Read Only Memory (ROM)

In a computer system, memory is a very essential part of the computer system and used to store information for instant use or permanently. Based on computer memory working features, memory is divided into two types. 

• Volatile Memory (RAM)
• Non-volatile Memory (ROM)

Before understanding ROM, we will first understand what exactly volatile and non-volatile memory is. Non-volatile memory is a type of computer memory that is used to retain stored information during power is removed. It is less expensive than volatile memory. It has a large storage capacity. ROM (read-only memory), flash memory are examples of non-volatile memory. Whereas volatile memory is a temporary memory. In this memory, the data is stored till the system is capable of, but once the power of the system is turned off the data within the volatile memory is deleted automatically. RAM is an example of volatile memory.

What is Read-Only Memory (ROM)?

ROM stands for Read-Only Memory. It is a non-volatile memory that is used to stores important information which is used to operate the system. As its name refers to read-only memory, we can only read the programs and data stored on it. It is also a primary memory unit of the computer system. It contains some electronic fuses that can be programmed for a piece of specific information. The information stored in the ROM in binary format. It is also known as permanent memory.

Features of ROM (Read-Only Memory):

• ROM is a non-volatile memory.
• Information stored in ROM is permanent.
• Information and programs stored on it, we can only read.
• Information and programs are stored on ROM in binary format.
• It is used in the start-up process of the computer.

Types of Read-Only Memory (ROM):

1. MROM (Masked read-only memory)
2. PROM (Programmable read-only memory)
3. EPROM (Erasable programmable read-only memory)
4. EEPROM (Electrically erasable programmable read-only memory)

Now we will discuss the types of ROM one by one:

1. MROM (Masked read-only memory): We know that ROM is as old as semiconductor technology. MROM was the very first ROM that consists of a grid of word lines and bit lines joined together transistor switches. This type of ROM data is physically encoded in the circuit and only be programmed during fabrication. It was not so expensive.

2. PROM (Programmable read-only memory): PROM is a form of digital memory. In this type of ROM, each bit is locked by a fuse or anti-fuse. The data stored in it are permanently stored and can not be changed or erasable. It is used in low-level programs such as firmware or microcode.

3. EPROM (Erasable programmable read-only memory): EPROM also called EROM, is a type of PROM but it can be reprogrammed. The data stored in EPROM can be erased and reprogrammed again by ultraviolet light. Reprogrammed of it is limited. Before the era of EEPROM and flash memory, EPROM was used in microcontrollers.

4. EEPROM (Electrically erasable programmable read-only memory): As its name refers, it can be programmed and erased electrically. The data and program of this ROM can be erased and programmed about ten thousand times. The duration of erasing and programming of the EEPROM is near about 4ms to 10ms. It is used in microcontrollers and remote keyless systems. 

Advantages of ROM:

• It is cheaper than RAM and it is non-volatile memory.
• It is more reliable as compared to RAM.
• Its circuit is simple as compared to RAM.
• It doesn’t need refreshing time because it is static.
• It is easy to test.

Disadvantages of ROM:

• It is a read-only memory, so it cannot be modified.
• It is slower as compared to RAM.

Difference between PROM and EPROM.

PROM	EPROM
The data stored in PROM is permanently stored and cannot be changed and erased.	The EPROM can be reprogrammed and reusable multiple times.
PROM is not expensive compared to EPROM.	EPROM is more expensive than PROM.
A bipolar transistor is used in PROM.	A MOS transistor is used in EPROM.
PROM is more flexible than EPROM.	EPROM is less flexible than PROM.
PROM is used in low-level programs such as firmware or microcode.	EPROM is used in microcontrollers.

Sample Problems

Question 1. What is ROM?

Solution:

ROM is a non-volatile memory that is used to stores important information and program on it. 

Question 2.  In which ROMs have the capabilities to rewrite again?

Solution:

EPROM and EEPROM.

Question 3. In which format information stored on ROM?

Solution:

Binary format.

Question 4. Why ROM is called non-volatile memory?

Solution:

ROM does not lose information when power is removed. 

Question 5. What is called each bit combination that comes out of the output lines in ROM?

Solution:

Address.

Question 6. What kind of circuits are used in ROM?

Solution:

ROM is a combinational circuit. It is a combination of different ICs. 

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