Get Segment Base Address

Returns the 32-bit linear base address from the LDT
descriptor for the specified segment.

Call With:

   AX     = 0006H
   BX     = selector

Returns:

   if function successful
   Carry flag= clear
   CX:DX  = 32-bit linear base address of segment

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8022H   invalid selector

Notes:

o  Client programs must use the LSL instruction to query the
   limit for a descriptor.  Note that on 80386 machines, the
   client must use the 32-bit form of LSL if the segment
   size is greater than 64 KB.
o  Refer to the rules for descriptor usage in Appendix D.