PC Hardware

Lesson 3: ROM BIOS

In addition to the chip set, you will find other chips called ROM BIOS. A ROM BIOS chip contains data that specifies the characteristics of hardware devices, such as memory and hard disk and floppy disk drives, so the system can properly access them. This lesson explores ROM BIOS and what it does.

After this lesson, you will be able to:
  • Identify the different types of ROM.

  • Modify the CMOS settings in a computer.

  • Identify POST codes and take appropriate corrective action when a problem is identified.

Estimated lesson time: 30 minutes

ROM (Read only memory)

ROM (read-only memory) is a type of memory that stores data even when the main computer power is off. This is necessary so that the system can access the data it needs to start up. When stored in ROM, information that is required to start and run the computer cannot be lost or changed. The BIOS, software in the form of programs stored on ROM chips, is used during the startup routine to check out the system and prepare to run the hardware. The system BIOS is a ROM chip on the motherboard used by the computer during the startup routine (boot process) to check out the system and prepare to run the hardware. The BIOS is stored on a ROM chip because ROM retains information even when no power is being supplied to the computer. The downside of storing data in an older computer's ROM is that we have to change a chip to update information.


More recent systems use a technology called flash ROM or flash BIOS that allows code in the core chips to be updated by software available through the BIOS or motherboard supplier. Check the Internet site for the supplier if you suspect your ROM chip has flash ROM technology; the software and instructions are generally downloadable.

Upgrade a BIOS only when necessary! Be sure to follow all precautions included with the motherboard manual and instructions for the upgrade. Improper installation can render the motherboard useless.

BIOS (also referred to as firmware) can be subdivided into three classes, depending upon the type of hardware it controls.

  • The first class, called core chips, includes support for hardware that is common to all computers, is necessary, and never changes.

  • The second class, called updatable chips, encompasses hardware that is also common and necessary, but that might change from time to time.

  • The third class of chips includes anything that is not included in one of the first two classes.

Core Chips

Look on any motherboard: ROM chips for the core chips are found everywhere. They are distinctive because they are in DIP (dual inline package) form and are almost always labeled. These chips are commonly used for the keyboard, parallel ports, serial ports, speaker, and other support devices. Each ROM chip contains between 16 and 64 KB of programming.

Updatable Chips

Several devices on a computer often contain their own flash BIOS or updatable ROMS. These include SCSI controllers and video cards. Because this information is subject to change (for instance, you can upgrade a hard disk drive or change a video card), it is stored on a special chip called the CMOS (complementary metal-oxide semiconductor). This chip gets its name from the way it is manufactured, not from the information it holds.

Unlike other ROM chips, CMOS chips do not store programs; they store data that is used by the BIOS for the programs needed to talk to changeable hardware. The CMOS chip also maintains date and time information when power to the computer is off.

The CMOS chips can store about 64 KB of data. However, to store the data needed to boot a computer requires only a very small amount of memory: about 128 bytes. If the data stored on the CMOS is different from the hardware it keeps track of, the computer, or part of it, will probably not work. For example, if the hard disk drive information is incorrect, the computer can be booted from a floppy disk, but the hard disk drive might not be accessible. The technician or owner will have to reset the CMOS values before the computer can use the device if it is not properly defined in the CMOS registry.

The information contained in a CMOS chip will depend on the manufacturer. Typically, CMOS contains at least the following information:

  • Floppy disk and hard disk drive types

  • CPU

  • RAM size

  • Date and time

  • Serial and parallel port information

  • Plug and Play information

  • Power Saving settings

It is critical that the core information on a CMOS chip be correct. If you change any of the related hardware, the CMOS must be updated to reflect those changes.