[mythtv-users] Fwd: Further Notice of Seagate Hard Drive Class Action and Proposed Settlement

Ben Kamen bkamen at benjammin.net
Sat Mar 13 16:43:58 UTC 2010


On 3/12/2010 6:18 PM, Manuel McLure wrote:
>
> The point of parity on RAM is not necessarily to correct errors
> (although ECC can do that in _some_ cases) - it's to detect memory
> errors before they can cause extensive data corruption. The idea is
> that once you get a bad bit, there's no telling what consequences it
> could have. For example, a bad bit in a pointer address could cause
> data to be written to the wrong part of memory, clobbering data that
> then could get written to external storage. A bad bit in the counter
> for a loop could cause you to write 66536 bytes instead of 1000 bytes.
> That one bad bit could cause thousands of bytes of data to be
> corrupted. With simple one byte parity you stop the processor as soon
> as the bad bit is detected, and the corruption has little chance to
> spread. This actually may make a machine with bad RAM crash more often
> but greatly reduces the chance of extensive data corruption.
>
> On more modern systems, multiple bytes (usually 8) and their parity
> bits are combined - this is what's known as ECC. The ECC can detect
> and correct single bit errors, and can detect two bit errors but not
> correct them. In that case it will crash the system exactly in the
> same way as single byte parity does.

I understand that -- but what I'm saying is way back then was it was "so very important" -- it seemed to do nothing since even the most occasional errors would end up in a crashed machine.

Not a machine that would gracefully tell you "you have bad memory at location x, you should fix it".

The machine would just crash --- hard.

Only running memory tests would seemingly THEN find the bad RAM.

So in the end, it seemed like extra non-functional fluff.

Even the PC Jr's (IIRC) had 9bit RAM.. and it didn't help them a bit.

  -Ben


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