Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!cornell!rochester!ur-tut!ur-valhalla!micropen!dave
From: dave@micropen (David F. Carlson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Intel 386SX chip & its applications
Summary: rash generizations
Message-ID: <513@micropen>
Date: 28 Jun 88 18:42:33 GMT
References: <206900116@prism> <6734@cup.portal.com> <6859@cup.portal.com>
Organization: Micropen Dirent Writing Systems, Pittsford, NY
Lines: 41

In article <6859@cup.portal.com>, mslater@cup.portal.com writes:
> 
> As for 286 vs 386 speed, I've seen a number of benchmarks that show that for
> 16-bit code (8086 or 286 code) the 286 runs very nearly the same speed, and
> sometimes slightly faster, than a 386.  There are many instructions that
> require fewer clocks on a 286 than on a 386.  The primary advantage of the
> 386 is that it can fetch two 16-bit words at a time; however, this apparently
> is not such an advantage, presumably because the pipeline can be kept nearly
> full with only 16-bit fetches and because at every jump there is a 50/50
> chance that the second word fetched must be thrown away.  The 32-bit bus of
> the 386 obviously has a big advantage for programs that manipulate lots of
> 32-bit data words, but most programs are dealing with bytes or 16-bit words.

Most programs deal with word size for the machine.  Most useful programs I have
used have 32 bit ints and floats.  I can run "useful" programs on the 386SX,
whereas I lost much hair running most anything "useful" on the 80286.

> 
> If anyone else has any insights on 286 vs 386 performance, I'd appreciate any
> clarifications.  The data I've seen indicates that nearly all the performance
> gain of 386 systems over 286 systems, when running DOS or OS/2, is simply due
> to the faster clock, and that the 32-bit bus is of little help.
> Michael Slater,  Microprocessor Report    415/494-2677
> mslater@cup.portal.com       sun!portal!cup.portal.com!mslater

The stipulation of DOS or OS/2 is probably correct.  The 386 requires more time
to do segment loads due to 2 levels of tables indirection whereas the 286 has
just a GDT, LDT, etc.  The big point of the 386SX is that you don't have to
close the door on Xenix, UNIX, OS/2-386, etc. just because the 32bit memory
scene is such a mess now.  For most single user apps (workstation model), a
16 bit data bus is not as much of a bottleneck as is the 250Kbps of the WD
MFM disk controller.  Speed isn't the big question but future compatibility
is.




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

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