Int 21 Fn 2531  P - Phar Lap 386/dos-extender Vmm V3.0 - Read/write Ldt D  [E]

   AX = 2531h
   BX = segment selector
   ECX = direction (00h read, 01h write)
   DS:EDX -> 8-byte buffer for descriptor contents

Return: CF clear if successful
   CF set on error
       EAX = error code
       81h invalid selector
       82h DPMI running, or not a code or data segment