Int 18  - Diskless Boot Hook (start Cassette Basic)                        [B]

Desc:  called when there is no bootable disk available to the system

Notes: only PCs produced by IBM contain BASIC in ROM, so the action is
     unpredictable on compatibles; this interrupt often reboots the
     system, and often has no effect at all
   network cards with their own BIOS can hook this interrupt to allow
     a diskless boot off the network (even when a hard disk is present
     if none of the partitions is marked as the boot partition)

See Also: INT 2F/AX=4A06h,INT 86"NetBIOS"