Int 2Fh / 1680h         DOS Idle Call                                DOS 5.0+

    Informs the system that the program is idle--for example, waiting
    for user input. The function permits the system to suspend the
    idle program temporarily and transfer control to another program.

        Entry   AX = 1680h

        Return  AL = 00h | 80h   Call is supported | not supported


    Programs should use this interrupt when they are idle (provided
    that the Int 2Fh interrupt vector is not zero).

    This interrupt is non-blocking, meaning the system does not
    suspend the program unless another program is ready to be run. In
    most cases, the interrupt returns immediately and the program
    continues running. To make sure the system can suspend the
    program, a program that remains idle should repeatedly call the
    interrupt as part of its idle loop.