Linux Device Drivers, Second Edition

Using Digital I/O Ports

The sample code we use to show port I/O from within a device driver acts on general-purpose digital I/O ports; such ports are found in most computer systems.

A digital I/O port, in its most common incarnation, is a byte-wide I/O location, either memory-mapped or port-mapped. When you write a value to an output location, the electrical signal seen on output pins is changed according to the individual bits being written. When you read a value from the input location, the current logic level seen on input pins is returned as individual bit values.

The actual implementation and software interface of such I/O ports varies from system to system. Most of the time I/O pins are controlled by two I/O locations: one that allows selecting what pins are used as input and what pins are used as output, and one in which you can actually read or write logic levels. Sometimes, however, things are even simpler and the bits are hardwired as either input or output (but, in this case, you don’t call them “general-purpose I/O” anymore); the parallel port found on all personal computers is one such not-so-general-purpose I/O port. Either way, the I/O pins are usable by the sample code we introduce shortly.

An Overview of the Parallel Port

Because we expect most readers to be using an x86 platform in the form called “personal computer,” we feel it is worth explaining how the PC parallel port is designed. The parallel port is the peripheral interface of choice for running ...