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  Interrupt:   21h     Function:   5Ch
 
  Title:       Lock or Unlock File Region [Version 3.0+]
 
  See also:    Set File Pointer
 
  Description:
 
     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)
     BX = Handle                         If function unsuccessful
     CX = High part of region offset       Carry flag: set
     DX = Low part of region offset        AX = Error code (01h, 06h,
     SI = High part of region length            21h, or 24h)
     DI = Low part of region length        See: ◄Error codes►
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