Date: Thu, 27 Jun 1996 20:35:37 -0700 From: "Michael L. VanLoon -- HeadCandy.com" <michaelv@HeadCandy.com> To: Joe McGuckin <joe@ns.via.net> Cc: hackers@freebsd.org Subject: Re: What is EDO memory? Message-ID: <199606280335.UAA05788@MindBender.HeadCandy.com> In-Reply-To: Your message of Thu, 27 Jun 96 19:40:03 -0700. <199606280240.TAA03861@ns.via.net>
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>What is EDO? Why doesn't it come in parity?
>Is it faster than standard ram?
This is my understanding of it...
EDO (Extended Data Out) DRAM is capable of bursting faster than
regular DRAM (Dynamic RAM).
For example, it might take six clock cycles to get the first word,
then four cycles for each additional word, once the intial address is
setup by the memory logic. This is usually designated like: 6-4-4-4,
indicating the bus cycles required for a burst of four words.
This is why we have cache -- all those clock cycles are cycles where
the CPU is twiddling its thumbs waiting for data. Cache runs faster
than DRAM, so can retreive data without all those wait states if you
get a cache hit. Most decent cache SRAM can do somewhere between
2-1-1-1 and 3-2-2-2.
EDO RAM starts a transaction like regular DRAM, but after the initial
address setup, is able to burst data through with less wait states,
say two cycles instead of four for each following word: 6-2-2-2. EDO
was originally touted as a way to make motherboards without cache.
But, it's obvious the performance, in spite of being better than DRAM,
just isn't as good as with a cache *and* EDO.
Another kind of RAM just starting to catch on is SDRAM, or Synchronous
DRAM. Supposedly it's supposed to be able to do like EDO, but at the
full speed of the bus for bursts, so something like: 6-1-1-1.
Although the setup time is still slower than cache, the following
words are burst in quickly.
SDRAM might actually be useful in low cost computers without cache.
However, for the best performance, you'll still want cache in your
machine (for faster setup times, and so the CPU can run from the cache
while a DRAM refresh is in progress, or if a bus-master device is
writing to memory while the CPU is doing something else).
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Michael L. VanLoon michaelv@HeadCandy.com
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