What is ROM and types of ROM ?

What is ROM and types of ROM?

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As the name implies, a read-only memory (ROM) is a memory unit that that performs the read operation only; it does not have a write capability. This implies that the binary information stored in a ROM is made permanent during the hardware production of the unit and cannot be altered by writing different words into it (non-volatile). Whereas a RAM is a general- purpose device whose contents can be altered during the computational process, a ROM is restricted to reading words that are permanently stored within the unit. The binary information to be stored, specified by the designed, is the embedded in the unit to form the required interconnected pattern.

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 ROMs come with special internal electronic fuses that can be “programmed” for a specific configuration. Once the pattern is established, it stays within the unit even when power is turned off and on again. ROMs are used for applications in which it is known that the information never needs to be altered, for example monitor program controlling a machine. These, however are slower than RAM.

There are various types of ROM which are given below: -

1)   Programmable Read Only Memory (PROM) is a ROM that can be programmed to record information using a facility known as PROM-programmer. Once the chip has been programmed, the recorded information cannot be changed i.e., PROM become same as ROM.

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2)  Erasable Programmable Read Only Memory (EPROM) is another type of ROM that can be erased and the chip can be reprogrammed to record different information using a special PROM- program facility. Erasure is achieved by exposing the chip to ultraviolet light. When an EPROM is in use, information can only be “read” and the information remains on the chip unit it is erased. EPROM are of two kinds- UPROM (ultraviolet PROM) and EAPROM (electrically alterable PROM).

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3)  Electrically Erased PROM (EEPROM). This type of ROM can be programmed and erased by electrical signals. It does not require exposure to ultraviolet light to erase its contents as EPROM memory does, and provides an easy means to load and store temporary or permanent information in a form of ROM memory. Information loaded in this memory can be retained for many years without any power supplied; one of its new applications is as a back-up to RAM memory whose contents are lost in a power failure. When power is returned, the EEPROM can be used to replace the lost contents of the RAM memory and the microcomputer can continue working just as if nothing had happened. Even newer devices are combining RAM and EEPROM memory in a single integrated circuit.

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What is ROM and types of ROM ?