DOS reserves some device names for its own use.  You should not use them as
filenames.  Their names and meanings are described below:

CON  - This specifies the console screen and keyboard.  If it is used as
       an input device, pressing the Ctrl-Z key will generate an "end-of-file"
       indication and end the use of CON as an input device.

PRN or LPT1  - This specifies the first parallel printer.  The printer device
               may be used as an output device only.  LPT2 and LPT3 are also
               names reserved for use with the IBM PC.

AUX or COM1  - This is normally the serial port, however when the optional
               modem is installed this is designated as the modem port and the
               serial port is reassigned to the device COM2!

COM2  - The serial port gets reassigned to this device when the modem option is
        installed.

NUL  - This is a nonexistent device that can be used when testing programs
       or where only certain portions of the output should be displayed or
       printed.  As an input device, an "end-of-file" is generated on the
       first read, as a high speed output device the write operation is
       simulated, but no output is generated.
