You should zero out some of the fields of the _go32_dpmi_registers structure before you call _go32_dpmi_simulate_int(). Random values in these fields can cause your program to behave erratically. The fields in point are ss, esp and eflags. When ss and esp are zeroed, the DPMI host will provide a 30-word-long stack for the interrupt handler; this is usually enough, but if it isn't, then you should point ss and esp to larger buffer in conventional memory (possibly part of the transfer buffer).