X-bit labs

First in DirectX 10.1: ATI Radeon HD 3850/3870 (page 8)

Category: Video

by Alexey Stepin , Yaroslav Lyssenko, Anton Shilov

[ 01/25/2008 | 10:31 AM ]

PCI Express 2.0, PowerPlay and CrossFire X

Like the competitor’s offer, the new ATI Radeon chip supports the PCI Express 2.0 interface that provides two times the bandwidth of PCI Express 1.0/1.1: 8GB/s in each direction as opposed to 4GB/s with the previous versions of the standard. The higher data-transfer rate is enabled only when supported by the chipset, of course. Otherwise, it is reduced to the bandwidth of PCI Express 1.0/1.1. This won’t have a big effect on performance in games because a bandwidth of 4GB/s is quite enough even if the graphics card carries at least 512 megabytes of onboard memory. And if the card has less memory, the increased bandwidth of the new PCI Express version won’t help much. The speed of the GPU’s access to system memory will anyway be much lower than to local memory that is clocked at a higher frequency and connected to the GPU with a 256-bit or broader bus. Like with Nvidia GeForce 8800 GT, we can expect some incompatibility with certain mainboards supporting PCI Express 1.0/1.0a. The lack of such problems is guaranteed for PCI Express 1.1 mainboard only.

The ATI RV670 is also the world’s first desktop processors that features the same power-saving technologies as are implemented in ATI’s chips intended for mobile applications. Particularly, ATI PowerPlay technology allows the new chip to flexibly adjust its frequencies and voltages and even disable unused blocks when the GPU is idle. As opposed to other desktop GPUs, the power-saving features are implemented in the RV670 on the hardware level, which ensures a quicker response to any change in the GPU load and avoid mistakes when determining the load type. AMD claims the 55nm tech process together with PowerPlay helped keep the power consumption of the new cards at 105W for the ATI Radeon HD 3870 and 95W for the ATI Radeon HD 3850. That’s a real breakthrough in comparison with the ATI Radeon HD 2900. We’ll try to check this out in our own tests.

Describing the ATI Radeon X1950 Pro we noted two CrossFire connectors, supposing that they could be used to build multi-processor systems consisting of more than two graphics cards. This supposition comes true now: the ATI Radeon HD 3800 series supports CrossFireX technology that allows uniting 3 or even 4 graphics cards into a single graphics subsystem. Such configurations can hardly become widely popular as not all systems can accommodate 3 or 4 graphics cards. Still, this may be an interesting feature for some users who could only build multiple-card subsystems out of GeForce 8800 GTX/Ultra so far. Theoretically, four Radeon HD 3870 cards in CrossFireX mode can deliver very high performance at a rather low, for such a complex system, power draw, about 400W. For comparison, a pair of Radeon HD 2900 XT cards consumes about 320W. The cost of four Radeon HD 3870 or of two Radeon HD 3980 X2 is going to be rather low, too, which should ensure CrossFireX systems a small but stable niche in the enthusiasts community especially as AMD promises the option of overclocking of such solutions and the support of up to 8 monitors simultaneously whereas Nvidia’s SLI can output to one monitor only in multi-GPU mode.

We’ve been describing the theoretical capabilities of the ATI Radeon HD 3800 series so far. Now it’s time to get to practical matters. We’ll tell you about the design features of the new solutions starting from the highest-performance model that has the index of 3870.