INT 40H: Diskette BIOS Revector

 This vector (0:0100) points to the code that handles INT 13H calls that
 apply to diskette drives.

 When hard disks were first introduced on the PC, the BIOS disk API code
 was on the hard disk controller and installed via ROM-Scan.  At boot time,
 it would put the original INT 13H vector into the INT 40H vector.  Then
 when a service call requested access to a diskette drive, (i.e., DL<80H),
 it would pass control on to the original INT 13H handler (now at INT 40H).

 On AT-class computers, the hard disk and diskette BIOSes are often merged
 and handled via the system BIOS (rather than a ROM-Scan add-on).  However,
 both INT 13H and INT 40H vectors are maintained for compatibility.

See Also: BIOS Data Area
          ROM-BIOS Functions