Subject: Re: Nine bits v. Eight

From: [email-masked] (Joel Polowin)
Date: Thu, 27 Apr 95 10:46:58 -0400

> Date: Tue, 25 Apr 1995 17:08:57 -0400 (EDT)
> From: Friedman <[email-masked]>
> Subject: CCL:Nine bits v. Eight
> 
> We are purchasing a pentium 100MHZ PC to speed up our runs using Hyperchem
> and Gaussian 92W/DFT. I was alerted by several people that many of the newer
> PCs use 8 bit SIMMs now. I've gotten differing opinions on whether parity
> checking is needed. I would appreciate hearing from anyone with an informed
> opinion as to the risks (no pun) involved. Hypercube and Gaussian boffins
> especially welcome.

I relayed the question to our hardware expert, Edmund Lam.  He responded:

---------
In the past, PC used 9 bit wide RAM.  This architecture was used to ensure
detection of RAM errors.  In recent times, the quality of RAM and mainboards
have curved the need for parity detection.  

MACs have always run 8 bit wide RAM.  Workstations such as the Sparcs also
use 8 bit wide RAM.

The extra bit can be used to detect a memory error, but is not sufficient to
correct it. 
----------

My own experience with parity checking was a PC that I ended up lugging
back and forth to and from the shop for nearly a year because it kept 
giving me "Parity error -- system halted" crashes at home but worked fine at
the shop.  Shop kept claiming it was a problem with the RAM, and replacing
the chips.  Took them a year to clue in that the problem was actually 
with the motherboard; after they replaced *that*, the machine gave no
more trouble.  Whether that little story argues FOR or AGAINST the merits
of parity-checking is something I'll leave for others to sort out.  :-)

Regards,
Joel

------------
Joel Polowin, Ph.D.   Manager, Scientific Support
Email to: [email-masked] 

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