[PATCH] Handle mtsprg and mfsprg properly for BookE

[Jeff Baker]

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
>