Simulate Real Mode Interrupt

Simulates an interrupt in real mode.  The function transfers
control to the address specified by the real mode interrupt
vector.  The real mode handler must return by executing an
IRET.

Call With:

   AX     = 0300H
   BL     = interrupt number
   BH     = flags
           Bit    Significance
           0      reserved for historical reason, must be
           zero
           1-7    reserved, must be zero
   CX     = number of words to copy from protected mode to
           real mode stack
   ES:(E)DI= selector:offset of real mode register data
           structure in the following format:

           Offset   Length  Contents
           00H      4       DI or EDI
           04H      4       SI or ESI
           08H      4       BP or EBP
           0CH      4       reserved, should be zero
           10H      4       BX or EBX
           14H      4       DX or EDX
           18H      4       CX or ECX
           1CH      4       AX or EAX
           20H      2       CPU status flags
           22H      2       ES
           24H      2       DS
           26H      2       FS
           28H      2       GS
           2AH      2       IP (reserved, ignored)
           2CH      2       CS (reserved, ignored)
           2EH      2       SP
           30H      2       SS

Returns:

   if function successful
   Carry flag= clear
   ES:(E)DI= selector:offset of modified real mode register
           data structure

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8012H   linear memory unavailable (stack)
           8013H   physical memory unavailable (stack)
           8014H   backing store unavailable (stack)
           8021H   invalid value (CX too large)


Notes:

o  32-bit programs must use ES:EDI to point to the real mode
   register data structure.  16-bit programs should use
   ES:DI.
o  The CS:IP in the real mode register data structure is
   ignored by this function.  The appropriate interrupt
   handler will be called based on the value passed in BL.
o  If the SS:SP fields in the real mode register data
   structure are zero, 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 register data
   structure will be pushed on the real mode stack's IRET
   frame.  The interrupt handler will be called with the
   interrupt and trace flags clear.
o  Values placed in the segment register positions of the
   data structure must be valid for real mode; i.e. the
   values must be paragraph addresses and not selectors.
o  All general register fields in the data structure are
   DWORDs so that 32-bit registers can be passed to real
   mode.  Note, however, that 16-bit hosts are not required
   to pass the high word of 32-bit general registers or the
   FS and GS segment registers to real mode even when
   running on an 80386 or later CPU.
o  The target real mode handler must return with the stack
   in the same state as when it was called.  This means that
   the real mode code may switch stacks while it is running,
   but must return on the same stack that it was called on
   and  must return with an IRET.
o  When this function returns, the real mode register data
   structure will contain the values that were returned by
   the real mode interrupt handler.
o  It is the caller's responsibility to remove any
   parameters that were pushed on the protected mode stack.