PCI Express FAQ for Graphics

PCI Express
PCI Express (PCIe) is a new I/O bus technology that, over time, will replace Peripheral Component Interconnect (PCI), PCI-X, and Accelerated Graphics Port (AGP). By providing advanced features and increased bandwidth, PCIe addresses many of the shortcomings of PCI, PCI-X, and AGP. PCIe retains full software compatibility with PCI Local Bus Specification 2.3, and it replaces the parallel multidrop bus architecture of PCI and PCI-X with a serial, point-to-point connection bus architecture.
Two PCIe devices are connected by a link, and each link is made up of one or more lanes. Each lane consists of two low-voltage, differential signal pairs carrying 2.5 Gbps of traffic in opposite directions. One pair is used for transmitting, and the other pair is used for receiving. To further increase the bandwidth of a link, multiple lanes can be placed in parallel (x1, x2, x4, x8, x12, x16, or x32 lanes) between two PCIe devices to aggregate the bandwidth of each individual lane. In the future, the signaling rate of the link can be increased to provide even more bandwidth.
PCIe hardware is backwards compatible with PCI software on the Microsoft Windows 2000 and Microsoft Windows XP operating systems. The PCI features supported by current Windows operating systems will continue to work with PCIe without any need for modifications in the applications, drivers, or operating system; however, the advanced PCIe features will be natively supported only in Windows Vista and later versions of Windows.

PCI Express Graphics
It is well known that graphics can always use more bandwidth than what is available. Graphics data transfers cause maximum traffic on the PCI bus. The continual increase in graphics demand and complexity eventually made the PCI bus insufficient, which led to the invention of AGP. Now we are pushing the limits of what AGP can deliver, and we need a better solution. PCIe surpasses AGP in bandwidth availability, with more room for expansion in the near future. By increasing the number of lanes in a link, graphics adapters can take advantage of increased bandwidth and faster data transfer. Graphics adapters will be using the X16 link, which will provide a bandwidth of 4 Gbps in each direction.
Given the higher bandwidth offered by PCIe, systems are already moving away from AGP to PCIe. There will not be many systems that provide both AGP and PCIe connectors. The first X16 graphics adapters and PCIe systems should be available in summer 2004.
PCI Express Graphics in Vista
The Windows Vista Display Driver Model (WDDM) will have specific requirements for PCIe graphics adapters. It will require that the 64-bit addressing mode be supported by the GPU. However, a minimum of 40 bits of physical address bits must be implemented. The unimplemented bits should be forced to zero. These requirements are not applicable to the Windows XP display driver model.
PCIe Graphics & AGP
In addition to the bandwidth considerations mentioned above, there are several other differences between AGP and PCIe.
By definition, AGP requires a chipset with a graphics address relocation table (GART), which provides a linear view of nonlinear system memory to the graphics device. PCIe, however, requires that the memory linearization hardware exist on the graphics device itself instead of on the chipset. Consequently, driver support for memory linearization in PCIe must exist in the video driver, instead of as an AGP-style separate GART miniport driver. Graphics hardware vendors who want to use nonlocal video memory in their Windows XP driver model (XPDM) drivers must implement both memory linearization hardware and the corresponding software. All PCIe graphics adapters that are compatible with the WDDM must support memory linearization in hardware and software.
AGP was dedicated to graphics adapters, and no other device class used it. PCIe is intended to be used by all device classes that previously used PCI. With AGP, a number of video drivers were directly programming the chipset, which gave rise to severe ill effects such as crashing and memory corruption in the graphics stack. Because PCIe will be used for all devices in the system, it is even more important that video drivers not program the chipset directly.

Information From
http://www.microsoft.com/whdc/system/platform/64bit/IA64_ACPI.mspx