Inside The 875P Chipset
875P Memory Controller Hub
Persisting with its Hub Architecture, 875P actually consists of several components. The first, and arguably most important, is the 875P Memory Controller Hub. The job of the MCH is that of an intermediary between processor, memory, graphics card, ICH, and now Ethernet interface. Previously, support for integrated networking was tacked on as an extension of the I/O Controller Hub (ICH) and as such, shared its 266MB/s with IDE, USB, and PCI devices.
Technologies like Gigabit Ethernet have the potential to severely tax that link, not only reducing network performance, but also potentially limiting other subsystems as well. Intel’s Communications Streaming Architecture (CSA) sees the Ethernet link moved up to the MCH and granted a dedicated 266MB/s link. As a result, CPU utilization is reportedly decreased, multiple data streams are more easily managed and ultimately, platform performance is enhanced.
CSA is linked to MCH
875P Block Diagram
The E7205 “Granite Bay” chipset was Intel’s first desktop product with AGP 8x support. However, it was and continues to be prohibitively expensive, especially considering its relatively small performance advantage. For the time being, “Granite Bay” remains a workstation-centric product, ideal for those who can utilize its 4GB memory ceiling. But now that 875P is here, we’ve also got AGP 8x in the high-end enthusiast space, though it should be noted that VIA and SiS have supported it for some time already with their P4X400 and 648 chipsets, respectively. We’ve yet to see a notable performance enhancement attributable to AGP 8x, but perhaps the upcoming generation of DX9 games will exemplify the bandwidth advantages of the 2.1GBps link.
More significant is the 800MHz bus supported by 875P. Technically a 200MHz bus, quad-pumped, the new frequency enables a theoretical 6.4GB per second of bandwidth between the MCH and Pentium 4, up from 4.3GB per second. “Granite Bay” is an ideal match for 533MHz front side bus processors because it supports two channels of DDR266 memory, creating a balance. The same does not hold true for the 800MHz chips, which are capable of utilizing significantly more bandwidth. So, Intel has equipped 875P with two, 64-bit channels of DDR400 memory that are able to match the front side bus, in theory.
The final addition to 875P, and the feature that distinguishes 875P from the upcoming 865 family of chipsets, has been dubbed Intel’s Performance Acceleration Technology. In essence, PAT facilitates tighter timings within the MCH, internally speeding up memory accesses regardless of the installed modules. “Springdale” silicon isn’t validated to take advantage of these improvements, giving 875P a small performance boost. With several new features in place, Intel has positioned the 875P MCH as a replacement for i850E, which Intel still considers its flagship Pentium 4 chipset. It should also be noted that Intel’s roadmap currently doesn’t show plans for a version of 875P with integrated graphics. Variants of “Springdale” will include graphics, but these will likely be directed towards the mainstream market.