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There was once a time
when running an intensive 3D game at a resolution of 1600x1200 was
wishful thinking. Frame rates were laughable,
at best. And what about Anti-aliasing and anisotropic filtering?
No can do.
Believe it or not, that
was a scant three and a half years ago, when
NVIDIA's GeForce2 Ultra roamed the earth devouring
hapless RADEON and Voodoo5 cards, before swallowing the
once dominant 3dfx
whole. Now where are we? Blasting
through the latest first-person shooters at that
mythological 1600x1200, toying with those extra knobs
and switches enabled by gratuitous fill rate and
ever-widening memory buses. There's no doubt about
it - hardware is outpacing software and it doesn't
show any indication of relenting. Sure, Doom III
is on the way and so is Half-Life 2, eventually.
Yet
NVIDIA and ATI are both determined to
enable even more realism; an era of interactive 3D that rivals "Finding Nemo" for graphical glory.
Clearly, that caliber
of content is a ways off. It's no matter, though; we're
perfectly happy playing multiplayer Halo on RADEON 9800
XT and GeForce FX 5950 Ultra cards until that happens, right?
But it's a different story entirely for the big swinging
GPU manufacturers anxious to convince you that dropping $500
for cutting edge graphics is justified.
Also consider that the FCC
voted in 2002 to require electronics manufacturers to
include digital tuners in all new television sets.
What does that have to do with PC graphics? Well,
HDTV is on the way, and nobody wants to be left behind.
We've already seen ATI's HDTV Wonder announcement, which
precedes mainstream acceptance of a technology already
available from companies like DVICO and MIT. Once
high-definition video starts finding its way into more
homes, it seems natural that HD editing will follow.
And ATI already demonstrated an example of HD editing in
conjunction with Intel and Pinnacle Systems at IDF in
February. According to ATI's press release, one
very important architectural feature enabled that
demonstration - PCI Express.
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PCI Express |
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A
little more about the technology... |
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PCI-SIG
(the Peripheral Component Interconnect Special Interest Group),
defines PCI Express as "...an open specification designed from the start to address the wide range of current and future system interconnect requirements of multiple market segments in the computing and communications industries. The PCI Express Architecture defines a flexible, scalable, high-speed, serial, point-to-point, hot pluggable/hot swappable interconnect that is software-compatible with PCI."
That's
a pretty dense specification summary, so we'll go over
it at a bit more length. Firstly, PCI Express is
an open specification, meaning anyone can implement PCI
Express. Although Intel will be the first
manufacturer to debut the technology, representatives at
NVIDIA have commented that the firm will also support
PCI Express in an upcoming version of the nForce3
chipset once Intel unveils the Grantsdale and Alderwood
chipsets. Moreover, SiS already announced its own
965 South Bridge, which incorporates two PCI Express 1x
connectors, in addition to eight-channel audio,
four-port Serial ATA, and integrated Gigabit Ethernet.
Secondly, PCI Express is flexible. That is, its
architectural design facilitates the ability to scale by
adding "lanes." Initial implementations of PCI
Express will employ single-lane and 16-lane designs.
Should the need arise, there's also a 32-lane
specification for even higher levels of bandwidth.
As it stands, the 16x slot offers 4GB per second of
throughput, both up and down the pipeline, over double
the amount of AGP 8x.
Finally, PCI Express is high-speed, serial, and
point-to-point. Existing platforms that employ PCI
share the interface's 133MB per second, however, PCI
Express enables a >200MB per second between each 1x slot
and the platform's core logic. The PCI-SIG
explains the interface's serial nature by saying, "Serial bus architectures deliver more bandwidth per pin than parallel bus architectures, and they scale more easily to higher bandwidths. Serial bus architectures enable a network of dedicated point-to-point links between devices as opposed to the multi-drop basis of parallel bus architectures. This eliminates the need for bus arbitration, provides deterministic low latency, and greatly simplifies hot plug/hot swap system implementations."
Of
course, PCI Express won't replace PCI overnight.
Rather, the two interfaces will co-exist for some time.
There also shouldn't be any problems when it comes to
software support. PCI Express graphics cards won't
require any programming considerations from third-party
software developers. Only those toying with BIOS
files and drivers will have to concern themselves with
the difference.
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NVIDIA's Approach |
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The bridge chip |
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Now,
there's been a fair bit of debate back and forth between
ATI and NVIDIA over the "best" way to move from today's
AGP standard to the impending takeover of PCI Express.
Jen-Hsun Huang, NVIDIA's CEO, already commented to the
effect that the NV4x family is to be natively PCI
Express. However, the PCI Express cards slated to
emerge before NV4x will require NVIDIA's own HSI (High
Speed Interconnect) bridge chip for compliance.
Four products with the bridge chip have already been
announced - the GeForce PCX 5950, the GeForce PCX 5750,
the GeForce PCX 5300 (a GeForce FX 5200 with PCI Express
support) and a GeForce PCX 4300 (an older GeForce4 MX
440 adapted for PCI Express).
According to NVIDIA, its decision to use the HSI bridge
is influenced by a couple of factors. First, it
allows the firm to manufacture one GPU with support for
two interfaces. In the case of its NV3x family,
AGP is supported natively. The HSI bridge provides
quick access to PCI Express operability. When NV4x
rolls around with PCI Express support, the same HSI
bridge can be reversed for an AGP variant of the board.
It also claims that a bridge chip is cheaper to
manufacture, since it precludes the need for two
versions of the same GPU - one with AGP support and the
other sporting PCI Express. The cost of taping out a
graphics processor is reportedly about $1 million
dollars, so arguing in favor of economics seems solid
given an entire product range comprised of multiple
chips that would need to exist in both AGP and
PCI Express form.
"But
what about the augmented bandwidth numbers?" you ask.
According to NVIDIA's spokespeople, the current
generation of graphics processors is manufactured with
enough tolerance to withstand an accelerated AGP bus -
the equivalent of AGP 16x, they claim. Older
architectures, presumably the GeForce MX 440, are
limited to an equivalent of AGP 12x made possible by
employing short traces from the GPU to bridge chip. The
following diagram illustrates a typical PCI Express
usage model. Note that the bandwidth numbers can
be altered, and the total 4.2GB per second can be
allocated however needed. And the 266MB per second
upstream limitation commonly believed to ail AGP cards
is not a factor at all, as that only holds true for
platforms limited to PCI-writes. The HSI bridge
supports AGP writes, alleviating that potential
bottleneck.
Theoretical PCI Express
Bandwidth
Effective PCI Express
Bandwidth
Typical PCI Express
Usage, Per NVIDIA
ATI claims that NVIDIA's
implementation introduces the possibility of latency,
which NVIDIA vehemently denies. Instead, by
maximizing in-flight requests, utilizing 64-byte request
sizes instead of 32, and bolstering AGP speeds, the PCX
family purportedly circumvents latency. Then
again, the argument is hard to debate given the lack of
PCI Express hardware and a general absence of software
demanding enough to observe adverse amounts of latency.
NVIDIA is confident in the
readiness of its PCI Express lineup, and is reportedly
already validated on Intel's Grantsdale (desktop),
Tumwater (workstation), and Alviso (mobile) chipsets.
The company demonstrated a PCI Express platform playing
back HDTV content during the most recent IDF, but it
should be noted that even today's AGP cards are capable
of that.
ATI's Approach and Conclusion
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