Read-only memory (ROM) is non-volatile main memory - this means that its contents are not lost 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 Basic Input Output System (BIOS) 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 runs a second program known as the bootup or bootstrap program that loads the computer's operating system from the hard drive into the RAM.