I/O management in Unix

There are two main categories of I/O units in UNIX, block devices and character devices.

In addition, there are sockets that are used for network communication.

Block devices

• Devices that address blocks of a fixed size, usually disk memories.

• Data blocks are buffered in the buffer cache.

• Block devices are usually called via the file system, but are also available as special files (for example /dev/hde1).

Character devices

• Terminals and printers, but also everything else (except sockets) that do not use the block buffer cache.

• There are for example /dev/mem that is an interface to physical memory.

• Device drivers are called via a switch table.

• There is one switch table for block devices and one for character devices.

• A hardware device is identified by its type (block or character) and a device number.

• Device numbers consist of two parts: major device number and minor device number.

• Major device number is used as an index in the switch table to locate the correct device driver.

• Minor device number is forwarded to the device driver and used to select the correct subunit. (For example correct file system partition if the disk is divided into several partitions).