INT 15h,  90h (138)      Device Busy                                     many

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

       On entry:      AH         90h
                      AL         Device type (see below)

       Returns:       Carry      0 - Wait not satisfied
                                 1 - Minimum wait time satisfied

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       Notes:         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:

      Device Type (AL)    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.