Read-Only Memory (ROM): Definition & Types

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  • 0:00 Definitions
  • 1:15 Booting Up Your Computer
  • 2:25 Types of ROM
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: Paul Zandbergen

Paul has a PhD from the University of British Columbia and has taught Geographic Information Systems, statistics and computer programming for 15 years.

Computers have various types of memory. Read-only memory (ROM) contains the instructions for what needs to happen when a computer is powered on. The data in ROM cannot be modified, or only with great difficulty, since these instructions do not need to be changed very often.


As the name suggests, read-only memory, or ROM, stores information that can only be read. Modifying it is either impossible or very difficult. ROM is also a type of non-volatile storage, which means that the information is maintained even if the component loses power.

In a typical computer system, ROM is located on the motherboard, shown in the right of the picture.

It contains the basic instructions for what needs to happen when a computer is powered on. This is typically referred to as the firmware of a computer. The firmware represents the basic code to get the computer started. Once the computer is up and running, the CPU takes over. Firmware is also referred to as BIOS, or basic input/output system. On most modern computers, the read-only memory is located on a BIOS chip, shown on the left. The BIOS chip is normally plugged into the motherboard.

You have probably heard of the term CD-ROM, which stands for compact disc read-only memory. This is another type of ROM that is impossible or difficult to change; however, the term ROM by itself is used to indicate the memory that stores the firmware for a computer.

Booting Up Your Computer

ROM plays a critical part in booting up, or starting up, your computer. So, what exactly happens?

When you press the power button, the BIOS chip wakes up and checks the various components of your computer to make sure they are all present and functioning properly. This is why it's called firmware as opposed to software; the set of instructions on the read-only memory interact directly with the various hardware components.

The BIOS instructs the computer-processing unit (CPU) to start reading code at various locations. It checks the various peripherals and the system clock. This process is also called the power-on self-test (POST). While this is happening, you will start hearing sounds from your computer. For example, the hard drive starts spinning and various lights may start flashing as part of the test; however, your monitor is still completely black at this point. Once this test is completed, the CPU takes over and starts launching the operating system. This is when you see the start-up sequence on your monitor.

The instructions stored on ROM are specific to the hardware and therefore, don't need to be changed often. Sometimes, firmware does need to updated, such as when you install additional memory or a new hard disk drive.

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