What is BIOS?

All motherboards include a small block of Read Only Memory (ROM) which is separate from the main system memory used for loading and running software. The ROM contains the PC’s BIOS and this offers two advantages.

The code and data in the ROM BIOS need not be reloaded each time the computer is started, and they cannot be corrupted by wayward applications that write into the wrong part of memory. A flash-upgradeable BIOS may be updated via a floppy diskette to ensure future compatibility with new chips, add-on cards etc.

The BIOS is comprised of several separate routines serving different functions. The first part runs as soon as the machine is powered on. It inspects the computer to determine what hardware is fitted.

Then it conducts some simple tests to check that everything is functioning normally—a process called the power-on self-test. If any of the peripherals are plug-and-play devices, the BIOS assigns the resources. There’s also an option to enter the Setup program.

This allows the user to tell the PC what hardware is fitted, but thanks to automatic self-configuring BIOS, this is not used so much now.

If all the tests are passed, the ROM tries to boot the machine from the hard disk. Failing that, it will try the CD-ROM drive and then the floppy drive, finally displaying a message that it needs a system disk. Once the machine has booted, the BIOS presents DOS with a standardized API for the PC hardware. In the days before Windows, this was a vital function. But 32-bit “protect mode” software doesn’t use the BIOS, so it is of less benefit today.

Most PCs ship with the BIOS set to check for the presence of an operating system in the floppy disk drive first, then on the primary hard disk drive. Any modern BIOS will allow the floppy drive to be moved down the list so as to reduce normal boot time by a few seconds.

To accommodate PCs that ship with a bootable CD-ROM, most BIOS allow the CD-ROM drive to be assigned as the boot drive. BIOS may also allow booting from a hard disk drive other than the primary IDE drive. In this case, it would be possible to have different operating systems or separate instances of the same OS on different drives.

Windows 98 (and later) provides multiple display support. Since most PCs have only a single AGP slot, users wishing to take advantage of this will generally install a second graphics card in a PCI slot. In such cases, most BIOS will treat the PCI card as the main graphics card by default.

However, some allow either the AGP card or the PCI card to be designated as the primary graphics card. While the PCI interface has helped by allowing IRQs to be shared more easily, the limited number of IRQ settings available to a PC remains a problem for many users.

For this reason, most BIOS allow ports that are not in use to be disabled. It will often be possible to get by without needing either a serial or a parallel port because of the increasing popularity of cable and ADSL Internet connections and the ever-increasing availability of peripherals that use the USB interface.

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