Intel's Core Duo meets the desktop

It's all about Core

Intel spent a long time talking about its brand new Core microarchitecture at the Intel Developer Forum Spring '06. Yonah (officially named Core Duo) is the predecessor to the new Core microarchitecture chips to which it lends its name, and it is predominantly a mobile part, but also forms an important part of the company's Viiv platform for low-power, high-capability media PCs. Core Duo, despite the name, doesn't sport the Core microarchitecture, which will appear first in Conroe - now called Core 2 Duo - in just a couple of month's time.

The chip was originally unveiled at last year's Intel Developer Forum Fall event in conjunction with the Napa platform for notebooks, but wasn't officially given its Core Duo brand name until this year's Consumer Electronics Show in Las Vegas, where Intel announced its Leap Ahead and also its Viiv platform.

Yonah is Intel's first mobile processor manufactured on its 65-nanometre process technology, and its roots are quite obvious. It's based on the very successful Banias and Dothan cores that came under the guise of Pentium M and the Centrino platform. There were some short comings with the Banias/Dothan chip family, though - these were mainly related to the feature set.

Core Duo solves a lot of the short comings, but there is one major feature omission from Yonah's architecture: it doesn't support Intel's EM64T 64-bit extensions. With that said, it is worth noting that the lack of 64-bit extension support in Core Duo will not prevent you from running Windows Vista. All versions of Windows Vista will be available in both 32-bit and 64-bit editions, with the exception of the Starter Edition.

Aside from the lack of EM64T support, which could become a relative non-issue, the Core Duo chip seems to be reasonably complete but there are a few other architecture-related problems. Much like AMD's Athlon 64 X2 architecture, Core Duo is intentionally designed to be a dual core processor. This is a change from the way that Intel has implemented any of its previous architectures - both Presler and Smithfield processor cores communicated with each other via the front side bus. Consequently, this introduces a lot of latency.

Yonah is still a high latency platform due to the lack of an on-die memory controller - this is somewhere where AMD's Athlon 64 architecture excels. However, Intel manages to hide a lot of the latency with its Smart Cache technology. This essentially allows both cores to use the same L2 cache.

In fact, not only do the two cores share L2 cache, the communication between the two cores is handled by an arbitration bus unit. The use of an arbitration bus unit eliminates the need for cache coherency traffic over the front side bus. This comes at the expense of raising the latency between the core and L2 cache from 10 clock cycles to 14.

The features table includes support for SSE3 instructions, improved SSE2 instruction capabilities, Execute Disable Bit for improved virus protection and also Intel's virtualisation technology. Along with the standard array of power management features, there are improved power management technologies included in Yonah, too.

Intel has gone a step further than previous implementations by enabling the two cores to manage their own low-power states independently. This also extends to the shared L2 cache, where the chip can deactivate portions of cache if the running applications don't need to use all of it. Obviously, these innovations help to reduce the thermal design power, as all of Intel's current Core Duo models have a TDP of 31W. By comparison, the faster Dothan chips have a TDP of 27W. Bear in mind that the Dothan is only a single core, while Yonah is a dual-core CPU.

A lot of this can be linked with the manufacturing process shrink from 90-nanometres to 65. Another direct result of the process shrink was the fact that Intel was able cram two cores into roughly the same die space as the single-core Dothan chip. Dothan was a mere 84mm² - Yonah only increases the die size to 90mm².

Core Duo is not compatible with previous Pentium M motherboards in much the same way that the Pentium M chips are not compatible with Core Duo motherboards. This is mainly because the pin arrangement is physically different. But that is not the only reason, because there are also a number of new chipset functions introduced in Intel's 945 and 975-series chipsets. We have managed to get hold of one of AOpen's i975Xa-YDG motherboards, which essentially enables Yonah to function in a desktop environment.