4.1 Device Terminology

A peripheral device is piece of hardware accessed by a computer system. Common peripheral devices include disks, tapes, CD-ROMs, screens, keyboards, printers, mouse devices and network interfaces. User programs perform control and I/O to these devices through system calls to operating system modules called device drivers . A device driver hides the details of device operation and protects the device from unauthorized use. Devices of the same type may vary substantially in their operation, so to be usable, even a single-user machine needs device drivers. Some operating systems provide pseudodevice drivers to simulate devices such as terminals. Pseudoterminals, for example, simplify the handling of remote login to computer systems over a network or a modem line.

Some operating systems provide specific system calls for each type of supported device, requiring the systems programmer to learn a complex set of calls for device control. UNIX has greatly simplified the programmer device interface by providing uniform access to most devices through five functions ” open , close , read , write and ioctl . All devices are represented by files, called special files , that are located in the /dev directory. Thus, disk files and other devices are named and accessed in the same way. A regular file is just an ordinary data file on disk. A block special file represents a device with characteristics similar to a disk. The device driver transfers information from a block special device in blocks or chunks , and usually such devices support the capability of retrieving a block from anywhere on the device. A character special file represents a device with characteristics similar to a terminal. The device appears to represent a stream of bytes that must be accessed in sequential order.