Interrupt Flag Management


            The popf  and iret  instructions may not modify the state of
            the interrupt  flag since most DPMI implementations will run
            programs with  IOPL < DPL.  Programs must execute cli or sti
            to modify the interrupt flag state.

            This means  that the  following  code  sequence  will  leave
            interrupts disabled:

                 ;
                 ; (Assume interrupts are enabled at this point)
                 ;
                         pushf
                         cli
                         .
                         .
                         popf            ; Interrupts are still OFF!

            Note that since some implementations of DPMI will maintain a
            virtual interrupt state for protected mode DOS programs, the
            current value  of the  interrupt flag  may not  reflect  the
            current virtual  interrupt state.   Protected  mode programs
            should use the virtual interrupt state services to determine
            the current interrupt flag state.

            Since cli  and sti  are privileged  instructions, they  will
            cause a  protection violation  and the  DPMI  provider  will
            simulate the  instruction.  Because of the overhead involved
            in processing  the exception,  cli and sti should be used as
            little as possible.  In general, you should expect either of
            these instructions to require at least 300 clocks.