DOS Fn 6cH: Extended Open/Create

                                                         Compatibility: 4.0+ 
 Expects: AH    6cH
          BX    extended open mode flags (see below)
          CX    file attribute (used only if file gets created)
          DX    action: 00-0h = if file exists, fail the call
                        00-1h = if file exists, open it
                        00-2h = if file exists, replace and open it
                        000-h = if doesn't exist, fail the call
                        001-h = if doesn't exist, create and open it
          DS:SI address of an ASCIIZ▲ string of a filespec to open
          ──────────────────────────────────────────────────────────────────
 Returns: AX    error code if CF is set to CY
                file handle if no error
          CX    action taken: 0 file was opened
                              1 file was created and opened
                              2 file was replaced and opened
          ──────────────────────────────────────────────────────────────────
    Info: This function opens (or creates and opens) a file.  Think of it
          as a smart 3dH Open which can optionally 3cH Create the file if
          it doesn't exist.

          Additionally, this call can disable normal INT 24h critical error
          handling and can force all writes to go direct to the disk.

    DS:DX points to an ASCIIZ string in the form...
             d:\path\filename.ext0
          If the drive or path is omitted, defaults are assumed.

       BX On entry, BX contains extended mode flags as follows:

           1 1 1 1 1 1
          ╓5┬4┬3┬2┬1┬0┬9┬8╥7┬6┬5┬4┬3┬2┬1┬0╖       See Open Mode for more
          ║0│d│f│0 0 0 0 0║i│ shr │0│ r/w ║       info on bits 0-7
          ╙─┴╥┴╥┴─┴─┴─┴─┴─╨╥┴─┴─┴─┴─┴─┴─┴─╜ bits
             ║ ║           ║ ╚═╦═╝   ╚═══╩═► 0-2: read/write access mode
             ║ ║           ║   ╚═══════════► 4-6: sharing mode
             ║ ║           ╚═══════════════►   7: inheritance
             ║ ╚═══════════════════════════►  13: 0=normal INT 24H use
             ║                                    1=fail critical errors
             ║                                      on open and all I/O
             ╚═════════════════════════════►  14: 0=normal file buffering
                                                  1=directly to disk

          If bit 13 is set (2000H), then I/O through this handle will never
          cause an INT 24H critical error.  If a drive door is open (etc.),
          then the function which caused the error will fail with a regular
          DOS error.  Use Fn 59H to find out what happened.

          If bit 14 is set (4000H), then the normal DOS output buffering is
          bypassed and all output goes directly to the disk (as if you had
          called Fn 68H after each write).  This can hurt I/O performance,
          but it maximizes safety.

       CX When creating a new file, set CX to the desired  file attribute
          (read-only, hidden, etc.).  Use 0000H for normal files.  If the
          file exists, CX is ignored.

       DX This controls the open/create action, depending upon whether the
          file exists at the time of the call.  Common combinations are:
              0001H = open the file if it exists; otherwise fail the call
              0011H = open the file if it exists; otherwise create it
              0012H = if the file exists open and truncate it to length 0;
                      otherwise create it and open it

   Notes: ■ Perhaps the most useful feature here is the ability to avoid
            special handling for INT 24H critical errors.  Of course, you
            can only discard your INT 24H handler if you are sure that the
            application is used only under DOS 4.0+.

See Also: Handle-Oriented File I/O
          INT 24H
          DOS Functions
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