I/O and device drivers

Device drivers in the UNIX system provide an interface to peripheral devices that is hardware-independent, just as they do in DOS systems. A programmer can write a program that outputs to disk, terminals, or printer without concern for the details of interfacing with the hardware. The hardware interface only needs to be written once and then utilized many times.

Device I/O in the UNIX system differs from the DOS implementation in two significant ways:

• Multiple tasks access the device concurrently. This gives the device driver writer the opportunity to optimize hardware accesses between tasks. For example, a driver for a hard disk can delay access to disk blocks far away from the current head position by batching these requests for execution at a later time. Because the UNIX operating system is multithreaded, another process can execute while the first process waits for the disk.

• Data sent to the device may not actually be written to the device immediately because of operating system buffering. This increases the efficiency and throughput of the operating system, although it may delay an individual write operation for peripherals such as disks and serial devices. Flushing the buffers forces data to the device but can greatly decrease the overall O/S efficiency.

• I/O port access is possible only in kernel mode. This is addressed by linking the device driver into the kernel.