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This type of RAM is common in that it is used in your L2 cache (I'll get to this
in a second). SRAM is also pretty expensive (compared to DRAM). Why is it so expensive?
Because it's very fast. It's not as fast as your CPU, but it's way faster than DRAM. SRAM
uses transistors to create bi-stable (capable of holding one of two stable states)
circuits in the chip. Because voltage is only necessary when changing the state of the
bi-stable circuitry (i.e. a one to a zero), it doesn't need to be refreshed. SRAM is also
called NVRAM or Non-Volatile RAM (if you pull the chip out of its socket and lay it on the
table, it still retains the information it had before you pulled it out). Generally SRAM
operates at around 12ns.
L2 or Level 2 cache is RAM used specifically to hold data that was predicted by the
caching algorithm to be needed next for processing in the the CPU. Because this is a
prediction, there are two terms associated with it. Cache hit, and cache miss. There are
also a couple of other terms, write through, and write back, but I'm still having problems
understanding this fully, so I won't try to explain it. If any of you readers out there
have a good handle on it, please hook me up with a good explanation.
Cache is also used as the intermediary from the CPU to your DRAM. 256KB of L2 cache is
needed for 64MB of DRAM, and 512KB of L2 cache is needed for 128MB DRAM. In Pentium II
systems, the L2 cache is built into the processor assembly and can handle the caching
requirements for up to 2GB.
Async SRAM - Asynchronous SRAM
Not in synch with the clock, so the CPU must sometimes wait to get the data it needs.
Sync SRAM - Synchronous SRAM
In synch with the clock.
- 66MHz bus speed max.
PB SRAM - Pipeline Burst SRAM
The data is collected into packets called bursts. It is not quite as fast as Synch SRAM,
but it is much faster than Asynch SRAM.
- 4, 5, 6 and 8ns
- 75MHz bus speeds and up.
|Types of DRAM