Simulate Real Mode Interrupt


                 This function  simulates an interrupt in real mode.  It
                 will invoke  the  CS:IP  specified  by  the  real  mode
                 interrupt  vector   and  the  handler  must  return  by
                 executing an iret.

            To Call

                 AX = 0300h
                 BL = Interrupt number
                 BH = Flags
                      Bit 0  = 1 resets the interrupt controller and A20
                      line
                      Other flags reserved and must be 0
                 CX =  Number of  words to  copy from  protected mode to
                      real mode stack
                 ES:(E)DI = Selector:Offset of real mode call structure

            Returns

                 If function was successful:
                 Carry flag is clear.
                 ES:(E)DI =  Selector:Offset of  modified real mode call
                      structure

                 If function was not successful:
                 Carry flag is set.

            Programmer's Notes

                 o    The CS:IP  in the  real  mode  call  structure  is
                      ignored  by   this  service.     The   appropriate
                      interrupt handler  will be  called  based  on  the
                      value passed in BL.
                 o    If the  SS:SP fields  are zero  then a  real  mode
                      stack  will   be  provided   by  the   DPMI  host.
                      Otherwise, the  real mode SS:SP will be set to the
                      specified values  before the  interrupt handler is
                      called.
                 o    The  flags   specified  in   the  real  mode  call
                      structure will  be pushed  on the  real mode stack
                      iret frame.   The interrupt handler will be called
                      with the interrupt and trace flags clear.
                 o    When the  Int 31h  returns,  the  real  mode  call
                      register structure  will contain  the values  that
                      were returned by the real mode interrupt handler.
                 o    It is  up to  the caller  to remove any parameters
                      that were pushed on the protected mode stack.
                 o    32-bit programs  must use  ES:EDI to  point to the
                      real mode  call structure.  16-bit programs should
                      use ES:DI.
                 o    The flag to reset the interrupt controller and A20
                      line is  ignored by  DPMI implementations that run
                      in  Virtual   8086   mode.      It   causes   DPMI
                      implementations that  return to  real mode  to set
                      the interrupt  controller  and  A20  address  line
                      hardware to its normal real mode state.