GNU replacing obsolete assembly instructions in PowerPc architecture

[ilya german]

Thanks a lot for your answers !!!

On Mon, Mar 18, 2019 at 5:29 PM David Brown <david@westcontrol.com> wrote:

> On 18/03/2019 15:29, Segher Boessenkool wrote:
> > On Mon, Mar 18, 2019 at 12:09:01PM +0100, David Brown wrote:
> >> Assuming my memory is correct, of course, it means there is no
> >> difference between "mftb r6" and "mfspr r6, 284", or between "mftbu r5"
> >> and "mfspr r5, 285".
> >>
> >> However, I think the particular SPR numbers may vary between PPC cores -
> >> earlier I saw 268 and 269 from a google search, while some old code of
> >> mine has 284 and 285.  So you should prefer to write "mftb" and "mftbu"
> >> in your souce code, and let the compiler and assembler turn it into the
> >> correct SPR values for your chip.
> >>
> >> But don't worry if you see the mfspr instructions in the disassembly.
> >
> > The bit with value 16 in SPR numbers means it is privileged access (*)
> > (hypervisor, or supervisor on CPUs without hypervisor).  You need to
> write
> > "mtspr rX,284" to write to the time base.  Reading should use 268 (and
> > this is used with the extended mnemonic mftb; and there is no mttb
> extended
> > mnemonic: there used to be mttbl and mttbu extended mnemonics, using 284
> > resp. 285, but you are supposed to use mtspr now, and privileged code can
> > be expected to change faster).
> >
>
> Ah, OK.  The PPC stuff I have done is all single-program unhosted
> embedded code, which runs in privileged mode all the time, and it was a
> good many years ago.  Thank you for filling in the details in this
> thread, and correcting my errors.  (And of course, thanks for your work
> on the PPC target for gcc!)
>
> mvh.,
>
> David
>
>