The acronym for ROM is Read Only Memory. It means that when the data is burnt on a ROM, it cannot be removed nor it cannot be altered it can only read. ROM can save its contents even when the computer is turned off. Therefore ROM is known as nonvolatile. In most of the personal computers, a ROM stores programs such as the program that boots the computer. ROMs are widely used in calculators and other devices such as laser printers.

There are five basic ROM types: PROM, EPROM, EEPROM and Flash memory. ROM chips are costlier and time-consuming in small quantities. Developers created another type of ROM known as programmable read-only memory PROM. The PROM chips are inexpensively and easily available chips. The PROM chips have a grid aligned along the columns and rows as seen in ordinary ROM. The difference between these two ROM is that at every intersection a PROM chip has a fuse. When a charge is sent between a column, it will pass through the fuse of a cell and indicates a value 1. All the cells have a fuse, to change the value of a cell to 0, we send a specific amount of current to the cell. The higher voltage will breaks the connection by burning out the fuse. This process of burning of fuse is known as burning the PROM. Even though ROM & PROM are inexpensive chip, the cost adds up time to time.

Another improved version known as erasable programmable read-only memory EPROM answers this issue. EPROM chips can be rewritten many times. To Erase an EPROM we require a tool that emits ultraviolet (UV) light. The EPROMs are configured by EPROM programmer that provides voltage depending on the type of EPROM used. The grid of columns and rows in an EPROM consists of a cell at each intersection and has two transistors. These two transistors are separated using thin oxide layer. One transistor is known as the floating gate and the other transistor is known as the control gate. The floating gate only link to the row through the control gate. As long as there is a link is in place, the cell has a value of 1. To change the value to 0 a process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons. An electrical charge is usually of 10 to 13 volts is applied to the floating gate in order to change the levels. This charge causes the floating gate transistor to act as electron gun. The excited electrons are pushed on the other side of the thin oxide layer, by giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate.

The advanced type of ROM known as Electrically erasable programmable read-only memory (EEPROM) chips removes the drawbacks which are present in EPROMs: The chip need not to be removed to be rewritten, the EPROM chip need not have to be completely erased, the contents which has to be changed does not require additional equipment. Instead of using Ultra Violet light, we can simply return the electrons in the cells of an EEPROM to application of an electric field to each cell. This erases the content of EEPROM, which can be used for rewritten.

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