Read Only Memory (ROM) is an integrated circuit which is pre-programmed with specific functional data at manufacturing time. It is also called Firmware. These ROMs are not limited to computers only. Most of the electronic gadgets utilize its flexible functionality. In this post we will try to understand What is ROM, How it works, its types, applications, advantages and disadvantages.

What is Read Only Memory (ROM)

ROM is the acronym for Read Only Memory. ROM is a type of Primary Memory. As the name suggests its contents can be read only but cannot write on it. It is a non-volatile memory and so the data is retained even when the power is switched off.

The data that is required to be stored inside ROM is written during manufacturing phase. It stores such programs that are essential for the booting process of the computer. It generally cannot be altered. However, technologies are available to program these types of ROM.

Classification of Primary Memory

Fig.1 – Classification of Primary Memory

How Read Only Memory (ROM) Works

A ROM operates like an array. ROM chips contain a grid of rows and columns to turn ON or OFF. It uses a diode to connect the lines if the value is 1. If the value is 0, then these lines are not connected at all. Each element of the array corresponds to one storage element in the memory chip.

The address input to the chip is employed to pick out a specific memory location (corresponding to the array index). The value read from the memory chip corresponds to the contents of the selected element of the array.

Diode working in Read Only Memory (ROM)

Fig.2 – Diode Grid in Read Only Memory (ROM)

ROM consists of two basic components- Decoder and OR gates. In ROM, the input to decoder will be in binary form and output will be its decimal equivalent. All the OR Gates present in the ROM will take decoders output as their input.

Types of Read Only Memory (ROM)

ROM is differentiated on the basis of methods used to write data on ROM chips and the number of times they can be written. It can be classified into following types : –

  • Mask Read-Only Memory (MROM)
  • Programmable Read-Only Memory (PROM)
  • Erasable Programmable Read-Only Memory (EPROM)
  • Electrically Erasable Programmable Read-Only Memory (EEPROM)
  • Flash Read-Only Memory (Flash ROM)

Mask Read Only Memory (MROM)

MROM stands for Mask Read Only Memory. It is a memory chip that is manufactured with its contents. These are inexpensive and are the very first ROMs which were hard wired devices that contain a pre-programmed set of data or instructions.

Mask Read Only Memory (MROM)

 Fig. 3 – Mask Read Only Memory (MROM)

These chips contain a software mask that is burned onto the chip during design phase of the semiconductor manufacturing process.

The specification of the ROM is taken by the manufacturer from its customer in tabular form and in a particular format. The manufacturer then makes the corresponding mask for the paths to produce the desired output.

Applications of Mask Read Only Memory (MROM)

The Mask Read-Only Memory (MROM) are used for:

  • Network Operating Systems.
  • Server Operating Systems.
  • Storing fonts for laser printers.
  • Storing sound data in electronic musical instruments.

Advantage of Mask Read Only Memory (MROM)

The main advantage of Mask Read-Only Memory (MROM) is its low production cost. The cost of IC depends on its size, per bit. Mask ROM is more compact. It is significantly cheaper than any other kind of secondary memory when large quantities of same ROM are manufactured.

Disadvantage of Mask Read Only Memory (MROM)

Design errors are costly i.e. if an error in the code is detected, the MROM is useless and must be replaced in order to change the code. The life expectancy of MROM is also short, hence requires frequent replacement.

Programmable Read Only Memory (PROM)

PROM stands for Programmable Read Only Memory. PROM is manufactured as a blank memory. And as its name suggests Programmable, it is programmed after manufacturing. The user buys a blank memory and enters the desired contents using a PROM program.

Programmable Read Only Memory (PROM)

Fig. 4 – Programmable Read Only Memory (PROM)

The process of programming a PROM is called burning the PROM. There are tiny fuses in a PROM chip which are burnt open during programming. The data can be programmed only once and cannot be altered. So it is called one- time programming device.

Applications of Programmable Read Only Memory (PROM)

The Programmable ROM (PROM) are used in:

  • Mobile Phones for providing User Specific Selections.
  • Video game consoles
  • Implantable Medical devices.
  • Radio-Frequency Identification (RFID)tags.
  • High definition Multimedia Interfaces(HDMI)

Advantages of Programmable Read Only Memory (PROM)

The advantages of Programmable ROM (PROM) are: –

  • The programming can be done using many types of software and does not rely on hard wiring of the program to the chip.
  • Since it is not possible to un-blow the fuse, so the authenticity of the data remains intact and it is impossible to remove or alter the contents.

