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From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: comp.sys.amiga
Subject: Re: Prolog on 68020
Message-ID: <17223@glacier.STANFORD.EDU>
Date: Fri, 20-Nov-87 20:31:31 EST
Article-I.D.: glacier.17223
Posted: Fri Nov 20 20:31:31 1987
Date-Received: Sun, 22-Nov-87 21:46:18 EST
References: <5338@cisunx.UUCP> <2801@cbmvax.UUCP> <177@vertigo.UUCP> <21872@ucbvax.BERKELEY.EDU>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Distribution: na
Organization: Stanford University
Lines: 14
Keywords: prolog, Amiga, M68020, cache
Summary: Use of high bits in address for other purposes won't work on 68020.


     Won't work.  The M68020 has an on-chip cache which stores 32-bit
address values.  Even if the external memory addressing only uses fewer
bits, the cache always checks all 32.   On a system where the high address
byte is unused by the memory, the following sequence will produce  
unwanted effects:

	Read address A.
	Write new value to A + 0xff000000
	Read address A - get the OLD value!

Like it or not, the M68020 is a 32-bit machine.

				John Nagle