INT 15h,  91h (139)      Interrupt Complete                              many

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

       On entry:      AH         91h
                      AL         Device type (See service 90h)

       Returns:       Carry      Set if error, else cleared

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       Notes:         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.