What is Read-Only Memory (ROM)? Definition and Types

What is Read-Only Memory (ROM)?

Memory is an essential part of a computer system that is used for temporarily or permanently storing data. There are two types of memory in a computer, RAM, and ROM.

ROM stands for Read-Only Memory, which we also refer to as non-volatile memory. It's a type of non-volatile storage because it retains information even when you turn off your computer or shut it down. In short, data stored in ROM is stable, and we use it to store static programs and files intended for long-term storage. The information (patterns) remain in the cells even without power.

Like Random Access Memory (RAM), it serves as the computer's primary memory unit. Don't confuse the two, though – there are several key differences between them. ROM is a type of solid-state semiconductor memory used to run an operating system by reading pre-stored data. The name "read-only memory" refers to the fact that we can't change the data or programs stored within it.

It is also the primary storage unit for a computer. ROM contains a binary programming format that stores specific information, which helps the computer regenerate it each time it boots up. Additionally, ROM handles a large amount of input and output and protects program or software instructions. Since ROM is written after manufacturing, the data in it cannot be erased.

Types of Read-Only Memory (ROM)

The oldest type of ROM is the Mask ROM, also known as Mask Read-Only Memory (MROM). This type of ROM is fixed and programmed onto the chip, and in order to change or update its contents, it must be physically replaced from the computer.

As time passed, types of ROM also evolved, and there are a total of four types of ROM. Let's briefly discuss all the types.

• MROM (Mask Read-Only Memory)

As mentioned, Mask Read-Only Memory (MROM) is the oldest type of ROM. It has become obsolete in the digital age and is seldom used today. The “mask” in its name refers to an opaque plate, called a mask, that is layered over the integrated-circuit components on a thin electronic chip for data manipulation. It embodies a form of intellectual property protection, in which the integrated-circuit manufacturer is the only one with access to the mask (also known as photolithography) for reproducing the product.

In short, it is a type of memory chip where content and programs are stored during manufacturing. Afterwards, no one can reprogram or alter the data. People use Mask Read-Only Memory to save the final code of a project, as they stay there forever, and no one can erase the data. MROM can store data without power.

• PROM (Programmable Read-Only Memory)

PROM is a blank version of ROM that users can program once. Manufacturers keep it blank so users can store their desired content or program into it using a tool called a programmer or PROM burner, only once. It was designed in 1956 by Wen Tsing Chow and has been used in video game consoles, mobile phones, microcontrollers, HDMI, and many other automotive innovations.

This type of ROM allows the user to program and add data as needed. However, once the ROM is programmed, the data cannot be changed. Therefore, PROMs are also called one-time programmable devices.

PROM is a type of digital memory, also known as a Field-Programmable Read-Only Memory (FPROM). Today, PROMs have been largely supplanted by other types of ROM, such as EPROM and EEPROM.

• EPROM (Erasable Programmable Read-Only Memory)

EPROM stands for Erasable Programmable Read-Only Memory, which is a type of PROM (Programmable Read-Only Memory) that can be programmed and erased multiple times. It is a specific kind of memory also referred to as EROM (Erasable Read-Only Memory). Data can be stored in an EPROM, and it can be erased by exposing it to ultraviolet light. To do this, you would need a device called a PROM programmer or EPROM burner.

You have to expose it to ultraviolet light for about 30 to 40 minutes to erase it, and this is used in microcontrollers that have a limited number of reprogrammings.

• EEPROM (Electrically Erasable Programmable Read-Only Memory)

In 1978, George Perlegos designed the EEPROM at Intel. It was a nonvolatile ROM chip that could store a small amount of erasable data. Unlike earlier EPROMs, users could modify the data in an EEPROM without having to remove the chip from their computer, by using electrical voltage to erase and reprogram individual bytes.

Reads and writes are more flexible than with RAM but are slower. EEPROMs can be erased and reprogrammed thousands of times. Erasing and reprogramming a ROM takes less than 10 milliseconds (ms).

Advantages and Disadvantages of Read-Only Memory (ROM)

Advantages of ROM:

Some of the attractions of ROM include:

• ROM is inexpensive and can store data permanently.
• ROM is a non-volatile memory, meaning data cannot be erased.
• ROM memory is easier to test.
• It is more reliable than RAM.
• Its circuits are simpler than those of RAM.
• ROM is static and does not require refreshing.

Disadvantages of ROM:

Some of the drawbacks of RAM include that it

• It is a read-only memory, so data cannot be modified.
• Compared to RAM, ROM is a slower type of memory.
• If data is erased incorrectly from ROM, the memory can become unusable.
• In some types of ROM, users can only write to the memory once.