In computers and other digital systems quite often it is required to store a bulk of information for digital computation, and for this purpose we use a device called a memory or a storage unit.
A memory unit consists of large number of storage registers and the information is stored semipermanently in it during data processing and some times the information may be stored permanently. The storage registers of memory unit are made up of a set of binary memory cells.
Each binary memory cell stores one bit. The memory units are named according to the type of binary cells they are made up of. A set of binary cells combine together to form a group cells and such a group is used to store a word. The words to be written or taken out are first entered into a register called Memory Buffer Register. All the addresses of various words are written in a specific register called Memory Address Register.
Memory unit is arranged in such a way that it has number of addressed locations each location storing a word.
The memory unit that communicates directly with the central processing unit is called main memory. The device that provide back up storage is called auxiliary memory. These storage devices plus algorithms needed to control or manage the stored information combined together form the memory system of the computer. CPU should have immediate and uninterrupted access to memory and it should be able to operate at its maximum speeds.