Read-only memory ( ROM ) is a class of storage medium used in computers and other electronic devices. Data stored in ROM can only be modified slowly or with difficulty, so it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to need frequent updates). Strictly, read-only memory refers to memory that is hard-wired, such as diode matrix and the later mask ROM . Although discrete circuits can be altered in principle, ICs cannot and are useless if the data are bad. Despite the simplicity, speed and economies of scale of mask ROM, field-programmability often make reprogrammable memories more flexible and inexpensive. As of 2007, actual ROM circuitry is therefore mainly used for applications such as microcode, and similar structures, on various kinds of processors. Other types of non-volatile memory such as erasable programmable read only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM or