Path: utzoo!mnetor!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Still More 68020 Questions
Message-ID: <3161@cbmvax.UUCP>
Date: 18 Jan 88 20:58:21 GMT
References: <9055@ccicpg.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 54

in article <9055@ccicpg.UUCP>, harald@ccicpg.UUCP ( Harald Milne) says:

>> The '020's cache is turned on by the OS, and left alone after that.  The OS
>> also sets the 68020 flag in ExecBase, and the 68881 flag if appropriate.  The
>> GetCC() vector is also set for the 68020, and any internal OS uses of
>> exception stack frames (if any) are set so the OS will run just fine.

> 	But the cache must be purged at some point. If you exit a task, the
> code in the cache can hit again, with new code loaded into the same location,
> the result is disasterous. Is the cache ever purged? The OS must handle this
> somehow.

I'm not exactly sure what happens here.  You'll never run into trouble 
during task swaps, since everything running is running at a unique place
in memory.  If a program exits and a new one is loaded into that same
memory location, obviously the cache must be purged, providing is could
possibly contain any reminant of that removed program at that point.  This
isn't a very large cache we're talking about.  Maybe Andy knows the truth
here...

> 	I think that's a bit ambiguous.

> 	The 68020 is claimed to run at 86% of the speed of the 68000.

> 	For some reason, this sound like it's running without the cache
> enabled.

I read the AmigaWorld article.  I just can't think of any reason why this
slowdown should be, other than as an "implementation detail" of the CSA
board.  Or perhaps the Dhrystone benchmark with the cache disabled hits
some particularly bad features of any 68020 to 68000 interface.  Have to
try that benchmark on our board.

> 	The only explanation I have for this behaviour, is the need for an extra
> address cycle to get the other 16-bit quanity. (It would take 2 address cycles
> to get a 32-bit quanity from 16-bit memory, correct?)

Right.  Whether you're a 68000 or a 68020.  There's the rub.

>> 		"I can't relax, 'cause I'm a Boinger!"

> 	I just can't relax either. Damm this Amiga. 8^)

I did relax a few weeks ago, but I had to fly a few thousand miles from 
my Amigas in order to achieve this feat.

> Work: Computer Consoles Inc. (CCI), Advanced Development Group (ADG)
>       Irvine, CA (RISCy business! Home of the CCI POWER 6/32)
> UUCP: uunet!ccicpg!harald

-- 
Dave Haynie  "The B2000 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!"