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Date: Thu, 23 May 2002 08:50:32 -0700
From: Jonathan Mini <mini@FreeBSD.org>
To: John Baldwin <jhb@FreeBSD.org>
Cc: cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org,
	Peter Jeremy <peter.jeremy@alcatel.com.au>
Subject: Re: cvs commit: src/sys/kern kern_mutex.c
Message-ID: <20020523085032.R25907@stylus.haikugeek.com>
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	Peter Jeremy <peter.jeremy@alcatel.com.au>
References: <20020523070814.Q25907@stylus.haikugeek.com> <XFMail.20020523102452.jhb@FreeBSD.org>
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John Baldwin [jhb@FreeBSD.org] wrote :

> 
> On 23-May-2002 Jonathan Mini wrote:
> > John Baldwin [jhb@FreeBSD.org] wrote :
> > 
> >> In Intel's documentation they said that all current look-alikes worked as
> >> well.  It's the opcode for 'rep mov'.  I am curious if some implementations
> >> trash %ecx, and if we are paranoid we could always clobber %ecx in the
> >> constraints.  [ ... ]
> > 
> > The rep prefix doesn't modify any registers.  You do need to make
> > sure that %ecx was zero when you dispatch a rep mov (unles you're
> > dispatching a rep mov $0, %ecx), or you'll get an infinite loop.
> 
> Bah, it's rep nop, not rep mov. 

Ahh. rep nop makes a lot more sense.

> rep does modify %ecx when used with, say
> string instructions.  It decrements %[e]cx once for each interation.

Um.. no. It's the string instruction that decrements cx. rep just, well,
repeats. =)

-- 
Jonathan Mini <mini@freebsd.org>
http://www.haikugeek.com

"He who is not aware of his ignorance will be only misled by his knowledge."
                                                        -- Richard Whatley

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