From: Ojala Pasi 'Albert' (albert_at_cs.tut.fi)
Date: 2003-06-16 12:12:06
> just wanted to chime in on SRAM vs. DRAM: SRAM is much faster,
> for reasons that are not simple to explain, but boil down to the fact
> that DRAM needs a special read access sequence,
> more complex than the one for SRAM. That's why cache memory is SRAM.
The read access sequence (RAS+CAS) is a historic relic and is there
more because of the interface (multiplexed addresses) than actual
necessity. However, implementing exactly the same kind of asynchronous
interface than in SRAM's is not very easy. There are pseudo-static
RAM's that have a very similar interface though.
> As for consumption, well, that depends.
Yes, it depends whether you need to access the memory or not,
how ofter you access it, and how fast you want it to be :-)
> On one hand, fast SRAM memory, like the one used for cache RAM,
> is highly energy-consuming, because in order to have fast access times,
> relatively high currents in the flip-flop branches are necessary
This is no different in SRAM's than DRAM's. The memory matrix is read
(virtually) in the same way in both, and faster access requires bigger
drivers/amplifiers, thus increased power consumption.
Why SRAM is used as cache RAM is IMHO because SRAM does not need
refresh cycles and thus SRAM accesses are always predictable.
Another reason is that the pseudo-static RAM's and such, which have
non-multiplexed address (i.e. single-cycle access) are not widespread
enough.
> So does SRAM consume less power than DRAM? Funnily enough, each expert
> seems to hold a different opinion on the subject, but my personal
> experience seems to indicate that SRAM consumes less energy than
> DRAM of comparable transistor technology and speed.
DRAM always needs refresh cycles, as infrequent as they might be, thus
the static power consumption (when there are no accesses) can not quite
approach the SRAM figures.
So, it all depends..
-Pasi
--
"Let her think she's running things. That way, when it's important,
and you say different, by the time she gets over the shock of it,
[..] it will be too late for her to badger you about changing it."
-- Abell Cauthon in The Wheel of Time:"The Shadow Rising"
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