There are certain times in the evolution of technology that serve as inflection points that forever change the course of events. For the computing sector and communications, the adoption of PCI Express will serve as one of these inflection points. PCI Express, a groundbreaking new general input/output architecture, allows computers to evolve far beyond the limitations imposed by their current infrastructure. In addition to this, PCI Express provides many new and exciting features such as Active State Power Management, Quality of Service, Hot Plug and Hot Swap support, and true isochronous capabilities.
Throughout this book there are references to three separate general I/O technologies. These technologies include: PCI (Peripheral Component Interconnect), PCI-X (a derivative of PCI), and the new architecture PCI Express. PCI-X is not an abbreviation for PCI Express. PCI-X evolved from PCI and is backward-compatible with PCI from a hardware and software perspective. PCI Express is neither an evolved nor an enhanced form of PCI or PCI-X, but does preserve the configuration, programming, and ordering models that PCI has established over the last ten years. At the physical level there are many differences between PCI/PCI-X and PCI Express. These differences are justifiable since they address many of the challenges that general I/O faces today, such as bandwidth limitations, physical space constraints, quality of service issues, and the inability to address immediate and future usage models.
These differences are also equally justified through the new abilities and opportunities they bring to the platform such as evolutionary and revolutionary design opportunities. Aside from the opportunity of introducing a brand new general I/O architecture, there are several motivations for writing this book. One of the primary motivations is to give the reader an easy-to-follow, introductory overview of PCI Express technology. This book is not a replacement for reading the PCI Express Base Specification. The opinion of the authors is that this book makes the PCI Express Base Specification easier to comprehend by giving it a context with extra background and insights into many areas of the technology. The second motivation is to prepare the industry for a transition to PCI Express architecture by discussing systemlevel impact, application-specific transitions, and the general timeline for consumer market introduction.