Get Raw Mode Switch Addresses

Returns addresses that can be called for low-level mode
switching.

Call With:

   AX     = 0306H

Returns:

   Carry flag = clear (this function always succeeds)
   BX:CX  = real-to-protected mode switch address
   SI:(E)DI= protected-to-real mode switch address

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; 16-bit programs should call the
   address returned by this function in SI:DI, while 32-bit
   programs should call the address returned in SI:EDI.

o  The mode switch procedures are entered by a FAR JMP to
   the appropriate address with the following parameters:

AX = new DS
CX = new ES
DX = new SS
(E)BX= new (E)SP
SI = new CS
(E)DI= new (E)IP

The processor is placed into the desired mode, and the DS, ES, SS,
   (E)SP, CS, and (E)IP registers are updated with the
   specified values; in other words, execution of the client
   continues in the requested mode at the address provided
   in registers SI:(E)DI.  The values specified to be placed
   into th
e segment registers must be appropriate for the destination mode; if
   invalid selectors are supplied when switching into
   protected mode, an exception will occur.

The values in (E)AX, (E)BX, (E)CX, (E)DX, (E)SI, and (E)DI after the
   mode switch are undefined. (E)BP will be preserved across
   the mode switch call so it can be used as a pointer. On
   an 80386 or later CPU, the FS and GS segment registers
   will contain zero after the mode switch.

If interrupts are disabled when the mode switch procedure is invoked,
   they will not be re-enabled by the DPMI host (even
   temporarily).

o  It is up to the client to save and restore the state of
   the task when using this function to switch modes.  This
   usually requires using the state save/restore procedures
   whose addresses are returned by Int 31H Fn 0305H
   (see page 94).

o  Clients may find it more convenient to use Int 31H
   Functions 0300H, 0301H, and 0302H for mode switching than
   this function.