Direct Hardware Access
Refer to Appendix C for a full description of all DPMI services. INT-2Fh/AX = 1686h is used to check if the CPU is running DPMI and is in Protected mode. This service returns a false flag if Windows is loaded in Real mode or DPMI is not running. You would only need to perform this check if you were running Windows in Real mode, but version 3.1 of Windows won't even run in Real mode, so these days this test isn't required.