Path: utzoo!utgpu!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!ucsdhub!ucsd!ucbvax!decwrl!labrea!rutgers!att!pacbell!well!ejf
From: ejf@well.UUCP (Erik James Freed)
Newsgroups: comp.sys.next
Subject: Re: NeXT Memory - No Error Checking or
Message-ID: <7575@well.UUCP>
Date: 6 Nov 88 21:42:17 GMT
References: <549@gt-eedsp.UUCP> <207400001@inmet> <956@accelerator> <553@gt-eedsp.UUCP> <967@accelerator>
Reply-To: ejf@well.UUCP (Erik James Freed)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 20

>Adding SEC/DED increases DRAM cost by 7/32, less than 25%. 
>Adding parity per 32 bits increases DRAM cost by 1/32, about 3%.
>Adding parity per byte increases DRAM cost by 1/8, 12.5%.
I am not sure that this is a pro-parity or anti-parity addition
but I would like to add that  when you look at an individual feature
it is too easy to say "well it would only add X% to the cost" but
the real issue is what do you do when you are faced with 100s of these
decisions? A few percent for each feature can easily double the price.
remember that each of these costs must include a factor of 4-5 to
allow for a profitable margin. Anyone who has to make these kind
of decisions quickly realizes why these seemingly small ommisions
are so important to holding the line on price. There have been
a lot of unsuccessful products/companies that didnt address this
important issue.

>I am not an expert on memory hardware either, but I know that
>parity and ECC does cause a performance hit. 
Actually you can implement parity so that it does not hurt performance
by having parity checking not hold up a memory cycle but interrupt 
the cpu after the memory cycle if an error is found.