BIOS Boot Function

1. To determine the device from which to boot. Depending on who wrote the BIOS, this is done in one of several ways:
   • By configuration options within the BIOS setup function. The user specifies which device to boot via a setup parameter.
   • By configuration options from a boot menu. This is conceptually identical to the preceding method. The user selects which device to boot. The difference is the method used for getting to the menu of boot devices: In the previous method, the boot device is simply one of many options available in the setup function; in the current method, there is a separate boot-device menu that is shown if the user selects an alternate "special" key, that is a key other than the one that invokes the setup function.
   • By testing each potential boot device in some specified order until a device is found that is ready for and capable of being booted. The order depends on the specific BIOS implementation; a common order might be:
     • Floppy disk
     • CD-ROM (or DVD-ROM)
     • Hard Disk
     • Network (PXE Boot)
     The first device that is found ready to boot is booted and the remaining devices are not checked.
   • A combination of the above, where the user uses setup to select the order in which the devices are to be checked.
2. To boot the device selected by the previous step. This generally means reading the first block from the device into memory and then transferring to the first byte of that block which is assumed to contain executable instructions. (The network boot is more complicated and will not be covered here.)

   When the hard disk is booted, the first block of the device is called the Master Boot Record (MBR).

Next: Master Boot Record (MBR)

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