Creating a Third Generation I/O Interconnect

[Prasanth]

This paper looks at the success of the widely adopted PCI bus and describes a higher performance next generation of I/O interconnect, called PCI Express* Architecture, that will serve as a standard local I/O bus for a wide variety of future computing platforms. Key PCI attributes, such as its usage model and software interfaces are maintained whereas its bandwidth-limiting, parallel bus implementation is replaced by a long-life, fully-serial interface. A split-transaction protocol is implemented with attributed packets that are prioritized and optimally delivered to their target. The new PCI Express Architecture comprehends a variety of form factors to support smooth integration with PCI and to enable new system form factors. PCI Express Architecture will provide industry leading performance and price/performance.

The PCI bus has served us well for the last 10 years and it will play a major role in the next few years. However, today’s and tomorrow’s processors and I/O devices are demanding much higher I/O bandwidth than PCI 2.2 or PCI-X can deliver and it is time to engineer a new generation of PCI to serve as a standard I/O bus for future generation platforms. There have been several efforts to create higher bandwidth buses and this has resulted in the PC platform supporting a variety of application-specific buses alongside the PCI I/O expansion bus.

The processor system bus continues to scale in both frequency and voltage at a rate that will continue for the foreseeable future. Memory bandwidths have increased to keep pace with the processor. Indeed, as shown in Figure 1, the chipset is typically partitioned as a memory hub and an I/O hub since the memory bus often changes with each processor generation. One of the major functions of the chipset is to isolate these ever-changing buses from the stable I/O bus.

Close investigation of the 1990’s PCI signaling technology reveals a multi-drop, parallel bus implementation that is close to its practical limits of performance: it cannot be easily scaled up in frequency or down in voltage; its synchronously clocked data transfer is signal skew limited and the signal routing rules are at the limit for cost-effective FR4 technology. All approaches to pushing these limits to create a higher bandwidth, general purpose I/O bus result in large cost increases for little performance gain. The desktop solution of Figure 1 is only part of the problem of diverging local I/O bus standards. To PCI’s credit it has been used in applications not envisaged by the original specification writers and variants and extensions of PCI can be found in desktop, mobile, server and embedded communications market segments.