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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH 6/15] bootwrapper: switch 64 bit cpus to 32 bit mode
Date: Wed, 11 Jul 2007 00:57:07 +0200
To: Milton Miller <miltonm@bga.com>
Cc: linuxppc-dev@ozlabs.org, Paul Mackerras <paulus@samba.org>,
	David Gibson <david@gibson.dropbear.id.au>
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> +	/* Check if the processor is running in 32 bit mode, using
> +	 * only 32 bit instructions which should be safe on 32 and
> +	 * 64 bit processors.
> +	 *
> +	 * Subtract the bottom 32 bits of MSR from the full value
> +	 * recording the result.  Since MSR[SF] is in the high word,
> +	 * the result will be not-equal iff in 32 bit mode (either
> +	 * the processor is a 32 bit processor or MSR[SF] = 0).
> +	 */

The other way around -- the processor is 64-bit and MSR[SF]=1.

Great trick btw! :-)

> +	mfmsr	r0		/* grab whole msr		*/
> +	rlwinm	r8,r0,0,0,31	/* extract bottom word		*/
> +	subf.	r8,r8,r0	/* subtract, same?		*/
> +	beq	0f		/* yes: we are 32 bit mode	*/

The code is fine though.


Segher