CACHE MEMORY

The word cache is pronounced as cash.The cache memory is placed in between CPU and main memory.It is main memory.It is a semiconductor memory.It consists of static RAMs.Its access time is about 10 ns which is much less than that of the main memory.The access time of the main memory is about 50ns.The capacity of the cache memory is 2 to 3 percent of that of the main memory.It stores instruction codes and data.which  are t be currently executed by the CPU.It is used to reduce the average access time for instructions and data,which are normally stored in the main memory. A cache memory also needs a cache controller.Cache controller ICs are available.

The modern 32-bit and 64-bit microprocessors operate at very high speed.Their clock rates are in the ranges of 400 MHz to 1000 MHz .The memory matching with high-speed microprocessor must be very fast.But very fast memory is very expensive. If a fast microprocessor operates with conventional main memory,it has to operate with several wait states.This will reduces the speed of the computer.A compromise is made,and data to CPU.The main memory stores program and data,which is to be processed by the CPU.The currently needed instructions and data of the program are loaded into the cache from the main memory.

There are two types of cache schemes:write-through and write-back.

In a write through cache the main memory is updates each time the CPU writes into the cache.The advantage of the write-through cache is that the main memory always contains the same data as the cache contains.This characteristics is desirable in a system which uses direct memory access scheme of data transfer.