[Techtalk] ECC Lesson :-) [Was: AlphaServer ... ECC error"]

[Julie]

Walt wrote:
> 
> I can't contribute much info, but I would like to
> understand what ECC involves, what it does,
> and if it's usually worth the extra cost, etc.
> 
> Even a cursory explanation would be interesting
> and helpful, Hamster.

ECC memory, as someone else explained, involves computing
some values (usually the actuall "error correcting code"
from which ECC gets it's name) and storing that in memory.

There are two ways for ECC to work.  The first is that it
is completely transparent to the system.  You store the
numbers, you get back the numbers, and if there is an error
in the middle, well, you never know.  Another way is for
the memory to generate an interrupt, but I don't think that
is ever used for PCs.  In that instance, the memory saves
information about the failing address.

Parity memory was really popular (at least in the circles
I navigated ...) many years ago.  But parity memory can only
detect single bit errors and is unable to correct them.  ECC
memory came along with the ability to correct single bit
errors and usually detect two bit errors.

The ECC strategy I was most familiar with involved Hamming
Codes.  Others exist as well.  See Google for more details.
-- 
Julianne Frances Haugh             Life is either a daring adventure
jockgrrl at austin.rr.com                 or nothing at all.
					    -- Helen Keller