It is not necessarily the processors PIR register value, nor does
it have any other meaning.
You can set the PIR to whatever you want, it's R/W.
It doesn't have to be. Book3 2.02 says:
"Read access to the PIR is privileged; write access, if provided, is
described in the Book IV, PowerPC Implementation Features document for
Yeah. We were talking specifically about 970 though (or I was at
In other words, it's up to the implemenation if it's writable or
not, and how it's writable: Chip in init scans, mtspr or other ways.
...and I goofed: PIR is R/O on 970, it's initialised via a bunch
of pin straps on the CPU package.
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