Q. Features of read-only memory?

ROMs are memories on which it's not possible to write data when they are on-line to computer. They can only be read. This is reason why it is known as read-only memory (ROM). Because ROM chips are non-volatile, data stored inside a ROM aren't lost when power supply is switched off unlike case of a volatile RAM chip. ROMs are also called as permanent stores.

ROMs can be used for storing system programs, micro-programs and subroutines. ROMs are non-volatile in nature and require not be loaded in a secondary storage device. ROMs are fabricated in large numbers in a manner where there is no room for even a single error. However this is an inflexible process and needs mass production. So a new kind of ROM known as PROM was designed that is also non-volatile and can be written only once and therefore name Programmable ROM (PROM). Supplier or customer can perform writing process in PROM electrically. Special equipment is required to perform this writing operation. Thus PROMs are more flexible and convenient than ROMs.

ROMs / PROMs can be written just once but in both cases whatever is written once can't be changed. However what about a case where you read mostly but write just very few times? This led to concepts of read mostly memories and the best illustration of these are EPROMs (Erasable PROMs) and EEPROMs (Electrically Erasable PROMs).