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Date: Sun, 1 Jun 1997 03:18:37 -0700 (PDT)
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To: roberto@keltia.freenix.fr
CC: hardware@FreeBSD.ORG, kato@FreeBSD.ORG
In-reply-to: <19970601112930.36404@keltia.freenix.fr> (message from Ollivier Robert on Sun, 1 Jun 1997 11:29:30 +0200)
Subject: Re: Intel Pentium II released
From: asami@cs.berkeley.edu (Satoshi Asami)
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 * A sysctl-controlled variable would be nice although I don"t see why anyone
 * would want to run without this feature :-)

Actually, write allocate may not help or even hurt if you are copying
large amount of data.  If it is implemented the straightforward way,
i.e., to read the rest of the bytes in the line when a write to a
currently uncached memory ("read-before-writes"), those reads will be
simply wasted as the rest of the line is going to be quickly
overwritten by writes from the CPU.  For this reason, we might even
want to disable write caching entirely during a large memory copy (no
need to keep them in cache if they are not going to be read again soon 
enough).

 * I have no documentation on both the assembler syntax (well, maybe somewhere
 * deeply buried) and the MMX registers so it is difficult for me to try it.

Well, even if you know the assembler syntax, you need an assembler
that understands it don't you. :)

So, please take a look at PR gnu/3157.  (The syntax is just like
regular mov instructions anyway.)  Also, you can get a sample test
program from:

  ftp://stampede.cs.berkeley.edu/pub/bcopy/bcopy-960524.tar.gz

as you probably don't want to change sys/i386/i386/support.s without
making sure it works first. :)

Satoshi