QPI (CSI) explained[11] [zz]

Speculations on CSI Clients

While the technical details of CSI are well documented in various Intel patents, there is relatively little information on future desktop or mobile implementations. These next two sections make a transition from fairly solid technical details into the realm of educated, but ultimately speculative predictions.

CSI will modestly impact the desktop and mobile markets, but may not bring any fundamental changes. Certain Intel patents seem to imply that discrete memory controllers will continue to be used with some MPUs [9]. In all likelihood, Intel will offer several different product variations based on the target market. Some versions will use integrated memory controllers, some will offer an on-package northbridge and some will probably have no system integration at all.

Intel has a brisk chipset business on both the desktop and notebook side that keeps older fabs effectively utilized – an essential element of Intel’s capital strategy. If Intel were to integrate the northbridge in all MPUs, it would force the company to find other products which can use older fabs, or shutter some of the facilities. Full integration also increases the pin count for each MPU, which increases the production costs. While an integrated memory controller increases performance by reducing latency, many products do not need the extra performance, nor is it always desirable from a marketing perspective.

For technical reasons, an integrated memory controller can also be problematic. Integrated graphics controllers share main memory to reduce cost. As a result, integrated graphics substantially benefits from sharing a die with the memory controller, as it does currently for Intel based systems. However, integrating graphics on the processor seems a little aggressive for a company that has yet to produce an on-die memory controller, and is a waste of cutting edge silicon – most high performance systems simply do not use integrated graphics.

Intel’s desktop version of Nehalem is code-named Bloomfield and it seems clear that the high performance MPUs, which are targeted at gamers, will feature on-die memory controllers. The performance benefits of reducing memory latency will probably be used as product differentiation by Intel to encourage gamers to move to the Extreme Edition line and justify the higher prices. However, on-die or on-package graphics is unlikely given that most OEMs will use higher performance discrete solutions from NVIDIA or AMD. The width of the CSI connection between the MPU and the chipset may be another differentiating factor. While a half-width link will work for mid-range systems, high-end gaming systems will require more bandwidth. Modern high performance GPUs use a PCI-E x16 slots, which provides 4GB/s in each direction. Hence, it is quite conceivable that by 2009 a pair of high-end GPUs would require ~16GB/s in each direction. Given that gaming systems often stress graphics, network and disk, a full width CSI link may be required to provide enough appropriate performance.

Other desktop parts based on Bloomfield will focus on low cost, and greater integration. It is very likely that these MPUs will be connected via CSI to a second die containing a memory controller and integrated graphics, all packaged inside a single MCM. A CSI link (probably half-width) would connect the northbridge to the rest of the chipset. This solution would let Intel use older fabs to produce the northbridge, and would enable more manufacturing flexibility – each component could be upgraded individually with fewer dependencies between the two. Intel will probably also produce a MPU with no integrated system features, which will let OEMs use chipsets from 3rd party vendors, such as NVIDIA, VIA and SiS.

Gilo, the mobile proliferation of Nehalem, will face many of the same issues as desktop processors, but also some that are unique to the notebook market. Mobile MPUs do not really need the lower latency; in many situations they sacrifice performance by only populating a single channel of memory, or operating at relatively slow transfer rates. An integrated memory controller would also require a separate voltage plane from the cores, hence systems would need an additional VRM on the motherboard. The clock distribution would also need to be designed so that the cores can vary frequency independently of the memory controller. Consequently, an on-die memory controller is unlikely because of the lack of benefits and additional complexity.

The implementations for Gilo will most likely resemble the mid-range and low-end desktop product configuration. The more integrated products will feature the northbridge and graphics in the same package as the MPU, connected by CSI. A more bare-bones MPU would also be offered for OEMs that prefer higher performance discrete graphics, or wish to use alternative chipsets.

While the system architecture for Intel’s desktop and mobile offerings will change a bit, the effects will probably be more subtle. The majority of Intel MPUs will still require external memory controllers, but they will be integrated on the MPU package itself. This will not fundamentally improve Intel’s performance relative to AMD’s desktop and mobile offerings. However, it will make Intel’s products more attractive to OEMs, since the greater integration will reduce the number of discrete components on system boards and lower the overall cost. In many ways the largest impact will be on the graphics vendors – since it will make all their solutions (both integrated and discrete) more expensive relative to a single MCM from Intel.