Most high-end Pentium processors use a nonblocking cache for L2 data store. For
example, the Pentium Pro and Pentium II microprocessors support up to four
nonblockingrequestssimultaneouslyontheIntelDIB(dualindependentbus)architecture.Cache Mapping
Some Pentium systems split the L1 cache and store data and instructions separately in
their own cache partitions. This requires amapping technique, which defines how the
cache contents are stored and referenced. A mapping technique sets the functional
features of the cache, including its hit ratio and transfer speed.
The three mapping techniques used with L1 caching are: Direct mapped cache Most motherboard mounted caches are of this type.
This mapping technique uses a simple 4-byte index to track which RAM
addresses are stored in the cache. This approach is the least complex of the
mapping techniques. It has drawbacks stemming from the method used for
indexing which can create duplicate references.
Fully associative cache The name of this mapping technique refers to the fact
that all data stored in cache is associated with its address in RAM, which is also
called itstag. Fully associative caching uses additional memory to hold the tags
associated with the data stored in cache. Complicated search algorithms are
used to locate the cached data. It can be slow, but it provides the best hit ratios.
N-way set associative cache The cache is divided into sets, which haven
cache lines each, typically 2, 4, 8, and more. This mapping technique, which is
a combination of the other two mapping techniques, provides better hit ratios
than direct mapped cache without the speed impact of a complicated search.
Processor-based L1 caches commonly apply either a 2-way or 4-way set
associative cache.Cache Mounts
Older PCs, namely 486s and early Pentiums, install SRAM chips directly on the mother-
board in individual sockets, which means the cache can be added, replaced, or upgraded.
Newer systems install external cache as fixed chips, usually soldered, directly on the
motherboard. If your PC mounts its cache in sockets, you may be able to add additional
SRAM to increase the size of the cache.
There are some motherboard types available that, although they have soldered SRAM
ontheboard,alsoallowcachemodulestobeaddedtoatleastoneopensocket,usuallywith
ajumpersettingortwo.IfyoucanaddSRAMtoyoursystem,itssizeandtypewillbesetby
the motherboard and chipset. Check your motherboard’s documentation or visit its manu-
facturer’s Web site to learn the specification of the cache you can add, if any.
A commonly used packaging form is theCOAST, which stands forcache on a stick.A
COAST module looks something like the SIMM (single inline memory module) memory
module and an Intel module is 4.35 inches wide and 1.14 inches high. However, this is notChapter 8: Cache Memory^165