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...
d:\path\filename.ext 0 
...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
taken.

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.

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DOS Fn 3dH: Open a File via Handle