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
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INT 40H: Diskette BIOS Revector