Xref: utzoo comp.unix.xenix:5227 comp.unix.questions:12067
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!batcomputer!itsgw!steinmetz!uunet!ispi!jbayer
From: jbayer@ispi.UUCP (Jonathan Bayer)
Newsgroups: comp.unix.xenix,comp.unix.questions
Subject: Re: Cache controllers, can Xenix use them?
Keywords: 386, cache
Message-ID: <482@ispi.UUCP>
Date: 6 Mar 89 15:00:42 GMT
References: <195@icc.UUCP>
Reply-To: jbayer@ispi.UUCP (Jonathan Bayer)
Organization: Intelligent Software Products, Inc.
Lines: 50

In article <195@icc.UUCP> wdm@icc.UUCP (Bill Mulert) writes:
>There are now a number of high performance 80386 motherboards in
>use in personal computers. Some of these machines have the Intel
>cache controller chip, and 32 to 64k of 30ns ram. Cacheing software
>for MsDos is available for those users. 
>
>My question is, is this cache controller usaeble by any of the
>Unix - Xenix kernels? Does'nt the kernel have to know about it
>in order to use it? Do any of Microport, SCO, Interactive
>support the chip? Would one be wasting ones money to buy a
>machine with a cache controller that would be running Xenix?


You are talking about two types of cache here.  The caching software is
mainly disk cache software, used to speed up access to the hard disk
(and sometimes the floppy).  Good disk caching software can make a slow
drive look much faster, and a fast drive look almost instantaneous.

The Intel cache controller and the cache ram (as well as other hardware
caches) are used as memory caches.  The software has no knowledge of it,
and usually has no need to know.  The only real exception is software
that is extremely time-dependent, such as copy-protection software.  Any
operating system will take advantage of the hardware cache.  These
caches work by relying on the fact that

		1.  Memory accessed once will usually be accessed again,
		    usually very soon.

		2.  As a general rule memory is usually accessed in a
		    fairly sequential order.  In other words, when the
		    first byte of an instruction is being loaded in to
		    the cpu the cache will be fetching the next byte in
		    memory in anticipation of the cpu.


Typical caches will have a hit rate of between 80 and 90 percent.  The
higher the hit rate the fewer wait states the cpu will have to endure,
and the faster the system throughput.

Not being a hardware guru I am sure that I have left out a few facts,
but the overall idea is complete.


JB

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