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Date: Tue, 07 Jul 2009 10:52:36 -0700
From: Peter Grehan <grehan@freebsd.org>
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To: Justin Hibbits <jrh29@alumni.cwru.edu>
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Cc: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>
Subject: Re: FreeBSD/ppc ABI
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Hi Justin,

> The trick is only needed in user space, as a shim between two
> functions, so I just need any volatile registers that I can use but
> aren't function arguments.  The target is G4/G5.  I only need 3 or 4
> volatile registers, but the more the better.

  You'll probably have to use the stack to spill the param register. 
Have  a look at the MCOUNT macro in ppc's <machine/profile.h> for an 
example of code that is an intercept between two routines and has to 
assume that all param registers are used

(http://svn.freebsd.org/viewvc/base/head/sys/powerpc/include/profile.h?view=markup&pathrev=153813 
is the version that has the code fragment commented).

later,

Peter.