The file system recognizes file names as special device files if the file names consist of exactly three uppercase letters, a single digit, and a colon (:). This format follows the convention established in the Microsoft® MS-DOS® operating system for serial and parallel ports. For example, “COM1:”, “PGR7:”, and “GPS0:” are valid device file names, but “MODEM1:”, “COM27:”, and “LPT1” are not.