BIOS - Basic Input Output System
The BIOS is a collection of commands usually stored in a ROM chip, which allows a CPU to communicate with the connected devices in a PC or other digital system.
The CPU in a PC needs a way of communicating with the many different hardware devices that are installed, each hardware device has its own special BIOS software which allows for this communication.
The main BIOS in a PC system is stored inside a microchip on the motherboard
(mainboard), and is the first piece of software your computer runs when you (boot up) turn it on.
When a PC is first powered-up the BIOS will run the POST (Power On Self Test), which initialises all of the basic hardware devices, including the CPU, RAM, video card, storage devices and DMA controllers. When a system is reset (warm-boot) the BIOS will effectively skip the POST test.
The configuration data used by the BIOS is stored in an EEPROM chip or a CMOS
chip which is backed up by a battery. This data includes all of the basic configuration data, including device types and also the date and time.
Another task assigned to the BIOS is determining which device to boot from, this is typically the main hard drive but can also be a USB
drive, CD-ROM or even a network adapter
. The way the BIOS determines if a device is bootable depends on the device type. For example, with a disk-like device such as a hard drive, flash drive, CD-ROM or tape drive, it will attempt to load the first sector and also check for the presence of a boot sector signature.
In the BIOS setup utility you can select which device you wish to boot from and also specify the boot sequence. For example, when installing a new operating system you may want to boot from the CD drive rather than the hard drive.
If no boot device is found then the BIOS, depending on the system, will either go directly to the BIOS setup environment or display a message such as "No boot device detected".
A Flash BIOS
can be updated with new information and many motherboard manufacturers publish updates for their respective BIOS on their websites.