Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!cornell!rochester!ur-tut!ur-valhalla!micropen!dave
From: dave@micropen (David F. Carlson)
Newsgroups: comp.unix.xenix
Subject: Re: 16-bit versus 32-bit memory performance
Message-ID: <398@micropen>
Date: 15 Dec 87 19:30:24 GMT
References: <388@ddsw1.UUCP> <620@omen.UUCP> <435@spdcc.COM> <109@slxsys.specialix.co.uk>
Organization: Micropen Direct Writing Systems, Pittsford, NY
Lines: 49
Summary: memory speed isn't the only thing

In article <109@slxsys.specialix.co.uk>, jpp@slxsys.specialix.co.uk (John Pettitt) writes:
> This should perhaps belong in comp.arch
> 
... other stuff deleted -- dfc
> 16 Bit memory subsystems have a devestating effect on the 80386 
> for 2 reasons.  Firstly 2 memory accesses are required rather than
> one thus doubling the access time.  Secondly most 16 bit memory cards
> are designed for 8 or 10 Mhz operation not 16 Mhz so a significant
> number of wait states are needed when used with a 386.   It would
> appear that a 'cache miss' on the Intel Inboard(tm) generates beteween
> 10 and 12 wait states thus making access to 16 bit ram slower than
> from the original 286.
> 

First, *the* standard of the AT class machines (which most of the non-ibm
386 boxes are) is the 8MHz 16 bit bus.  Thus, your 16MHz 386 runs all IO and
16bit memory operations at 8MHz.  Wait states a-plenty.  10MHz AT buses
have been tried but *many* xt class cards won't work with that rate, so
all AT-class buses on 386 boxes (that I know of) are strictly 8MHz.

> In conclustion - if you want a 32 bit CPU use 32 bit ram.  If you
> just want the instruction set use the P9 (80388) - if it ever appears.

The reason that I use 386 machines is that the 386 is a real architecture.
(Read 32 bit linear address space, virtual memory support, etc.) and the
286 machines that we did use had a fatally flawed architecture for supporting
multi-user systems (UNIX in particular.)  I use 4 Meg of static "32-bit"
ram and 3 meg of 16 bit ram.  Reason:  Most kernel activities and most
user processes are in fast ram.  Disk buffers (read slow io bound) are kept
in 16 bit memory.  It will always be faster to have extra ram pages in
a DP VM environment than to go with less ram and be forced to go into page
thrash.  That 28msec disk system is *much* slower that 16 bit memory any
way you slice it.  So, get enough *affordable* ram in any combination
that will keep your system from thrashing.  Intel Inboard had 1meg of fast
ram and that simply isn't enough to prevent thrash.  Solution is to muddle
through with 16 bit ram and like it!

Personally, I can't wait for the vaporware 80388 just to get the architecture
of a real computer.  And I can't afford 6 Meg of static column ram.  Besides,
8 MHz AT bus speed is sufficient for the tasks required of my system.  And 
certainly beats paging thrash!

> John Pettitt - 144.5 MHz: G6KCQ, CIX: jpettitt,  Voice: +44 1 398 9422

-- 
David F. Carlson, Micropen, Inc.
...!{ames|harvard|rutgers|topaz|...}!rochester!ur-valhalla!micropen!dave

"The faster I go, the behinder I get." --Lewis Carroll