Those services expect a real-mode function, so you should wrap your protected-mode function with a real-mode stub. To this end, call _go32_dpmi_allocate_real_mode_callback_retf() library function, and pass the rm_segment and rm_offset fields it returns to the service you want (in the above example, Int 33h function 0Ch) by calling _go32_dpmi_simulate_int(). See the docs in the libcref.i info file for further details about allocating wrapper function.