Introduction

With 320 stream processing units, DirectX 10.1 and PCIe 2.0 support, and a 256-bit GDDR4 or GDDR3 memory interface, AMD’s RV670 and R680 GPUs certainly boast nice specs on paper. The Radeon HD 3870 in particular is capable of delivering performance similar to AMD’s former flagship, the Radeon HD 2900 XT, for half the price, while the dual GPU Radeon HD 3870 X2 generally performs faster than the GeForce 8800 Ultra in DX9 applications. But as powerful as these GPUs are individually, things really begin to get exciting as you add more GPUs to the equation. This is where AMD’s CrossFireX technology comes in.

Back when the Radeon HD 3870 and 3850 were launched last year, AMD promised CrossFireX drivers before the end of Q1’08. Well folks, today is that day. AMD’s upcoming 8.3 Catalyst driver release (set to debut by Wednesday), not only supports CrossFireX, but also AMD’s Hybrid Graphics technology that’s designed to bring the world of CrossFire to integrated graphics platforms based on AMD’s RS780 chipset, which is also launching alongside Catalyst 8.3 today.

Today we’re going to take a look at two potential CrossFireX configurations, as well as Hybrid Graphics. We’ve only had the final 8.3 driver for a little over 72 hours, so unfortunately we haven’t had the time or resources to test more configs. Rest assured that we’ll take a look at them over the coming months.

So what is CrossFireX?

CrossFireX is AMD’s third generation CrossFire technology. AMD’s first attempt at CrossFire with the Radeon X800 family never really took off due to the limitations of the technology: cards were capped at a max resolution of 1600x1200 at 60Hz and the dedicated CrossFire master card with its own bulky interconnect cable really turned off enthusiasts. AMD’s second generation CrossFire tech debuted in the Radeon X1950 Pro and was warmly received thanks to its native CrossFire support that was built in to the GPU as well as featuring a new high-speed 24-bit connection.

The seeds for 3 and 4-way CrossFire were actually planted during this generation, with X1950 cards sporting dual CrossFire connectors. Unfortunately however, CrossFireX support is limited to AMD’s RV670 and R680 GPUs, so the implied suggestion that X1950 cards would one day support 4-Way CrossFire were unfortunately untrue.

In their defense though, AMD has made CrossFireX more flexible than previous multi-GPU solutions. One key feature that CrossFireX supports is the ability to mate any RV670 or R680-based graphics card with another. This means that you can combine two Radeon HD 3850s with a Radeon HD 3870, or a Radeon HD 3870 X2 with a 3850 for 3-Way CrossFire. Or you could pair two 3850s with two 3870s, or two 3870 X2 cards for 4-Way CrossFire. There are literally three dozen different combinations:

CrossFireX is also supported by a variety of different platforms, not only are AMD’s own chipsets supported, but Intel chipsets are supported as well:


As the first chat above indicates, in cases where cards with different memory sizes are combined together to run in CrossFireX mode, the card with the least amount of memory will become the limitation. So if three 512MB Radeon HD 3850 boards are paired with one 256MB card, all four boards will run as 256MB cards. The same applies to cards running at different clock speeds – all the cards will only run as fast as the slowest card.

Because of this, while all your cards don’t have to be identical to one another, or even utilize the same GPU, it is important that they’re not too out of sync with one another. i.e. you won’t want to mix a 256MB board with a pair of 512MB boards, or a 3850 card that doesn’t OC well with a factory overclocked 3870 or 3870 X2.