A memory unit is an internal part of any microcomputer system and its primary purpose is to hold programs and data. In a board sense a microcomputer memory system can be logically divided into three groups.

Processor memory refers to the microprocessor registers. These registers are used to hold temporary results when a computation is in progress. Also there is no speed disparity between these register and the microprocessor because they are fabricated using the same technology.

Primary or main memory is the storage area in which all programs are executed. The microprocessor can directly access only those items that are stored in primary memory.

Secondary memory refers to the storage medium comprising slow device such as magnetic tapes and disks. These devices are used to hold large data files and huge programs such as compilers and data base management systems which are not needed by the processor frequently. Secondary memory stores programs and data in excess of the main memory. There are two types of optical disks.