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This setting controls the order that the BIOS uses to look for a boot device from which to load the operating system during the DOS boot process. Older machines do not have this setting; they look at the floppy drive first (A:) and then the hard drive (C:). Most systems will at least let you choose between "A:, C:" (the default) and "C:, A:". Newer systems will allow you to boot from the CD-ROM as well; in this case there will be six different combinations listed. The default will normally be "A:, C:, CD-ROM".
Note: Booting off the
second floppy disk, B:, has not been an option on any PC I've ever seen.
Some BIOSes are getting even more advanced in terms of the boot sequence options they will allow. Some systems will now allow you to boot off a different IDE hard disk than the primary master (C:), or let you boot from a SCSI device instead of IDE even when both are used in the same system (normally the IDE device gets preference).
Changing the boot sequence to seek from the hard disk drive first instead of the floppy disk drive has some advantages, and also some disadvantages. There are two main advantages. First, if the floppy disk isn't bootable, you virtually eliminate the chance of a boot sector virus spreading to your hard disk from a floppy. Second, you have a measure of security and reliability since when the floppy disk is bootable, anyone can write their own system files and boot the PC with their floppy, bypassing the standard startup files on the hard disk.
The disadvantages of not having the floppy bootable all relate to convenience. First, if you ever do have a virus on your PC then you will normally need to boot from a clean floppy disk to disinfect your hard disk; you will have to go back into the BIOS and change the boot sequence, disinfect, and then change it back. Second, there are some software programs that require their own boot disks (though they are becoming quite rare). Third, if your hard disk ever fails, you won't have any way to boot the PC. Some viruses manifest themselves by making the hard disk disappear. Finally, if you are installing a new operating system, or building a PC, you really need to be able to boot from the floppy disk.
Note: See also the "Floppy Drive Seek" setting.