Intel hammers the final nail into RDRAM’s coffin with its wholesale adoption of dual-channel DDR technology. John Gillooly’s at the funeral.
Even!though the CPU wars have calmed of late, largely due to AMD's constant delaying of the Athlon 64 desktop CPU, both processor makers have continued to provide minor boosts and tweaks to their product lines. While AMD is due to take the double-pumped Athlon XP bus to an effective 400MHz 'real soon now', Intel has already made its move, unleashing a revised Northwood core that runs at an effective 800MHz on its quad-pumped bus.
Both of these boosts seemed unlikely until very recently. Intel had quite publicly made it know last year that RDRAM was on its last legs and the huge consumer support for DDR-RAM was the catalyst. But the Pentium 4 is a bandwidth hungry beast, a by-product of Intel's original development push for a highly scaleable architecture. DDR would mean that the maximum bandwidth available for the CPU would be 2.7GB/s when using memory based upon the latest JEDEC DDR333 standard. This was not a huge problem for the Athlon XP as DDR333 provided ample bandwidth for the then standard 266MHz FSB and would easily cope with a planned upgrade to a 333MHz FSB.
Intel launched some very fast DDR chipsets, but even the top-end i845PE was unable to top the performance of the i850E chipset running PC1200 RDRAM. To combat this Intel focused on dual-channel DDR technology, something that had little more than novelty value in NVIDIA's nForce chipset for the Athlon. However the Pentium 4 would be better placed to leverage dual-channel DDR. It's then 533MHz FSB would be ideally matched with dual-channel DDR266, something that eventually emerged as part of Intel's workstation chipset, the e7205 (formerly codenamed Granite Bay).
With the e7205 DDR finally managing a convincing performance win over RDRAM, Intel ramped up its plans for the second piece of this particular performance booster -- another increase in frontside bus speed. The initial plans were to up the FSB to 667MHz effective, matching it with dual-channel DDR333, but these were never to see the light of day, even with motherboards being launched that touted pre-emptive 667MHz FSB support. Instead Intel did an unlikely thing and started pushing DDR400, a draft standard that had looked until that moment like it would never actually enter the market.
DDR400 meant that Intel could ramp the Pentium 4 FSB up to 800MHz, which would provide ample performance boosts until the next generation Prescott core could be launched. An 800MHZ FSB Pentium 4 would need 6.4GB/s of memory bandwidth; DDR400 delivers 3.2GB/s in a single-channel configuration, and the magic 6.4GB/s in the dual-channel one.
This dream was to become a reality with a new 3GHz Pentium 4 (not to be confused with the 533MHZ FSB 3.06GHz model) and two chipsets, codenamed Springdale and Canterwood.
Enthusiastic backflipCanterwood, now known as i875P, is a refreshing and remarkable change for a company that once had a global policy of contacting every journalist who ever extolled the virtues of overclocking in order to educate them on the evils of the practice. It began life as a workstation chipset but has been released for the desktop with the enthusiast in mind.
ABOVE: Dual-channel vs single-channel benckmark. For a full explanation of the test see the Dual-channeling boxout.
Springdale on the other hand has become the i865 and given the responsibility of bringing dual-channel DDR to the Pentium 4 mainstream. The Intel 865 will be available in both a standard variant, the i865PE and an integrated graphics version, the i865G. The current i845 chipsets can support the 800MHz FSB but this will vary from motherboard to motherboard and it is advisable to check the manufacturer's Website for compatibility.
The chipsets are physically identical, but in a move that we are very familiar with in the video card and CPU world, Intel has chosen and tested only the best quality dies for the i875P, and used this to get better timings and other optimisations out of the chipset. This should not only make the i875P a little more expensive, it should ensure that the 875P will outperform the i865 across the board.
Each chipset supports the full range of 400, 500 and 800MHz FSB Pentium 4 CPUs, along with DDR266, 333 and 400 running in either single or dual-channel mode, but for optimum performance, especially with i875P, then the combination of an 800MHz FSB and DDR400 is the key.
Besides being the fastest combination, this lets one of the major features of the i875P to come into play. It's called Performance Acceleration Technology (PAT), and optimises chipset timings even further when running DDR400 and an 800MHz FSB CPU. The other differentiating feature of the i875P is that it supports ECC memory for entry-level workstation use.
Dual-channel DDR is not a new concept; it was first implemented by NVIDIA in its Athlon-supporting nForce chipset. However the benefits of this were quite small, only noticeable in 3D benchmarks when using the integrated graphics capability. nForce2 also supports the technology, but it does not make a huge impact to the Athlon XP, whose 266 or 333MHz FSB is matched with that of DDR266 and 333.
The Pentium 4 stands to benefit more because it needs dual-channel DDR to provide a bandwidth match between CPU and system memory. We have seen these benefits already with the e7205 chipset, and they continue with the i875P and i865. But the other, often ignored benefit of dual-channel DDR is that it provides extra bandwidth for other components that need direct memory access, predominantly the AGP controller and the IDE controllers.
This can make for much smoother system performance and can be tested for by running benchmarks that stress the whole system.
We have tested the i875P in dual and single-channel memory configurations using SPECviewperf 7, which is a workstation graphics benchmark that stresses the CPU, AGP and I/O subsystem at the same time. We also tested with our standard Unreal Tournament 2003 graphics benchmark, at high quality 1,024x768. For the test we used DDR333 to avoid the extra effects that come into play when PAT is enabled.
UT2003 does not show much variance between single and dual-channel DDR. This is due to the fact that it predominantly stresses the CPU and GPU, and the need for extra bandwidth is minimal. However the much more balanced SPECviewperf benchmarks show a significant performance difference between the two memory configurations, with the performance boost in the range of 11-15% when using dual-channel DDR.
South central ICHIntel is also using these new chipsets to introduce a raft of updates to its platform. Both Northbridges support the AGP 3.0 (AGP 8x) standard, something that Intel had only previously supported with the e7205, however most of the action happens on the new ICH5 Southbridge.
Available in two flavours, the ICH5 and ICH5-R, the Southbridge adds serial ATA support as well as a new technology called Communications Streaming Architecture (CSA). It still contains a parallel ATA controller, and like its predecessors supports the newer USB 2.0 standard. The ICH5-R also supports hard
Issue: 137 | June, 2012