Segment to Descriptor

Maps a real mode segment (paragraph) address onto an LDT
descriptor that can be used by a protected mode program to
access the same memory.

Call With:

   AX     = 0002H
   BX     = real mode segment address

Returns:

   if function successful
   Carry flag= clear
   AX     = selector for real mode segment

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8011H   descriptor unavailable

Notes:

o  The descriptor's limit will be set to 64 KB.
o  Multiple calls to this function with the same segment
   address will return the same selector.
o  The intent of this function is to provide clients with
   easy access to commonly used real mode segments such as
   the BIOS data area at segment 0040H and the video refresh
   buffers at segments A000H, B000H, and B800H.  Clients
   should not call this function to obtain descriptors to
   private data areas.
o  Descriptors created by this function can never be
   modified or freed.  For this reason, the function should
   be used sparingly.  Clients which need to examine various
   real mode addresses using the same selector should
   allocate a descriptor with Int 31H Fn 0000H and
   change the base address in the descriptor as necessary,
   using the Set Segment Base Address function (Int 31H
   Function 0007H).

o   Refer to the rules for descriptor usage in Appendix D.