX
Message-ID: <894@amadeus.TEK.COM>
Date: 23 Dec 87 20:53:21 GMT
References: <810@hyper.UUCP> <969@sask.UUCP>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 29
Keywords: compress,xenix,4.0,help
Summary: Compress Incompatibilities


I believe that part of the non-compatibility of compress on 80X86 machines
has to do with the Intel segmentation.  It turns out that VAX versions use
a 16 bit compression code which requires substantially more than 64K array
sizes.  Since most 80X86 systems are limited to a 64K array they generally
only support a 12-bit compression code.  I know of two fixes:

    1.	Compress  using the '-b' switch and limit the compression to 12-bits.
	This will actually speed the compression up some although the percent
	compression will be somewhat reduced.

    2.	Obtain a modified compress routine which is designed for a 64K
	segmented architecture.  This is generally done by splitting the
	internal arrays into several pieces --usually about 8; then using
	the upper three bits of the array subscript to determine which array
	to use.  Note that this have a severe speed penalty on the 80X86
	systems, generally decreasing the speed of the program to about half.
	If someone has problem locating this I can probably send you a copy
	over E-mail if you give me your full mail path.

As to someone mentioning a problem on the VAX systems running Ultrix, here at
Tektronix, we have not seen that problem on our systems.  It turns out that
the "#ifdef vax" statements use a Vax assembly instruction to generate the
odd code sizes (9-16 bits) needed by compress.  The Vax has a nifty instruction
which allows you to access the nth word where each word is x-bits (e.g. get the
15th 14-bit word from an array).  The normally compress code requires a lot
of shift left, shift rights and multiplies.  Hope this is of some help.

James E. Akiyama