Path: utzoo!mnetor!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Still More 68020 Questions
Message-ID: <3129@cbmvax.UUCP>
Date: 11 Jan 88 23:33:09 GMT
References: <8801081459.AA16543@decwrl.dec.com>
Organization: Commodore Technology, West Chester, PA
Lines: 35

in article <8801081459.AA16543@decwrl.dec.com>, plouff@nac.dec.com (08-Jan-1988 0946) says:
> 
> 1.  When the operating system recognizes a 68020, what exactly does it 
> do?  Is the '020 instruction cache turned on by ROM Kernel routines (or 
> elsewhere in the OS), or must the user run a separate program?

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.
> 
> 2.  Other than running self-modifying code, can one get in trouble 
> running with the cache enabled?

I've never found any reason to turn off the cache.  

Even though the C-A board (A2620 by name) has on-board RAM, making a run
from 16 bit only RAM rather moot, I have found that with all 32 bit RAM
disabled, the 68020 system will run slightly faster than the plain 68000.
This speed increase is probably attributable to caching and to the fact that
non-memory operations will still happen at 14.2MHz.  I don't understand
why the CSA board would run slower from 16 bit memory than the 68000, though
never having used the CSA board I'm not in a position to argue that point.
But it's obviously an implementation detail if anything, not a fundamental
problem with 68020s on the Amiga.

> Thanks in advance,

> Wes Plouff
> Digital Equipment Corp.		UUCP:	   ...!decwrl!nac!plouff
> Littleton, Mass.		Internet:  plouff%nac.dec@decwrl.dec.com
-- 
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!"