Get Raw Mode Switch Addresses


                 This function returns addresses that can be jumped to
                 for low-level mode switching.

            To Call

                 AX = 0306h

            Returns

                 If function was successful:
                 Carry flag is clear
                 BX:CX = Real -> Protected mode switch address
                 SI:(E)DI = Protected -> Real mode switch address

                 If function was not successful:
                 Carry flag is set

            Parameters To State-Save Procedures

                 Execute a  far jump to the appropriate address (real or
                      pmode) with:
                 AX   = New DS
                 CX   = New ES
                 DX   = New SS
                 (E)BX     = New (E)SP
                 SI   = New CS
                 (E)DI     = New (E)IP

                 The processor  will be placed in the desired mode.  The
                 DS, ES,  SS, (E)SP,  CS, and  (E)IP  will  contain  the
                 values specified.  The (E)BP register will be preserved
                 across the  call and  so can be used as a pointer.  The
                 values in  (E)AX, (E)BX, (E)CX, (E)DX, (E)SI, and (E)DI
                 will be  undefined.  On an 80386 or 80486 the FS and GS
                 segment registers  will contain  zero  after  the  mode
                 switch.

            Programmer's Notes

                 o    The address  returned in BX:CX must only be called
                      in real  mode to  switch into protected mode.  The
                      address returned  in SI:(E)DI  must only be called
                      in protected mode to switch into real mode.
                 o    16-bit programs  should call  the address returned
                      in SI:DI  to switch  from protected  to real mode.
                      32-bit programs  should call  the address returned
                      in SI:EDI.
                 o    It is  up to  the caller  to save  and restore the
                      state of  the task  when using  this  function  to
                      switch modes.   This  usually requires  using  the
                      state save function (see function 0600H).
                 o    The  parameters   must  contain   segment   values
                      appropriate for  the mode  that is  being switched
                      to.   If  invalid  selectors  are  specified  when
                      switching into  protected mode,  an exception will
                      occur.
                 o    Applications  may  find  functions  0300h,  0301h,
                      0302h, and 0304h more convenient to use than using
                      this type of mode switching.