Note: This section contains an overview for readers who are not familiar with the details of the UNIX I/O system. All these points are covered in much greater detail in the IRIX Device Driver Programmer's Guide (see "Other Useful Books" on page xxiii). It is a basic concept in UNIX that all I/O is done by reading or writing files. All I/O devices--disks, tapes, printers, terminals, and VME cards--are represented as files in the file system. Each physical device is represented by an entry in the /dev file system hierarchy. The purpose of each device special file is to associate a device name with a a device driver, a module of code that is loaded into the kernel either at boot time or dynamically, and is responsible for operating that device at the kernel's request.