From owner-freebsd-hackers  Tue Oct 28 17:28:04 1997
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To: Mikael Karpberg <karpen@ocean.campus.luth.se>
cc: hackers@FreeBSD.ORG
Subject: Re: Parity Ram 
In-reply-to: Your message of "Wed, 29 Oct 1997 02:28:30 +0100."
             <199710290128.CAA07569@ocean.campus.luth.se> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Tue, 28 Oct 1997 17:29:28 -0800
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>
>[...Discussion on ECC/parity/no-parity memory...]
>
>I seem to recall something about partiy and/or ECC memory having slower
>access rates, or something, and therefor being a bad thing preformace-wise
>but a good thing safety-wise?
>
>I don't know where I got this, but could anyone with knowledge in the
>subject maybe enlighten me on the amount of truth behind this?

   In order to update the memory, the ECC must be recalculated over the entire
64bit quadword. This escentially means that you have to read the memory first,
apply the changes/calculate the new ECC and then write it back. Obviously,
this makes memory writes quite a bit slower.

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project