INT 16 CU - Microsoft QBASIC internal - MICROSOFT COOPERATION WITH TSR???
        AX = 55FEh
        DX = 0000h
        ES:BX -> ??? function
        ES:CX -> ??? structure
Return: AX = result
            4D4Bh only the INT 1B handler will be installed.
            <> 4D4Bh handlers for INT 08, INT 09, INT 16, INT 1B, and
                  INT 1C are installed
Notes:  The pointer in ES:CX seems to point at a structure defining a callback
          function when new keyboard keys are pressed. If a TSR returns
          AX=4D4Bh, QBASIC will stop (with IRQ's and interrupts enabled).
SeeAlso: INT 16/AX=5500h,INT 1A/AX=3601h

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