Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!osu-cis!att!alberta!access!edm!rroot
From: rroot@edm.UUCP (Stephen Samuel)
Newsgroups: comp.sys.m68k
Subject: Re: opcode emulation (Was: Re: 68020 in a *68010* socket?)
Message-ID: <3142@edm.UUCP>
Date: 16 Jun 88 21:30:59 GMT
References: <4015@cbmvax.UUCP>
Organization: Unexsys Systems, Edmonton,AB.
Lines: 21

From article <4015@cbmvax.UUCP>, by daveh@cbmvax.UUCP (Dave Haynie):
> it would also be a nice way to support math in a 68000 based system.  The
> 68020 at the same speed would run slower than the 68000 on a 16 bit bus,
> though, for many operations.  All that 32 bit prefetching.  So if you're
> running 68000 code with no floating point, a plain 68020 drop in will 
> often hurt your performance and give you little in return (ok, it does
> get something back on tight loops if your OS lets you run with the cache
> on).
My understanding (last thing I heard) was that the '20 runs about 50%
faster with 16 bit mem than a 68000. This makes some sense.. Although
the '20 tends to prefetch in 32 bit blocks, this doesn't really cause
a big hit, in most cases, since you get it back on the next instruction
(I assume you get at least THAT much even with the cache disabled). The
time you'll get a REAL performance hit is when you have a program that
consists primarily of short (8 bit) branches all over the place with
no real work done anyways.
-- 
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