DOS Fn 3dH: Open a File via Handle

                                                         Compatibility: 2.0+ 
 Expects: AH    3dH
          AL    Access Mode (0=read, 1=write, 2=both, etc.)
          DS:DX address of an ASCIIZ string of a filespec
 Returns: AX    error code if CF is set to CY
                file handle if no error
    Info: Opens an existing file for read and/or write operations.

    DS:DX points to an ASCIIZ string in the form...
          ...which identifies the location and name of the file to open.
          If drive and/or path are omitted, defaults are assumed.

          If CF is set on return, an error code is in AX and no action was

   Notes: ■ The file must exist.  See Fn 3cH (create file).

          ■ The file is opened in the selected Access Mode / Open Mode.
            For compatibility with DOS 2.x and no network involvement, use:
            • AL = 0 to open for reading
            • AL = 1 to open for writing
            • AL = 2 to open for reading and writing

          ■ The read/write pointer is set to 0.  See DOS Fn 42H (Lseek).

          ■ CONFIG.SYS specifies how many handles are available or you may
            use Fn 67H (set handle count) to increase available handles.
          ■ Fn 6cH has additional options, such as disabling critical
            errors and forcing automatic write-through to disk.

See Also: Handle-Oriented File I/O
          DOS Functions