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SubjectRe: Memory Trauam

On Tue, 19 Nov 1996, Richard B. Johnson wrote:

> > So here's my dilemma: why didn't memtest 1.1 notice the bad ram? This
> > wasn't the type of problem that showed up intermittedly: after booting
> > my kernel INIT would ALWAYS fail.... ALWAYS. As would initrd.
> >
> > So what's the difference between how the real kernel accesses memory
> > and memtest? It seems that memtest isn't the checker it should
> > be. Many people on this list complain of bad ram, checked "OK" by
> > memtest but failing with the kernel.
> >
> You have discovered the oldest software problem. You can't test RAM using
> a program that runs in the RAM being tested! You can try, but you will
> probably not find the bad RAM.


While I was reading this I thought of another potential problem for mem
testers: cache.
You write something out, you read it back in, it's still in the
cache, so you're not actually ever reading from the SIMM again. There is
an instruction to clear the cache, but I don't know if it would be
implemented on all platforms because I really don't see the point in
having such an instruction. :)

Greg Alexander

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