On Oct 8, 2009, at 3:23 PM, Benjamin Herrenschmidt wrote:
<<
• Reference and change bit updates—The MPC850 does not generate an
exception for
an R (reference) bit update. In fact, there is no entry for an R
bit in the TLB.
The change bit (C) is bit 23 in the level-two descriptor,
described in Table 8-4.
Software updates C (changed) bits, but hardware treats the C bit
(negated) as a
write-protect attribute. Therefore, attempting to write to a page
marked unmodified
invalidates that entry and causes an implementation-specific DTLB
error exception.
^^^^^^^^^^^^^^^^^^^^^^
If change bits are not needed, set the C bit to one by default in
the PTEs.
How interesting....
I've looked at many 8xx docs and they all have the same text
(probably cut/paste :-)) I'd place some debug code in the C functions
to print out a few of the TLB Entry for various errors to see if this
really
happens, and for other errors, too. I guess I never stumbled into
this because I always thought I had to do everything from software,
so just made sure I did.
-- Dan
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