On Thu, 3 Apr 2003, Ralf Baechle wrote:
> > Hmm, why -- is such a change observable externally in any way? Of
> > course you can't switch the other way if the s-cache uses a line width of
> > 16 bytes. Maybe that's the case with the Magnum?
> It's a hardware problem with the memory controller I was told by one of
> it's developpers. That forced them to run the machine with an different
> line size for D-cache and I-Cache. There's various revs of the Magnum's
> memory controller and only one of them got all the cases right ...
Hmm, that's even more interesting -- how can instruction fetches be
distinguished from data reads externally??? Then again, the memory
controller shouldn't be able to observe inter-cache data moves. Strange.
> Maybe DECstation and other SGI hardware got that better?
No problem testing, I suppose.
> > Why? It isn't that obvious especially as a p-cache miss costs a single
> > cycle only.
> During my recent work on the cache code I found the execution time of
> cache flushing code to be quite a bit higher than previously assumed so
> larger lines would help reducing that also.
This can be benchmarked -- there may be some gain for p-cache flushes
> > > working truly correct we also should no longer see VCE exceptions on
> > > R4000SC processors - the reason why Indys are still a valuable test tool.
> > As are DECstations which use the opposite endianness -- so you can test
> > code both ways.
> A bunch of evaluation boards that support running in the other endianess
> and way exceed the performance of any R4000-based platform. Just having
> to flip a switch on the board is very handy.
I was referring to testing cache and VCE code specifically -- you won't
get that from usual evaluation boards.
Note that with evil /dev/mem maps you should still be able to force VCEs
if needed. ;-)
+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+ e-mail: firstname.lastname@example.org, PGP key available +