The ROM is Read Only Memory that can only be read by the CPU but it cannot be changed or modified as the name suggests. The Basic Input/Output System (BIOS) is stored in the ROM that checks and initializes various peripherals attached to the PC when the switch is turned ON. “BIOS” is a pre-programmed software that controls the computer’s start up processes and other basic components such as keyboard, monitor/ display, disk drives, etc. The programs that are preloaded in the ROM are also known as firmware. The memories, which do not lose their content on failure of power supply, are known as non-volatile memories. ROM is a non-volatile memory. This area is only readable by the user and cannot be used for storing data.
ROM can be categorized into two types:
1. Permanent Rom
In this ROM, a bit pattern is permanently recorded by the masking and metallization process. It is an expensive and specialized process.
It is Programmable Read Only Memory. This memory has ni-chrome or polysilicon wires arranged in a matrix. These wires can be functionally viewed as diodes or fuses. This memory can be programmed by the user with special software that selectively burns the fuses according to the bit pattern to be stored. You know that it is not possible to modify or erase programs stored in ROM, but it is possible for you to store your program once in PROM chip. Once the programs are written, these cannot be changed and remain intact even if power is switched off. Therefore, programs or instructions written in PROM cannot be erased or changed.
2. Semi Permanent Rom
This stands for Erasable Programmable Read Only Memory, which overcomes the problem of PROM and Masked ROM. EPROM chip can be programmed time and again by erasing the information stored earlier in it. It uses the technology of charging gates, using high voltage. Information stored in EPROM can be erased by exposing it to ultraviolet light.
This stands for Electrically Erasable Programmable Read Only Memory. This concept is same as that of EPROM. The only difference is that the memory can be altered using electrical signals. The whole of the memory need not be erased.