> Line 1: mfspr   %r3,SPR_DBCR0
> 	Load DBCR0 into r3
> Line 2: oris    %r3,%r13,DBCR0_RST_SYSTEM@h
> 	Or the top half of DBCR0_RST_SYSTEM with the leftmost bits of r13 into
> 	r3.
> Line 3: mtspr   SPR_DBCR0,%r3
> 	Load r3 into DBCR0

> The first operation has no effect, because we immediately smash it
> with whatever random garbage happened to be in r13.

It looks effectless, yes, but a lot of machines have registers with
magic effects when they're read, independent of what you may do with
the data you get by reading them.

I don't know whether SPR_DBCR0 is such a register in this case, but
your "because..." makes it look as though you're not considering the
possibility.  (Of course, this could be because you did think about it
and determined that it's not such a register, in which case just ignore
me.)

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