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Re: [PATCH] Handle mtsprg and mfsprg properly for BookE


The IBM PPC440 doesn't support reading from the supervisory SPR's (you get garbage values back) - the mfsprg's won't be too useful. E.g.:

running the following code (MSR.PR=0) (PVR=0x41020481):

		li		%r7,-1
		li		%r8,-1
		li		%r6,1
		mtspr	276,%r6
		mfspr	%r7,260
		mfspr	%r8,276

leaves R7 with a 1 and R8 with a 0.

This is correct behaviour according to IBM.

Alan Modra wrote:
On Sun, Mar 13, 2005 at 03:34:33PM -0800, Alex Rosenberg wrote:

On Mar 9, 2005, at 7:58 PM, Alan Modra wrote:

There remains the question of which spr number should be used by
privileged programs for read access to sprg4 through sprg7.  Does the
non-privileged spr number work when privileged?  I would think it
probably does, but I'm just guessing..

I think the documentation is the problem here.


Originally, the user mode read-only SPRG were supposed to be referred
to as USPRG.


Makes sense. Consistent with the spr numbering for USPRG0 too.

I propose that gas adopt this syntax. ie.

mtsprg and mtsprg[0-7] writes spr272..279 (as we currently do)
mfsprg and mfsprg[0-7] read the same regs as mtsprg
mfusprg0 reads spr256 (as we currently do)
Add new mfusprg[3-7] insns which read spr259..263
Add a new mfusprg insn which reads spr256..263



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