ROM (Read Only Memory) Types : PROM - EPROM - EEPROM

 

Read Only Memory (ROM):

 

ROM is a non-volatile primary memory. It does not lose its content when the power is switched off. ROMs are also known as field stores, permanent stores, or dead stores.

 

The features of ROM are as follows:

 

ROM, as the name implies, has only read capability and no write capability. After the information is stored in ROM, it is permanent and cannot be corrected.

 

ROM comes programmed by the manufacturer. It stores standard processing programs that permanently reside in the computer.

 

ROM stores the data needed for the start-up of the computer. The instructions that are required for initializing the devices attached to a computer are stored in ROM.

 

The ROM memory chip stores the Basic Input Output System (BIOS). BIOS provide the processor with the information required to boot the system. It provides the system with the settings and resources that are available on the system.

 

BIOS is a permanent part of the computer. It does not load from disk but instead is stored in a ROM memory chip. The program code in the BIOS differs from ordinary software since it acts as an integral part of the computer.

When the computer is turned on, the BIOS do the following things:

 

Power On Self Test (POST) is a program that runs automatically when the system is booted. BIOS perform the power-on self-test. It checks that the major hardware components are working properly.

 

BIOS setup program, which is a built-in utility in BIOS, lets the user set the many functions that control how the computer works. BIOS display the system settings and finds the bootable devices. It loads the interrupt handlers and device drivers.

It also initializes the registers.

 

Bootstrap Loader is a program whose purpose is to start the computer software for operation when the power is turned on. It loads the operating system into RAM and launches it. It generally seeks the operating system on the hard disk. The bootstrap loader resides in the ROM. The BIOS initiates the bootstrap sequence.

 

Types of Read Only Memory (ROM):

 

ROMs are of different kinds. They have evolved from the fixed read only memory to the ones that can be programmed and re-programmed.

They vary in the number of re-writes and the method used for the re-writing. They can be categorized as follows:

1.    Programmable ROM (PROM)

2.    Erasable Programmable ROM (EPROM) and

3.    Electrically Erasable Programmable ROM (EEPROM).

 

All the different kinds of ROM retain their content when the power is turned off.

 

 

Programmable Read Only Memory (PROM):

 

There are two types of ROMs - manufacturer programmed and user programmed. A manufacturer programmed ROM is one in which data is burnt in by the manufacturer of the electronic equipment in which it is used. 

 

For example, a personal computer manufacturer may store the system boot program permanently in a ROM chip located on the motherboard of each PC manufactured by it. 

 

Manufacturer programmed ROMs are used mainly in those cases where the demand for such programmed ROMs is large.  Manufacturer programmed ROM chips are supplied by the manufacturers of electronic equipment and it is not possible for a user to modify the programs or data stored in ROM chip.

 

A user programmed ROM is a one in which a user can load and store “read only” programs and data. That is, it is possible for a user to “customize” a system by converting his or her programs to micro programs and storing them in a user programmed ROM chip.

 

Such a ROM is commonly known as Programmable Read Only Memory (PROM), because a user can program it. Once the user programs are stored in a PROM chip, they can be executed usually in a fraction of the time previously required.

 

PROMs are programmed to record information using a special device known as PROM programmer. PROM memories have thousands of fuses (or diodes). High voltage (12 V) is applied to the fuses to be burnt. The burnt fuses correspond to 0 and the others to 1.

 

However, once the chip has been programmed, the PROM becomes a ROM. That is, the information recorded in it can only be read and it cannot be changed. PROM is also a non-volatile storage.

 

 

Erasable Programmable Read Only Memory (EPROM):

 

Once the information is stored in a ROM or PROM chip, it cannot be altered. Erasable Programmable ROM overcomes this problem.

 

As the name implies, it is possible to erase information stored in an EPROM chip and the chip can be reprogrammed to store new information.

 

EPROMs are often used by R&D personnel who frequently change the micro programs to test the efficiency of a computer system with new programs.

 

EPROMs are also useful for those applications in which one may like to store a program in a ROM that would normally not change but under some unforeseen conditions, one may like to alter it.

 

When an EPROM is in use, information stored in it can only be “read” and the information remains in the chip until it is erased. EPROM chips have to be removed from the computer for re-writing.

 

EPROM chips are of two types - one in which the stored information is erased by exposing the chip for some time to ultraviolet light and the other one in which the stored information is erased by using high voltage electric pulses.

 

The former is known as Ultra Violet EPROM (UVEPROM) and the latter is known as Electrically Erasable PROM (EEPROM). It is easier to alter information stored in an EEPROM chip as compared to an UVEPROM chip. EEPROM chips do not have to be removed from the computer for re-writing.

 

EEPROM is also known as a flash memory because of the ease with which programs stored in it can be altered. Flash memory is used in many new IO and storage devices like USB pen drive and MP3 music player. 

 

 

Flash Memory:

 

Flash Memory is a kind of semiconductor-based non-volatile, rewritable compute memory that can be electrically erased and reprogrammed. It is a specific type of EEPROM.

It combines the features of RAM and ROM. It is a random access memory and its content can be stored in it at any time. However, like ROM, the data is not lost when the machine is turned off or the electric power is cut.

Flash memory stores bits of data in memory cells. Flash memories are high-speed memories, durable, and have low-energy consumption. Since flash memory has no moving part, it is very shock-resistant.

 

Due to these features, flash memory is used in devices such as digital camera, mobile phone, printer, laptop computer, and record and play back sound devices, such as MP3 players.