Path: utzoo!attcan!sobmips!uunet!aplcen!uakari.primate.wisc.edu!brutus.cs.uiuc.edu!psuvax1!psuvm!odx
From: ODX@PSUVM.BITNET (Tim Larson)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: /286 processor speeds:question
Message-ID: <89320.110617ODX@PSUVM.BITNET>
Date: 16 Nov 89 16:06:17 GMT
References: <5392@cps3xx.UUCP> <1336@uvm-gen.UUCP>
Organization: Penn State University
Lines: 35

In article <1336@uvm-gen.UUCP>, wirthlin@uvm-gen.UUCP (Ralph Wirthlin,Gamma
IV,31415926535,27182818) says:
>
>
>        An 8086 will also access memory 16 bits at a time on a word boundary
>(fetch).  As someone pointed out, however, the 286 has reduced the
>number of cycles required for a few of the, previously, expensive
>instructions such as div or mul.  I also believe (don't have the
>tech manual handy) that the pre-fetch queue on the 286 is larger than
>the 8086, thus increasing the throughput.
>        The relative speed of the 286 over the 86 is obviously complicated by
>the type of program being run.  Is the program cpu or memory-bound?
>
This is the most important point -- what is your application doing?  Word
processing, database, program development, etc. are all IO-intensive (most
office type programs are).  Games, etc. may not be so IO-intensive.  The
biggest difference, then, is whether you have an 8-bit data bus, 16-bit data
bus, or a 32-bit data bus (like MicroChannel or EISA).  The 8086 can handle
16 bits of data (unlike the original 8088), but is most often (always?) stuck
in an 8-bit XT style bus.  The 80286 is most often stuck in a 16-bit AT style
bus and thus IO gets a performance boost of roughly 2X.  Put an 80286 in
an XT and what do you get?  Very modest performance improvement at the same
clock rate (is this why so-called accelerators have faded into the
background?).

Recall, too that the AT bus is a superset of the XT bus and XT cards in an
AT slot won't do much to improve performance.  An AT with all XT cards will
not be a lot faster than an XT at the same clock rate.  Note that this is
the way many *inexpensive* clones stay inexpensive, and the reason that they
often are a disappointment to their buyers.

Moral: buyer beware!

-Tim Larson
odx@psuvm.bitnet