Data and instructions entered into a computer system through input units have to be stored inside the computer before actual processing starts. Similarly, results produced by a computer after processing have to be kept some where inside the computer system before having being passed on to an output unit. Moreover, a computer must also preserve intermediate results for ongoing processing. Storage unit of a computer system caters to all these needs. It provides space for storing data and instructions, intermediate results, and results for output.
In short, a storage unit holds (stores):-
The data and instructions required for processing (received from input units).
Intermediate results of processing.
Final results of processing, before of system releases them to an output unit.
Storage unit of all computers is comprised of following two types of storage:-
1. Primary storage. Primary storage of a computer system, also known as main memory, stores pieces of program instructions data, intermediate results of processing, and recently produced results of those jobs(s) on which the computer system is currently working. The central processing unit can access these prices of information directly at a very fast speed because they are represented electronically in the main memory chip's circuitry. However, primary storage is volatile, and it loses the information in it as soon as the computer system switches off or resets. Moreover, primary storage normally has limited storage capacity because it is very expensive. Primary storage of modern computer systems is made up of semiconductor devices.
2. Secondary storage. Secondary storage of a computer system, also known as auxiliary storage, takes care of the limitations of primary storage. It supplements the limited storage capacity and volatile characteristic of primary storage. This is because secondary storage is much cheaper than primary storage and it can retain information even when the computer system switches off or resets. A computer system uses secondary storage to store program instructions, data and information of those jobs on which the computer system is currently not working but needs to hold them for processing later. The most commonly used secondary storage medium is magnetic disk.