The disk drive that the operating system is loaded from during the boot process, usually A: (the floppy disk) or C: (the hard disk), or on some systems a CD or DVD drive. In some diskless workstations it may be a network drive.
In a PC-compatible computer, the disk drive that the BIOS uses to automatically load the operating system when the computer is turned on. Generally, the default boot drive is the primary floppy disk drive A: in PC-compatible computers with MS-DOS, Windows 3.x, or Windows 95 operating systems. If a floppy disk is not found in that drive, the BIOS will check the primary hard disk next, which is drive C:. The BIOS for these operating systems can be reconfigured to search drive C: first by using the BIOS setup program.