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 store 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.

Block Diagram of ROM

As shown in below diagram, there are k input lines and n output lines in it. The input address from which we wish to retrieve the ROM content is taken using the k input lines. Since each of the k input lines can have a value of 0 or 1, there are a total of 2 k addresses that can be referred to by these input lines, and each of these addresses contains n bits of information that is output from the ROM.

A ROM of this type is designated as a 2k x n ROM.

Block Diagram of ROM

Internal Structure of ROM

The internal structure of ROM have two basic components.

• Decoder
• OR gates

Internal Structure of ROM

A circuit known as a decoder converts an encoded form, such as binary coded decimal, or BCD, into a decimal form. As a result, the output is the binary equivalent of the input. The outputs of the decoder will be the output of every OR gate in the ROM. Let’s use a 64 x 4 ROM as an example. This read-only memory has 64 words with a 4 bit length. As a result, there would be four output lines. Since there are only six input lines and there are 64 words in this ROM, we can specify 64 addresses or minimum terms by choosing one of the 64 words that are available on the output lines from the six input lines. Each address entered has a unique selected word.

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 and cannot modified.
• 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)

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.

For more refrence you can refer Difference between PROM and EPROM – GeeksforGeeks article.

FAQs on Read Only Memory(ROM)

Q.1: Can I store my data in ROM?

Answer:

No, During manufacturing ROM is pre-programmed. ROM cannot be easily modified by Programmers. It is designed to reserve data that must unchanged, firmware and, system instructions.

Q.2: How long can data be retained in ROM?

Answer:

Data saved in ROM can be saved for many years, maybe even decades. The information saved in the ROM chip endures for a long time as lengthy as the physical integrity of the chip is preserved.

Q.3: In which format information stored on ROM?

Answer:

In Binary format information stored on ROM.

Q.4: Why ROM is called non-volatile memory?

Answer:

ROM is called non- volatile memory because ROM does not lose information when power is removed. 

Q.5: Is data in ROM secure?

Answer:

Yes, data or information saved in ROM is secure from unauthorized modifications. Since ROM is read-only, the data cannot be easily changed. ROM provide safety for critical instructions and data.

Q.6: What kind of circuits are used in ROM?

Answer:

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