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  Interrupt:   21h     Function:   6Ch
 
  Title:       Extended Open File [Version 4.0+]
 
  See also:    Open File
 
  Description:
 
     Given an ASCIIZ path name, opens, creates, or replaces a file in
     the designated or default directory on the designated or default
     disk drive. Returns a handle that can be used by the program for
     subsequent access to the file.
 
     Input                            Output
 
     AH = 6Ch                           If function successful
     AL = 00h                             Carry flag: clear
     BX = Open mode                       AX = Handle
                                          CX = Action taken
          Bits   Significance                  1 = File existed and
                                                   was opened
          0-2    Access type                   2 = File did not exist
                 000 = read-only                   and was created
                 001 = write-only
                 010 = read/write       If function unsuccessful
          3      Reserved (0)           Carry flag: set
                                        AX = Error code
          Bits  Significance (if set)
 
          4-6   Sharing mode
                000 = compatibility
                001 = deny read/write
                      (deny all)
                010 = deny write
                011 = deny read
                100 = deny none
          7     Inheritance
                0 = child process
                    inherits handle
                1 = child does not
                    inherit handle
          8-12  Reserved (0)
          13    Critical error
                handling
                0 = execute Int 24h
                1 = return error
                    to process
          14    Write-through
                0 = writes may be
                    buffered and
                    deferred
                1 = physical write at
                    request time
          15    Reserved (0)
 
     CX = File attribute (bits may be
          combined; ignored if open)
 
          Bits   Significance (if set)
 
          0      Read-only
          1      Hidden
          2      System
          3      Volume label
          4      Reserved (0)
          5      Archive
          6-15   Reserved (0)
 
     DX = Open flag
 
          Bits   Significance
 
          0-3    Action if file exists
                 0000 = fail
                 0001 = open file
                 0010 = replace file
          4-7    Action if file
                 doesn't exist
                 0000 = fail
                 0001 = create file
          8-15   Reserved (0)
 
     DS:SI = segment:offset of ASCIIZ
             path name
                                    -♦-