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Int 15h, 90h            Device Busy                                      many

    This service is called to notify the system that a device is
    waiting to be serviced.

        Entry   AH = 90h
                AL = Device type (see below)

        Return  CF = 0  Wait not satisfied
                   = 1  Minimum wait time satisfied

    ------------------------------------------------------------------

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

    The device types are classified into three generalized groups.
    These groups are:

        00h - 7Fh     Serially reusable devices.
        80h - BFh     Reentrant devices. ES:BX is used to differentiate calls.
        C0h - FFh     Devices that are time-out only with no posting.


    The defined device types are listed below:

        Type    Device                                     Time-out
        00h     Fixed disk                                    Yes
        01h     Diskette                                      Yes
        02h     Keyboard                                      No
        03h     Pointing Device                               Yes
        80h     Network                                       No
                (ES:BX, pointer to Network Control Block)
        FDh     Diskette drive motor start                    Yes
        FEh     Printer                                       Yes


    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. See Service 91h for a service that signals that a device is
    ready for a process.