Arranging boot order in BIOS
BIOS has a built-in setup utility for initial computer configuration. To enter it, you have to press a certain key combination (Del, F1, Ctrl+Alt+Esc, Ctrl+Esc, or some other, depending on your BIOS) during the POST (power-on self test) sequence that starts immediately after you turn your computer on. Usually the message with the required key combination is displayed during the startup test. Pressing this combination takes you to the menu of the setup utility that is included in your BIOS.
The menu can differ in appearance, sets of items and their names, depending on the BIOS manufacturer. The most widely known BIOS makers for PC motherboards are Award/Phoenix and AMI. Moreover, while items in the standard setup menu are mostly the same for various BIOSes, items of the extended (or advanced) setup heavily depend on the computer and BIOS version.
Among other things, the BIOS menu allows you to adjust the boot order. Boot order management differs for various BIOS versions, e.g. for AMI BIOS, AWARDBIOS, and brand-name hardware manufacturers.
Computer BIOS allows booting operating systems not only from hard disks, but also from CD-ROMs, DVD-ROMs, and other devices. Changing the boot order may be required, for example, to make your rescue media (CD, DVD or USB stick) device the first booting device.
If there are several hard disks installed in your computer labeled as C:, D:, E:, and F:, you can change the boot order so that an operating system is booted from, for example, disk E:. In this case, you have to set the boot order to look like E:, CD-ROM:, A:, C:, D:.
This does not mean that booting is done from the first device in this list; it only means that the first attempt to boot an operating system will be from this device. There may be no operating system on disk E:, or it may be inactive. In this case, BIOS queries the next device in the list.
The BIOS numbers disks according to the order in which they are connected to IDE controllers (primary master, primary slave, secondary master, secondary slave); next go the SCSI hard disks.
This order is broken if you change the boot order in BIOS setup. If, for example, you specify that booting has to be done from hard disk E:, numbering starts with the hard disk that would be the third in usual circumstances (it is usually the secondary master for IDE hard drives).
Some motherboards have a so called boot menu opened by pressing a certain key or key combination, for instance, F12. The boot menu allows selecting the boot device from a list of bootable devices without changing the BIOS setup.