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Read only memory

Read only memory (ROM) is non-volatile primary storage. It keeps its contents when the computer is turned off.

ROM can be read from but not written to, hence the term read only. This makes ROM ideal for storing instructions and data that are needed for the computer to run. These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten.

The BIOS (Basic Input Output System) is an example of a program that is stored in ROM. The BIOS runs as soon as the computer is switched on. It checks that the hardware is functioning correctly then loads the computer's operating system. Because the BIOS is always needed it is stored in ROM.

Random access memory

Random access memory (RAM) is volatile primary storage. Once the computer is switched off the data and instructions held in RAM are lost. RAM is given the term ‘random access’ because data and instructions can be stored and accessed from any location within the memory).

RAM is used to hold data and instructions that are currently in use. In a modern PC, RAM is used to hold the operating system and any open documents and programs that are running.

The contents of RAM can be changed at any time by overwriting it with other data and instructions. For example, a user might close one document and open another or run a different program.

The more RAM a computer has, the more data and program instructions it can hold simultaneously. RAM can also be upgraded easily, unlike other types of primary storage.