Int 16 Fn FF98  U - PC Tools V7+ Desktop - Open \desk.ovl File And Seek T  [U]

   AX = FF98h
   DX = byte offset in file of overlay header

Return: BX = file handle for DESK.OVL file
Desc:  open the DESK.OVL file, seek to the specified offset, read in the
     overlay header, and seek to the offset specified by the header

Format of overlay header:
Offset Size    Description
 00h 12 BYTEs  NUL-padded ASCII overlay filename
 0Ch   DWORD   offset within DESK.OVL file of actual overlay