Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.m68k
Subject: Re: opcode emulation (Was: Re: 68020 in a *68010* socket?)
Message-ID: <4067@cbmvax.UUCP>
Date: 20 Jun 88 17:14:43 GMT
References: <3142@edm.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 42

in article <3142@edm.UUCP>, rroot@edm.UUCP (Stephen Samuel) says:
> 
> From article <4015@cbmvax.UUCP>, by daveh@cbmvax.UUCP (Dave Haynie):
>> 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.  

> My understanding (last thing I heard) was that the '20 runs about 50%
> faster with 16 bit mem than a 68000. 

I've never seen any statictics on it, or seen any predictions made by
Motorola.  I have, however, actually used 68020s running only 16 bit
memory in Amiga systems.  And with the cache off, they do run slower.
Somewhere around 20% slower overall; I didn't really try to find out
what was causing the slowness, as I had no intention to ever really use
the system with the cache off.  Or without 32 bit wide memory, either.
But I was interested in seeing the effects of 32 bit over 16 bit memory,
and it does make a difference.  The '020 slowdown was initially a 
surprise, but since then, in learning more about the '020 and seeing other
systems behave similarly, I'm comfortable with it.

> 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). 

I think if the cache is off, unused prefetched stuff is just thrown out.  With
the cache on, I see maybe an overall 20% increase in speed.  With 32 bit memory
in this system (14.3MHz vs. 7.2MHz) I see maybe a 300% total increase in most
operations (there's still some 16 bit memory that has to be used by the OS).

> 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.

That's one of 'em, anyway.

>  Stephen Samuel 			Disclaimer: You betcha!
>   {ihnp4,ubc-vision,seismo!mnetor,vax135}!alberta!edm!steve
>   BITNET: USERZXCV@UQV-MTS
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"