Basic Input Output System (BIOS) is a set of basic programs for testing the computer equipment during its running, booting of operating system and also to support exchange of data between devices. Basic Input Output System is stored on ROM (read-only memory). The programs of BIOS define a general productivity of a computer and in most cases are not inaccessible for users.
Actually, BIOS is a small microchip with a set of micro commands that control the operation of all devices on a motherboard. When you switch your computer, a processor sends a signal to BIOS that initiates the loading of low-level micro program “BOOT-ROUTINE”. At first, a subprogram POST (Power-On Self Test) boots; it checks the capability of the processor, operating memory and also assistive devices. Then BOOT-ROUTINE searches and initialized the other BIOS that can be installed on other motherboards. For example, almost all modern video cards have their own BIOS.
Then BIOS allocates the system resources between found devices. They are assigned with numbers of IRQ, channels of direct access to DMA, addresses of input/output ports and other. Finally, there will be a boot table where you can find the main accessible devices and allocated resources.
Then BIOS launches a micro program “BOOTSTRAP LOADER” which is the bootstrap loader. This program scans the devices that may contain media with boot record. This leads to downloading of an appropriate operating system.
This is the most common sequence of BIOS operation but there are a lot of supplements and changes, connected with operation of each particular BIOS.