Fundamentals of Computer > Computer Memory > Types of ROM

Basically, there are two categories of ROM:

Manufacturer-programmed Read Only Memory

In this type of ROM, data is stored permanently by the manufacturer. It is not possible for a user to modify the programs stored inside the ROM chip.

For example, a PC manufacturer may store the system boot program permanently in the ROM chip used on the motherboard. Similarly, a printer manufacturer may store the printer controller software in the ROM chip used on the circuit board of the printer.

This ROM is also called MROM (mask read only memory) or just ROM. 

User-programmed Read Only Memory

In this type of memory, the user can store “read-only” programs and data on it. Such a ROM is commonly known as PROM (programmable read-only memory), because, a user can program it.

Other kinds of user-programmed ROM are EPROM (erasable programmable read-only memory), EEPROM (electrically erasable programmable read-only memory).

 

Programmable Read Only Memory (PROM):

This ROM is user-programmable i.e. it is not programmed during the manufacturing process but are custom-programmed by the user. Once programmed, a PROM cannot be erased or reprogrammed. Thus, if the program in the PROM is faulty or has to be changed, the PROM has to be thrown away. For this reason, this memory is often referred to as ‘one time programmable’ ROM.

Erasable Programmable Read Only Memory (EPROM):

An EPROM can be programmed by the user and it can also be erased and reprogrammed as often as desired. Once programmed, the EPROM is a nonvolatile memory that will hold its stored data indefinitely.  It can be erased by exposing it to ultraviolet light. Once erased, the EPROM can be reprogrammed.

EPROMs are used to store programs, which are permanent but need updating. Since, the contents of EPROM are erased by exposing the memory chip for some time to ultraviolet light; it is often termed as ultra violet EPROM (UVEPROM).

Electrically Erasable Programmable ROM (EEPROM):

This ROM can be electrically programmed, erased and reprogrammed.Contents in the EEPROM can be erased by using high voltage electric pulses. In EEPROM, instead of erasing the entire contents of the chip, its contents may be erased block-wise. It can be reprogrammed electrically either partly or fully. Due to the ease with which stored programs can be altered, EEPROM is also known as flash memory, which is used in many new I/O and storage devices.

Difference Between EPROM and EEPROM:

• EEPROM has the ability to electrically erase and reprogram individual words in the memory array.
• It is possible to erase a selected memory cell in EEPROM but there is no way to erase a selected memory cell in EPROM, once erased, all contents of entire EPROM is erased.
• It is easier to alter stored information from an EEPROM chip, as compared to EPROM.
• An EEPROM can be erased and reprogrammed quickly and easily than the EPROM (10 ms vs. 30 minutes).

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