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  Description:  Lock or Unlock File Region     DOS 3 Only
 
  Locks or unlocks a specified region of a file that was previously opened
  or created with Int 21H Functions 3CH, 3DH, 5AH, or 5BH. This function
  is not available unless the file-sharing module (SHARE.EXE) is loaded.
 
  Input                                     Output
  ══════════════════════════════            ══════════════════════════════
  AH=5CH                                    If function successful
  AL=00H  if locking region                 Carry flag=clear
     01H  if unlocking region               If function unsuccessful
  BX=handle                                 Carry flag=set
  CX=high part of region offset             AX=error code
  DX=low part of region offset
  SI=high part of region length
  DI=low part of region length
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