by a number uniquely identifying the particular device. For hard drives, a letter is used to identify devices and is followed by a number to identify partitions. Thus a file system may "know" an area on a disk as , for example, or "see" a networked terminal session as associated with . On disks using the typical PC master boot record, the device numbers of primary and the optional extended partition are numbered 1 through 4, while the indexes of any logical partitions are 5 and onwards, regardless of the layout of the former partitions (their parent extended partition does not need to be the fourth partition on the disk, nor do all four primary partitions have to exist). Device names are usually not portable between different Unix-like system variants, for example, on some BSD systems, the IDE devices are named , , etc. devfs devfs is a specific implementation of a device file system on Unix-like operating systems, used for presenting device files. The underlying mechanism of implementation may vary, depending on the OS. Maintaining these special files on a physically-implemented file system such as a hard drive is inconvenient, and as it needs kernel assistance anyway, the idea arose of a special-purpose logical file system that is not physically stored. Defining when devices are ready to appear is not trivial. The devfs approach is for the device driver to request creation and deletion of devfs entries related to the devices it enables and disables. PC DOS, TOS, OS/2, and Windows A device file is a reserved keyword used in PC DOS, TOS, OS/2, and Windows systems to allow access to certain ports and devices. MS-DOS borrowed the concept of special files from Unix but renamed them devices. Because early versions of MS-DOS did not support a directory hierarchy, devices were distinguished from regular files by making their names reserved words. This means that certain file names were reserved for devices, and should not be used to name new files or directories. The reserved names themselves were chosen to be compatible with "special files" handling of PIP command in CP/M. There were two kinds of devices in DOS: Block Devices (used for disk drives) and Character Devices (generally all other devices, including COM and PRN devices). DOS uses device files for accessing printers and ports. Most versions of Windows also contain this support, which can cause confusion when trying