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A Northbridge too Far
The northbridge was another area that AMD’s architects chose to focus on, to save power. The memory controller now runs at memory frequencies, rather than core clock rates, which generally lowers power, without a large impact on performance. Like Barcelona, Griffin features two independent 64 bit DDR2 memory controllers, complete with a page access predictor, which has been slightly modified to err on the side of power efficiency. The memory controller also includes a prefetcher and write buffer, which has again, been tuned specifically for mobile workloads. One substantial change is that the drive strength for the DDR2 I/O pins has been reduced, which saves power, although certain configurations, such as 4 SO-DIMM slots are no longer supported . This is a very reasonable trade-off, as most notebooks simply don’t need the same configurations that a desktop or server would require.
Griffin uses HyperTranpsort 3.0 to communicate with the chipset and the rest of the world. The additional bandwidth that HyperTransport 3.0 offers will improve performance for integrated graphics, especially on Windows Vista/DirectX 10 and other stressful workloads. HT3 also introduces some new power saving features that are controlled by the hardware. The HT link itself is 16 bits wide, in each direction; however, the link can change width depending on the bandwidth that is actually needed. For example, when a user is just working on a low I/O (disk and graphics) bandwidth application, like a spreadsheet or word processor, each HT3 link could narrow itself to 1, 2, 4 or 8 bits wide (and each direction is managed independently). The HT3 link can even be disconnected and certain components powered down when not in use, for further power saving. The HT3 hardware has a variety of options for power saving. For instance, the clock recovery circuitry and delay locked loops on the receiver can be turned off, which increases wake up latency, but saves more power. Similarly, training sequences can be sent periodically across a ‘sleeping’ link to reduce wake up latency.
Thermal management was another area where Griffin improved upon the K8. Each core integrates two thermal sensors, and there is an embedded thermal controller in the MPU itself. When OEM-defined limits are exceeded, the embedded thermal controller can reduce the processors frequency and voltage. The embedded thermal controller can also interface with an external monitor for the memory system. The embedded controller can then throttle memory to keep energy consumption below pre-specified levels for the next 128 cycles. The previous generation K8 had a single analog sensor, and no integrated controller, which meant that the management capabilities varied from system to system.
Figure 2 – AMD Puma Platform
AMD’s mobile platform, known as Puma, also includes chipsets, both from AMD itself and from partner companies such as NVIDIA or VIA. AMD offering in this area uses the RS780M northbridge and the SB700 southbridge. The former connects to the MPU with HT3 and adds PCI-E Gen2 for discrete graphics and other peripherals. The northbridge also contains two display controllers, one for HDMI with HDCP and the other for older displays (TV out is also included). One neat feature the RS780 offers is switching between integrated and discrete graphics on the fly. High-end notebooks can use discrete graphics when plugged into AC power for gaming performance, but switch over to integrated graphics for better power consumption when using the battery. The southbridge offers support for NAND flash, serial and parallel ATA, HD audio and legacy PCI. It appears that it will be up to OEMs to integrate 3rd party Wifi, and possibly WiMAX or 3G chipsets.
The enhancements in Griffin and Puma will make AMD’s microprocessors substantially more attractive for mobile applications. Most of the issues with the previous generation have been fixed, which should produce much more consistent power consumption across different AMD based notebooks. Whether Griffin will be competitive with Penryn is somewhat uncertain. Certainly from a performance standpoint, Griffin will lag behind Intel’s offerings. However, the situation for power and battery life, which is probably more important, is much less clear. In some areas, AMD’s power management is more sophisticated than what Intel currently offers – particularly the separate voltage planes for each core. However, the K8 microarchitecture is not as highly tuned for performance/watt as Intel’s competing microarchitecture. Although Griffin cannot be expected to dominated the mobile world, it is clearly a good first step along the road for AMD and first silicon has already booted Windows.