ROM (Read Only Memory)

It is the programming chip, where all of the system information is kept at the time of manufacture. We can see the information of ROM at the time of booting the computer. We can only read the information of ROM; we can't write/edit or delete the information available on it. The ROM allows you to read the content of that memory as many times as desired but does not allow modifications. The contents of this memory can not be edited by the users. ROMs are non-volatile and non-destructive. They are permanent.
When we turn on the computer, the CPU gets its first instructions from the ROM. These instructions are burnt into the chip with the help of special equipment before the chip is installed inside the computer. Users do not have access to the ROM, so they can't change these instructions. It is electrically independent. The data stored in ROM is called firmware.
Mainly, there are three types of ROM is below :
a) PROM (Programmable Read Only Memory)
This is basically a blank ROM chip that can be written to, but only once. It is much like a CD-R drive that burns the data into the CD. Some companies use special machinery to write PROMs for special purposes.
b) EPROM (Erasable Programmable Read Only Memory)
This is just like PROM, except that you can erase the ROM by shining a special ultraviolet light into a sensor a top the ROM chip for a certain amount of time. Doing this wipes the data out, allowing it to be rewritten.
c) EEPROM (Electrically Erasable Programmable Read Only Memory)
The PROM's whose contents can be erased electrically are called EEPROM. EEPROM can be programmed up to 10,000 times. It is also called flash BIOS. This ROM can be rewritten through the use of a special software program. Flash BIOS operates this way, allowing users to upgrade their BIOS.