ROM refers to Read Only Memory. It is a type of memory in the computer that we can only read the data on it, but we cannot write on it. ROM is mainly used to store instructions used to boot up a computer. This type of memory is non-volatile. Therefore, data stored in it remains intact even when there is a sudden power loss. ROM memory chips contain rows and columns and a diode present at each point of intersection. If the lines are connected the value is 1(one) and, it is 0(zero) if there is no connection at all.
PROM (Programmable Read Only Memory).
PROM is a computer memory chip that can be programmed only once by the user after its creation. A programmer purchases a blank PROM and stores the desired information into the chip. The information on it becomes permanent and cannot be modified. A PROM chip has fuses that get burned open during programming. It, therefore, becomes impossible to reprogram the memory chip. Earlier computers are which made of PROMs in their Bios systems.
EPROM (Erasable Programmable Read Only Memory).
Unlike the PROM which is programmed only once, EPROM is reprogrammable. Therefore, a programmer can erase or modify the data written on it and replace it with the desired information. Data on an EPROM is erased by exposing the chip to ultra-violet rays for up to a duration of 40minutes. An EPROM can easily be identified from other memory chips due to the transparent fused quartz window present on top of the package. EPROM is more commonly used as it gives manufacturers and programmers a chance to modify and reprogram the memory chip.