Int 21 Fn 2539  P - Phar Lap 386/dos-ext Vmm V3.0 - Get Offset Of .exp Fi  [E]

   AX = 2539h
   BX = MS-DOS file handle for open file

Return: CF clear if successful
       EAX = offset of .EXP header in file
   CF set on error
       EAX = error code (02h file error)
       EBX = file error code
       02h DOS error seeking
       03h DOS error reading
       04h invalid file type
       05h invalid file format
       ECX = DOS error code if EBX=02h or 03h
   current file position in file modified