Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!cis.ohio-state.edu!ucbvax!mtxinu!taniwha!paul
From: paul@taniwha.UUCP (Paul Campbell)
Newsgroups: comp.sys.m68k
Subject: Re: Daystar Digital Review: 030 vs. 040 Perf. (Very Long)
Message-ID: <886@taniwha.UUCP>
Date: 30 Jun 91 19:46:48 GMT
References: <1991Jun28.230007.10651@oakhill.sps.mot.com>
Reply-To: paul@taniwha.UUCP (Paul Campbell)
Organization: Taniwha Systems Design, Oakland
Lines: 53

In article <1991Jun28.230007.10651@oakhill.sps.mot.com> jtr@oakhill.sps.mot.com (Jim Reinhart) writes:
>
>>SOFTWARE COMPATIBILITY WILL BE A MAJOR PROBLEM ON 68040 
>>ACCELERATORS AS WELL AS APPLE'S NEW 68040 MACHINE.  APPLE 
>>WILL HAVE TO MAKE MAJOR PATCHES TO THE MAC OS TO HANDLE 
>>PROBLEMS WITH MEMORY MANAGEMENT AND EXCEPTION HANDLING. IN 
>
>There are indeed differences between the O.S. programming for 
>the 68030 and 68040.  However, Daystar's assertion that this 
>is a "MAJOR" problem is not generally supportable.  First, 
>the differences have been documented long enough (~1.5 years 
>in print) for vendors to make appropriate plans.  Second, the 
>only major differences (or 'problems' in Daystar terminology) 
>concern the virtual exception processing model and cache 
>management.  This impacts only a small portion of O.S. code 

While I mostly agree with Mot. and disagree with Daystar on almost
all these issues, I have to take exception to this response. There is a
quite large body of code (mostly INITs and things that do trap
patches) that runs on existing Macs and doesn't run on '040s,
the main reason has to do with programmers who modify code, or load
data containing code and execute it without being aware of the
impacts of the caches in the '020/'030 and '040. The reason this happens
is for 2 reasons:

	1)	People who are running '040s without writethrough data
		caches (ie a copyback cache) which means that data
		may be in the cache but that memory has an old value

	2)	The '040 caches are much bigger, code that happens to run
		today on an '030 fails on an '040 because the '030 
		instruction cache entries happened to be replaced because
		it's much smaller.

So if you are a Mac programmer: if you load code in a resource, or copy it
anywhere, or if you modify code (a good example is the trick of putting
the address of a trap address after a jmp instruction when you are trap
patching) remember to flush the caches, it's easy, you have to do both
of them and it's done using the traps:

	_FlushInstructionCache
	_FlushDataCache

If in doubt flush the caches! On the other hand CPUs like the '040 depend
on caches for their performance - so don't flush the caches unnecessarily.

	Paul Campbell

-- 
Paul Campbell    UUCP: ..!mtxinu!taniwha!paul     AppleLink: CAMPBELL.P

Tom Metzger's White Ayrian Resistance has been enjoined to stop selling Nazi
Bart Simpson t-shirts - Tom of course got it wrong, Bart is yellow, not white.