Int 15h, 91h            Interrupt Complete                               many

    Signals the system that a device is ready to be serviced.

        Entry   AH = 91h
                AL = Device type (See service 90h)

        Return  CF  Set if error, else cleared

    ──────────────────────────────────────────────────────────────────

    This service is not available for the PC, PCjr, and XT dated
    11/08/82.

    When a device is needed by a process, but access to it is delayed,
    the system can perform another task to achieve maximum performance
    until the device is freed up for the process to use. In a
    multi-tasking environment, that is the purpose for this service.
    The operating system can keep track of which process is waiting
    for which device, and perform another task while the device is
    busy. After a device is ready to be serviced, then this function
    is called to notify the system that a device is ready.

    For example, Int 09h will call this service after a key has been
    entered, to signal the operating system that a key has been
    entered. Thus if the system was performing another task while a
    previous process was waiting for a keyboard character, then the
    previous process could regain control of the system and get the
    keyboard character just entered.