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BIOS short overview

The BIOS on x86 computers provides functions necessary for booting the machine. The BIOS functions are stored in a chip, hence they are available immediately after switching on the machine.

The BIOS functions are implemented as calls of certain software interrupts. There are software interrupts for accessing the keyboard, the screen and storage devices (disks, diskettes, cd/dvd).

Standard boot process from disk:
Booting from disk is performed as follows:

  1. In the BIOS a certain order of the boot disks is specified.
  2. The BIOS loads the first sector (MBR) of the boot disk 1 into memory and starts the MBR code residing in the MBR.
  3. This MBR code searches the partition table for a partition entry marked as active.
  4. The MBR code loads the boot sector of the active partition into memory and starts the boot sector code.
  5. The boot sector code loads additional sectors from disk and starts them. This finally starts the operating system.

Standard boot process from diskette:
Booting from diskette is similar to booting from disk. The BIOS loads and starts the first sector of the diskette. However on diskettes the first sector is already the boot sector, there is no partition table on diskettes. The operating system is therefore started already in the first step.

Boot process from cd/dvd:
The boot process from cd/dvd is different in a some details from booting from disk. First of all the size of a cd/dvd sector is 2048 bytes (while disk sectors have a size of 512 bytes), and secondly the boot data are stored in sector 17 (boot record) and not in the first sector of the cd/dvd. The boot record contains links to the boot catalog which contains the images to start. The images can be diskette images or whole disk images.