Read Only Memory (ROM)

Read-Only Memory (ROM) is a type of semiconductor memory that provides permanent data storage permitting only the reading of stored data. The name has become synonymous with non-volatile memory because the removal of power does not result in data loss. These types of devices became popular with the advent of dedicated microprocessor based systems, which required the storage of a system program that did not change. Consequently, ROM became synonymous for program storage space, whilst RAM for data storage space.

ROM memory access occurs in the same way as RAM, in as much as the processor addresses it, and fetches the data from a particular address location. However, this data can only ever be read and never altered.

The first ROM technology was extremely expensive because the ones and zeros were established by a metallisation interconnect mask. The manufacturing set-up cost of this process was high and employed only for high-volume productions. The introduction of user-programmable ROM (PROM) made this technology cheaper because the technology consisted of fusible links that could be "burned" by the user using a "programmer" device.

There are obvious differences between ROM and RAM memory such as ROM memory is permanent and non-volatile, whilst RAM memory is temporary and volatile. There are also differences in the semiconductor technologies employed and their circuit configurations.