Fixed typographical error in code sample.

Corrected typographical errors.

Fixed some example errors, reformatted code examples, and clarified some wording now that Intel-based hardware support is no longer Darwin-specific.

Minor typographical fixes.

Added information on IOFilterInterruptEventSource as it relates to interrupt blocking.

Filled in missing material about PCI bridges and AGP devices. Added references to Designing PCI Cards and Drivers. Added a chapter about endianness and how it differs from byte-invariant addressing. Added revision history.