Block device

In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file. There are also special files in MS-DOS, OS/2, and Microsoft Windows. They allow software to interact with a device driver using standard input/output system calls, which simplifies many tasks and unifies user-space I/O mechanisms.

Device files often provide simple interfaces to peripheral devices such as printers and serial ports, but they can also be used to access specific resources on those devices, such as disk partitions. Finally, device files are useful for accessing system resources that have no connection with any actual device such as data sinks and random number generators.

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 MS-DOS: Block Devices (used for disk drives) and Character Devices (generally all other devices, including COM and PRN devices). PIPE, MAILSLOT, and MUP are other standard Windows devices.

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