What Is ROM? How Read-Only-Memory Works in Computers

By: Jeff Tyson & Marie Look  | 
computer memory
A computer's memory storage looks pretty different from ours. YvanDube / Getty Images

In the world of computers and electronics, there are a lot of terms that can be perplexing. One such term that often crops up is "ROM." But what is ROM, and how does it function within the framework of computer systems? Let's unpack this data storage concept.


What Is ROM?

ROM is an acronym for Read-Only Memory. It refers to a type of computer memory that stores data permanently.

A ROM memory chip contains hard-wired instructions that you can't change. It's also nonvolatile, which means it retains its contents even if the device loses power. This characteristic makes ROM ideal for storing critical system settings, firmware and other essential ROM data that should not be lost.



Standing for Random Access Memory, RAM is volatile, meaning RAM memory is erased when the computer loses power. ROM chips, on the other hand, are nonvolatile, meaning they retain their data even when you power down.

ROM vs. Hard Drive

Hard drives store data magnetically, and you can write over them multiple times. Unlike a hard drive, however, ROM stores data permanently, and you can't rewrite the ROM content without special equipment or procedures.


How ROM Works

Similar to RAM, a ROM chip works by storing data in memory cells organized into an array. Each memory cell contains a fixed arrangement of transistors that represent binary data, typically 0s and 1s.

During the manufacturing process, methods such as photolithography or electrical programming ensure the data is permanently physically encoded into these memory cells.


Reading ROM Memory

There are two basic components involved in addressing and reading memory cells in ROM.

Memory Cells

ROM consists of memory cells, which are the basic units for storing data. These cells are organized into an array and can hold a single bit of information, typically in the form of a 0 or 1.


Word Lines and Bit Lines

Addressing and reading memory cells in the ROM array involves word lines and bit lines.

To access a specific memory, the corresponding word line activates, selecting a particular row of memory cells. During the read operation, the selected memory cells on the activated word line transfer their stored data to the corresponding bit lines for further processing or output.


6 Different Types of ROM

There are several different types of ROM, each with its own unique characteristics and applications. The most common include:

  1. Read-Only Memory (ROM): This is the standard ROM that contains permanently stored data. It is typically used for critical system functions and cannot be rewritten.
  2. Programmable Read-Only Memory (PROM): PROM allows users to write data to the memory chip using special equipment. Once programmed, the data is fixed and cannot be altered.
  3. Erasable Programmable Read-Only Memory (EPROM): EPROM chips can be erased and reprogrammed multiple times using high voltages or exposure to ultraviolet (UV) light.
  4. Electrically Erasable Programmable Read-Only Memory (EEPROM): EEPROM chips can be rewritten electrically without the need for UV light, making them more convenient for reprogramming.
  5. Flash memory: This type of EEPROM uses in-circuit wiring for erasure by applying an electrical field. Flash memory works faster than traditional EEPROMs because it writes data 512 bytes at a time instead of just one byte at a time.
  6. Mask ROM: Also known as "hard-wired ROM," Mask ROM is programmed during the manufacturing process (such as for storing firmware and system code) and you can't alter it afterward.


Example Applications of ROM

ROM finds applications in various hardware components, including computer systems, game consoles and embedded devices. Here are some common uses.

  • Operating systems: ROM often stores essential components of the operating system (OS), ensuring that they remain intact even after the power cycles off and on again.
  • Firmware: Devices such as BIOS (Basic Input/Output System) utilize ROM to store firmware, which initializes hardware components during the boot process.
  • System settings: Critical system settings and configurations are often stored in ROM.
  • Game consoles: Game cartridges use ROM chips to store game data permanently.


How PROM Works

Figure 2.

PROM chips (Figure 2) have a grid of columns and rows just as ordinary ROMs do. The difference is that every intersection of a column and row in a PROM chip has a fuse connecting them.

PROM functions by allowing users to write data to the memory chip after manufacturing, typically using specialized programming equipment.


Programming PROM Chips

PROM cells contain fusible links that are initially intact, representing a default state (usually all 1s). During programming, electrical pulses or currents are applied to specific locations on the chip, causing the fusible links to be selectively blown.

This changes the state of the corresponding memory cells to 0s. Once programmed, the data becomes fixed and the user can't alter it.

PROM Pros and Cons

Blank PROMs are inexpensive and are great for prototyping the data for a ROM before committing to the costly ROM fabrication process. However, PROMs are more fragile than ROMs. A jolt of static electricity can easily cause fuses in the PROM to burn out, changing essential bits from 1 to 0.


How EPROM Works

EPROM operates through a process of selective erasure and reprogramming. EPROM cells consist of floating-gate transistors that can trap or release electrons, representing binary data as either a charged or discharged state.

Programming EPROM Chips

During programming, high voltages are applied to specific memory cells, injecting electrons into the floating gate and altering the transistor's conductivity, thus storing data.


To erase the data, the EPROM chip is exposed to ultraviolet (UV) light, which clears the charge from the floating gates, returning the cells to their default state. Once the chip has been erased, new data can be programmed into the EPROM cells using the same high-voltage programming process.

EPROM Suitability

EPROM's ability to be erased and reprogrammed multiple times makes it suitable for applications requiring occasional updates or revisions, such as storing firmware and BIOS in electronic devices.


How EEPROMs and Flash Memory Work

EEPROM and flash memory operate on similar principles, utilizing floating-gate transistors to store data.

Both EEPROM and flash memory offer non-volatile storage solutions, making them suitable for applications requiring frequent data updates or modifications, such as storing system settings, firmware and user data in various electronic devices.


Programming EEPROM Chips

In EEPROM, data is stored by charging or discharging the floating gates of individual memory cells through electrical programming.

Unlike EPROM, EEPROM does not require exposure to UV light for erasure; instead, a high-voltage signal is applied to selectively remove the stored charge from the floating gates, allowing for multiple write-erase cycles.

Programming Flash Memory

Similarly, flash memory stores data by trapping or releasing electrons in floating gates, but it operates at a larger scale, organizing memory cells into blocks and sectors.

Flash memory employs a mechanism called tunneling to move electrons in and out of the floating gates during programming and erasure, respectively. Flash memory is designed to perform block erasure and programming, which makes it more efficient for mass data storage and retrieval.

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.


Frequently Answered Questions

Is ROM a Scrabble word?
Yes, ROM is in the scrabble dictionary.
What is ROM and examples?
ROM is an acronym for Read-Only Memory. It refers to computer memory chips containing hard-wired instructions that can not be changed. It is nonvolatile, meaning it retains its contents even when power is removed. An example of ROM is the cartridge used with video game consoles.