🚀 go-pugleaf

In article <1655@lpami.wimsey.bc.ca> lphillips@lpami.wimsey.bc.ca (Larry Phillips) writes:

> In Very Important Applications, I would go for ECC.  In other situations, I
> would go for no checking at all. Parity is useless.

>> DRAMs these days are much more reliable than 10 years ago... even 5 years
>> ago.

>  You rest my case. :-)

   This is more in the nature of an agreement than a flame, but there
are circumstatnces where ECC is LESS relaible than no checking
currently...specifically when speed limits are being pushed.

    Modern DRAMs are fairly well protected against cosmic ray induced
errors, and other transients, but if you use ECC circutry, the overall
reliability of a memory system has to include the possibility that the
ECC circutry returns the wrong value or (much more common) does not
assert the correct value soon enough.  On most ECC memory systems this
is the most frequent cause of uncorrected error (even though it most
frequently occurs when a bit has been flipped).  Worse, if such an
error occur when the memory is read correctly, it is not even detected
by most tranisent fault counters.  It takes a lot of extra logic to
look for signal changes on the buss immediately after the correct
value is expected to be asserted.

    I used to work at Stratus Computer, and (due to duplexed ECC
memory boards) we could detect and count both types of faults. (It's
only a failure if the program sees bad data...)  That is the minimum I
would recommend for life critical systems.  We did see both kinds of
faults, and I would imagine that without very careful design, today
ECC memory does not significantly improve reliability.

    (Before the flames start...If a system has a transient memory
failure every ten months with ECC and every three months without, that
is not a significant difference.  As a fault-tolerant designer, I
would want to push it out to several years, preferably a century or
two.  You can't do that with simple ECC.)

    If you have a friend with an IBM compatible with parity ask him
when he last had a parity error.  My guess is that with 256K or 1 Meg
parts, it should be significantly less than 1 per Megabyte per year.

--

					Robert I. Eachus

   Amiga 3000 - The hardware makes it great, the software makes it
                awesome, and the price will make it ubiquitous.