Devices differ on many different dimensions, as outlined in Figure 13.7

Devices differ on many different dimensions, as outlined in Figure 13.7:

• Most devices can be characterized as either block I/O, character I/O, memory mapped file access, or network sockets. A few devices are special, such as time-of-day clock and the system timer.
• Most OSes also have an escape, or back door, which allows applications to send commands directly to device drivers if needed. In UNIX this is the ioctl( ) system call ( I/O Control ). Ioctl( ) takes three arguments - The file descriptor for the device driver being accessed, an integer indicating the desired function to be performed, and an address used for communicating or transferring additional information.