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  Interrupt:   21h     Function:   3Dh
 
  Title:       Open File
 
  See also:    Close File, Extended Open File, Create File, @OpenFile
 
  Description:
 
     Given an ASCIIZ path name, the function opens the specified file
     in the designated or default directory on the designated or
     default disk drive. A handle is returned that can be used by the
     program for subsequent access to the file.
 
     Input                                   Output
 
     AH = 3Dh                                If function successful
     AL = Access mode                          Carry flag: clear
                                               AX = Handle
          Bits    Significance
                                             If function unsuccessful
          0-2     Access mode                  Carry flag: set
                  000 = read access            AX = Error code (02h, 03h,
                  001 = write access                04h, 05h, or 0Ch)
                  010 = read/write access      See: ◄Error codes►
          3       Reserved (0)
          4-6     Sharing mode
                  [Version 3.0+]
                  000 = compatibility mode
                  001 = deny all
                  010 = deny write
                  011 = deny read
                  100 = deny none
          7       Inheritance flag
                  [Version 3.0+]
 
     0 = Child process inherits handle
     1 = Child does not inherit handle
     DS:DX = segment:offset of ASCIIZ
             path name
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