Path: utzoo!mnetor!uunet!mcvax!ukc!stc!datlog!slxsys!jpp
From: jpp@slxsys.specialix.co.uk (John Pettitt)
Newsgroups: comp.unix.xenix
Subject: Re: 16-bit versus 32-bit memory performance
Message-ID: <109@slxsys.specialix.co.uk>
Date: 10 Dec 87 14:17:13 GMT
References: <388@ddsw1.UUCP> <620@omen.UUCP> <435@spdcc.COM>
Reply-To: jpp@slxsys.UUCP (John Pettitt)
Organization: Specialix International, London, UK.
Lines: 29
Summary: 32 bit cpu on 16 bit ram is a waste of money

This should perhaps belong in comp.arch

It would appear that most 8088,8086,186 and 286 systems are
limited by the number of cycles taken to execute instructions
(I.E the clock speed).  However the 80386 (at 16 and esp at 20 Mhz)
is limited by its memory bus bandwidth.  That is the memory subsystem
on most 286 boxes is fast enough have little or no real effect on
performance compared to a change in clock speed.   An 80386
however is largly limited by the rate that it can be 'fed' data
and instructions.   

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.

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.

(This posting written on a Dell 386 with 6 MB of 0 wait static 32 bit ram)

-- 
John Pettitt - 144.5 MHz: G6KCQ, CIX: jpettitt,  Voice: +44 1 398 9422
UUCP:  ...uunet!mcvax!ukc!pyrltd!slxsys!jpp  (jpp@slxsys.specialix.co.uk)
Disclaimer: I don't even own a cat to share my views !