Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!ucselx!crash!pnet01!jca
From: jca@pnet01.cts.com (John C. Archambeau)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: 386 vs 386SX
Message-ID: <6242@crash.cts.com>
Date: 10 Dec 90 23:16:05 GMT
Sender: root@crash.cts.com
Organization: People-Net [pnet01], El Cajon CA
Lines: 38

silver@xrtll.uucp (Hi Ho Silver) writes:
>In article <1990Dec5.211220.13194@msuinfo.cl.msu.edu> draper@buster.cps.msu.edu (Patrick J Draper) writes:
>$In my experience, running MS-DOS an SX is equally fast as a full 386. I
>$think that is because MS-DOS is a 16-bit system, and runs in 16 bit real
>$mode of 386's. Therefore there is no speed advantage from the 32 bit
>$bus. I may be wrong, and I'm sure someone will correct me if I am.
>
>   You're just a little off.  The 386DX does its instruction fetches
>32 bits at a time, whereas the 386SX (obviously) can't fetch more than
>16 bits at a time.  This gives the DX somewhat higher performance for
>the same clock rate on 16-bit software.
>
>   Keep in mind, also, that there is some software that is 386-specific,
>and the speed difference will be greater for that software than for
>16-bit software; also, there are some programs that detect if you have a
>386 installed and will transparently use it if so (PKZIP/PKUNZIP, for
>example).

This is where you're wrong.  There is no way in software to tell the
difference between the 386SX and 386DX.  It all looks the same to software. 
Even the prefetch instruction queue is the same length.  True, it can only get
16-bits of data at a time, but internally, a 386SX is identical to a 386DX. 
The only difference if I'm not mistaking is the bus interface unit (limited to
16-bits on an SX, 32-bits on a DX), and the address space (only 24-bit on an
SX, 32-bit on a DX) which is limited to 16 Mb physical memory.

Otherwise, it's the same thing to software.

     // JCA

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