Disadvantage of Programmable Read Only Memory (PROM)

The biggest disadvantage of PROM is that the data once burnt cannot be erased or changed when detected with errors.

Erasable Programmable Read Only Memory (EPROM)

EPROM stands for Erasable Programmable Read-Only Memory. It is a non volatile memory i.e. it can retain data even if the power supply is cut off. The basic limitation being encountered in PROM is that once it is programmed, it cannot be changed or altered. This limitation has been overcame by EPROM.

EPROM can be erased by exposing it to ultra violet light for a particular length of time using an EPROM eraser. After exposing, the chip returns to its initial state and can be reprogrammed.

This procedure can be carried out many times but repeated erasing and rewriting can eventually render the chip useless. Once written, data can be retained for about 10 years.

Erasable Programmable Read Only Memory (EPROM)

Fig. 5 – Erasable Programmable Read Only Memory (EPROM)

Applications of Erasable Programmable Read Only Memory (EPROM)

The applications of Erasable Programmable ROM (EPROM) includes:

  • As program storage chip in Micro controllers.
  • For debugging.
  • For program development.
  • As BIOS chip in computers.
  • As program storage chip in modem, video card and many electronic gadgets.

Advantages of Erasable Programmable Read Only Memory (EPROM)

The advantages of Erasable Programmable ROM (EPROM) are:

  • It is non-volatile.
  • It can be erased and re -programmed.
  • It is cost effective as compared to PROM.

Disadvantages of Erasable Programmable Read Only Memory (EPROM)

The disadvantages of Erasable Programmable ROM (EPROM) are:

  • The static power consumption is high as the transistors used have higher resistance.
  • It is not possible for a particular byte to be erased, instead the entire content is erased.
  • UV based EPROM takes time to erase the content.

Electrically Erasable Programmable Read Only Memory (EEPROM)

EEPROM is the short form for Electrically Erasable Programmable Read Only Memory. It is similar to EPROM and thus developed to overcome the drawbacks of EPROMs. It is erased and programmed electrically i.e. it uses electrical signals instead of ultra violet rays.

The erasing and programming of data takes 4 to 10 milliseconds. Any byte can be erased at a time instead of the entire chip. The chip can be erased and re programmed for around ten thousand times, though the process is flexible but slow.

Electrically Erasable Programmable Read Only Memory (EEPROM)

Fig. 6 –Electrically Erasable Programmable Read Only Memory (EEPROM)

Applications of Electrically Erasable Programmable Read Only Memory (EEPROM)

The applications of Electrically Erasable Programmable ROM (EPROM) includes:

  • As BIOS chip in computers
  • As storage for re-programmable calibration information in test-equipment.
  • As storage for in-built self learning functionality in remote operated transmitters.

Advantages of Electrically Erasable Programmable Read Only Memory (EEPROM)

The advantages of Electrically Erasable Programmable ROM (EEPROM) are:

  • The method of erasing is electrical and instant.
  • Chip can be reprogrammed infinite number of times.
  • Byte wise data can be erased instead of entire content on the board.
  • To change the data, additional devices are not required.

Disadvantages of Electrically Erasable Programmable Read Only Memory (EEPROM)

The disadvantages of Electrically Erasable Programmable ROM (EEPROM) are:

  • Different voltages are required for erasing, reading and writing the data.
  • The data retention period of EEPROM is limited i.e 10 years approx.
  • EEPROM devices are expensive compared to others.

Flash Read Only Memory (Flash ROM)

It is a universal flash programming non volatile utility, used in computer as a storage medium. It can be electrically erased and reprogrammed. In this, memory blocks of data (512 bytes) can be deleted and written at a particular time.

Flash ROM

Fig. 7 – Flash Read Only Memory (Flash-ROM)

Applications of Flash Read Only Memory (Flash ROM)

The applications of Flash Read-Only Memory (Flash ROM) are:

  • The latest technology computers use BIOS stored on a flash memory chip, called as flash BIOS.
  • Modems, pen drives, small cards use flash ROM.

Advantages of Flash Read Only Memory (Flash ROM)

The Advantages of Flash Read-Only Memory (Flash ROM) are:

  • High transferring speed.
  • It saves data when turns OFF, preserve its state without power.
  • Less prone to damage.
  • Comparatively economical to other drives in small storage capacities.

Disadvantages of Flash Read Only Memory (Flash ROM)

The disadvantages of Flash Read-Only Memory (Flash ROM) are:

  • Comparatively costly than hard disk.
  • Number of read/writes are limited.
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Arisha is an MCA and BSc (IT) and has work experience as PGT Computer Science. She is an author, editor and partner at Electricalfundablog.