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PCI Express Details
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Computer systems and their subsystems (such as
graphics cards) need some way to send and receive data. Since 1997 the
conventional interconnect system for graphics adapters in PC workstation systems
has been the AGP (advanced graphics port) standard in 1x, 2x, 4x, and 8x
configurations.
These interconnects offer a maximum data
bandwidth of slightly more than 2GBps, but they can only convey data in one
direction at a time; that is, from the host computer to the graphics
subsystem or vice versa. But this can be a significant limitation. For example,
in order to maintain real-time frame rates, applications that merge real-world
video with computer-generated imagery require the ability to transmit huge
quantities of data in both directions simultaneously. Another issue with
the conventional AGP bus standard is that it’s becoming increasingly difficult
to route these multibit parallel buses at the circuit board level while keeping
all of the tracks one length and impedance.
There is a solution, however, and that is to
move to a serial data communications technology like PCI Express. A
single PCI Express lane uses a set of differential pairs to provide
point-to-point connections between two devices: one to transmit and the other to
receive data (Figure 1, below).
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Figure 1: A single PCI Express lane allows for transmitting and
receiving using a set of differential pairs. |
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This single lane would be referred to as a
x1 (”by-one”) implementation. Such a lane can support up to
0.25GBps of real data communications in both directions
simultaneously, which equates to a total bandwidth of 0.5GBps.
The PCI
Express standard allows multiple lanes to be grouped together to increase
bandwidth: x1, x2, x4, x8, x12, x16, and x32 lane widths are supported by
the standard (x32 is not currently available). A x1 connection might be used for
a networking application; a x4 by a standard broadcast video capture card; a x8
by a high-definition video capture card or a low-end graphics subsystem; while a
x16—with a total bandwidth of 8GBps for 4GBps moving in both directions
simultaneously—is required to satisfy the extreme bandwidth requirements of
high-end 3D graphics subsystems.
Figure 2: A high-end PCI Express power connector for the additional
needs of a high-end graphics subsystem shown left.
For an overview of
PCI Express
or to Compare Bus Speeds click here. |
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The high data bandwidths that can be achieved
by PCI Express-enabled computers and graphics subsystems offer the potential for
huge productivity gains in graphics-intensive applications such as CAD and
digital content creation.
When you purchase a motherboard that says it is
"PCI Express-enabled", what exactly does this mean? Different motherboards and
supporting chipsets offer different capabilities. For example, take three PCI
Express motherboards currently on the market, one has a x16 connector and a x4
connector; the second has a x16 connector and a x1 connector; and a third has a
x16 connector and three x1 connectors. Additionally, some PCI Express
motherboards may not even have a x16 connector, making do with two x8
connectors.
Thus, it’s important to ensure that the
computer you purchase supports your specific PCI Express requirements and the
physical size constraints for length, width, and height for the add-in cards you
plan to use.
Similarly, when you purchase a new graphics
card, it’s not enough to simply select any card that says it’s PCI
Express-enabled. Much like trying to squeeze the output from a fire hose through
a drinking straw, some graphics cards simply cannot take full advantage of the
bandwidth offered by PCI Express. In fact, it’s possible to purchase a high-end
AGP 8x-based card that significantly outperforms a low- to mid-range PCI
Express-based offering.
And last but not least, there’s the fact that
the PCI Express specification requires the motherboard to supply only 75W of
power. This specification satisfies the requirements only of low-end and
medium-range graphics subsystems. In order to address the needs of high-end
graphics subsystems, the specification also allows for an additional (optional)
75W connector on the main computer’s power supply to connect to the graphics
card (Figure 2). The key word here is “optional,” which means that you
have to ensure the system can handle the power needs of your graphics card
before you bring them together.
PCI Express-enabled computers and graphics
subsystems offer the promise of tremendous performance and productivity gains.
However, you first need to research the various subsystems to ensure that they
are compatible, and especially that the graphics card can take full advantage of
the bandwidth provided by PCI Express technology.
Application Benefits. What are the true
benefits to CAD end users? With the speed and data bandwidth that PCI Express
provides, CAD professionals now have the ability to transmit data to and from
the graphics accelerator and process extremely large amounts of textures in
real-time while manipulating their designs in a 3D environment. By harnessing
the power of PCI Express, quick data transfer rates mean workflows will not be
interrupted with lagging display redraws.
Additionally, PCI Express helps accelerate CAD
applications that render real-time shader programs to photorealistic quality and
mirror the material properties assigned to pieces of the design. These new
photorealistic material shaders include glass, metal, water, wood, plastics,
cloths, and other organic items. This industrial-strength shader support, which
is made possible by PCI Express’ fast data transfer rate, ensures that what is
represented on your display is an accurate rendition of what you would see in
the physical world.
Figure 3: PCI Express helps accelerate CAD
applications that render
real-time shader programs to photorealistic images. It augments the 3D graphics
pipeline
| It was also impossible to simulate fluid
flow and dynamic calculations on a single AGP or PCI in the past. PCI
Express delivers the large bandwidth needed to perform these calculations
completely on a fully programmable graphics accelerator (Figure 3, above).
This means faster completion of these calculations and the ability to
execute real-time flow analysis within your design.
Current CAD software applications are
written so that heavy processing and computing tasks are sent to the CPU
first. This means the CPU calculates and compiles data, then sends results
to the graphics card for additional manipulation or final rendering. While
the CPU is churning, the graphics card is idle. Today’s graphics cards
include more onboard memory reserves and faster visual processing units (VPU)
that compute graphics-intensive data faster than a CPU.
PCI Express can be a huge help in this
situation due to its large data transfer capabilities. As software
developers design new applications that use the graphics accelerator to
process data and then push the resulting data to the CPU for further
processing, more bandwidth between the system and graphics accelerator
will be required. Thanks to the improved bandwidth offered by PCI Express,
design professionals will see a huge increase in overall productivity.
Computer hardware is in a constant state
of evolution. With PCI Express, savvy computer shoppers should be able to
choose graphics, motherboard, and processing solutions that give them more
room to grow.
Written by: Clifton Robin, 3Dlabs.
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