Some Hardware: The Parallel Port
Before walking through an example of a programmed I/O driver, it is helpful to look at some actual hardware. This serves the dual purpose of showing what kinds of devices tend to perform programmed I/O and providing a hardware example for the minimal driver to control.
How the Parallel Port Works
The parallel interface found on most PCs is based on an ancient standard from the Centronics Company. Although its original purpose was to communicate with printers, clever people have found many ways of attaching everything from disks to optical scanners to the parallel port. The DB-25 connector on this port carries a number of signals, the most important ones being
RESET. The CPU sends a pulse down this line when ...