OT memory too cheap to pass up

From: Alex Holden <alex_at_linuxhacker.org>
Date: Sat Jan 27 17:31:44 2001

On Sat, 27 Jan 2001, Tony Duell wrote:
> After all, in a reasonably complex system you're going to want to write a
> given word to a given address. The address is probably going to have to
> be relocated by some kind of MMU, and then applied to the DRAM in 2
> halves. And while the address is being processed in this way, it's
> possible that the data is already 'available', so the partity bit could
> be calculated at the same time. Then it doesn't take any longer to
> calculate and store parity -- in a sense the critical path could be the
> address relocation.

Nope, the memory controller and the memory management unit are completely
seperate modules, in fact they're usually not even on the same chip. I
suspect that what most (all?) memory controllers that support parity RAM
do is to pipeline the access, so it takes two cycles to store a word
instead of one (but the processor doesn't see any difference because the
controller can accept and compute the parity of the next store whilst it's
writing the previous one out to the RAM). On loads, the controller
simply has to generate an NMI if it detects a parity failure, so that
wouldn't require an intermediate stage.

> So I guess that it might well be the case that parity memory _for a
> particular machine_ might have to use faster chips than non-parity memory,

I'm pretty sure the chips used in parity modules are exactly the same spec
as those used in non parity.

> and this might be one reason why they're so much more expensive. But it's

That's just economies of scale. Joe Windoze User doesn't really want to
pay an extra 1/8 on top of the cost of his RAM to be informed when a stray
particle of solar radiation happens to flip a memory cell. ECC is more
useful because at least it can recover from a bit error, instead of simply
crashing (which admittedly is probably still better than silently
corrupting your data). Since so few people buy parity RAM modules, the few
who are willing to pay extra for them (medium-high end server
manufacturers, Unix workstation manufacturers, etc.) have to pay commodity
prices.

-- 
------- Alex Holden -------
http://www.linuxhacker.org/
 http://www.robogeeks.org/
Received on Sat Jan 27 2001 - 17:31:44 GMT

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