F
or some unexplainable reason, the major components of the PC—the microproces-
sor, the memory, motherboard data bus, the hard disk drive, and so on—all operate
at different speeds. One would think that they would all be coordinated to operate
together. Well, to a certain extent they do, but by and large they are all developed by
different companies who are in competition to develop the fastest, biggest, and best
computer component.
The two components that must work together closely and constantly are the CPU
(microprocessor) and primary memory (RAM). Unfortunately, RAM is faster than the
CPU. It is also the design goal of every PC to have the CPU idle as little as possible. If the CPU
requests data from RAM, the data must be located and then transferred over the data bus
to the CPU. Regardless of how fast RAM is, the CPU must wait while these actions are
carried out. This is where caching comes in.Cache on the PC
Cache memoryis very fast computer memory that is used to hold frequently requested
data and instructions. As you will see later, it is a little more complicated than that, but
cache exists to hold at the ready data and instructions from a slower device (or a process
that requires more time) for a faster device. On today’s PCs, you will commonly find
cache between RAM and the CPU and perhaps between the hard disk and RAM. Acache
isanybufferstorageusedtoimprovecomputerperformancebyreducingitsaccesstimes.
AcacheholdsinstructionsanddatalikelytoberequestedbytheCPUforitsnextoperation.
Caching is used in two ways on the PC: Cache memory A small and very fast memory storage located between the
PC’s primary memory (RAM) and its processor. Cache memory holds copies
of instructions and data that it gets from RAM to provide high-speed access
by the processor.
Disk cache To speed up the transfer of data and programs from the hard disk
drive to RAM, a section of primary memory or some additional memory placed
on the disk controller card is used to hold large blocks of frequently accessed data.SRAM and Cache Memory
Cache memory is usually a small amount of static random access memory or SRAM (see
Chapter 7 for more information on SRAM). SRAM is made up of transistors that don’t
need to be frequently refreshed (unlike DRAM, which is made u pof ca pacitors and must
be constantly refreshed).
SRAM has access speeds of 2ns (nanoseconds) or faster; this is much faster than
DRAM, which has access speeds of around 50ns. Data and instructions stored in
SRAM-based cache memory are transferred to the CPU many times faster than if the data
were transferred from the PC’s main memory. In case you’re wondering why SRAM isn’t(^158) PC Hardware: A Beginner’s Guide