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AMD's Radeon HD 4870 X2 graphics card


Double-barreled badness
— 12:01 AM on August 12, 2008

This should be fairly painless. We've already previewed the Radeon HD 4870 X2 for you, and today that card is becoming official and available for purchase. To mark the occasion, we've wrangled not one but two of these dual-GPU monsters and paired them up via CrossFire. We've also assembled a killer's row of GeForce GTX cards for a comprehensive look at the next generation of multi-GPU madness. Can we set a new record for power draw at the wall socket? Oh, I think you know we can.


An X2 refresher
You really should read our preview of the Radeon HD 4870 X2 if you haven't already. There we explained the ins and outs of "CrossFire on a stick" technology and even took some naughty pictures of a naked 4870 X2 card. There's not a lot more to be said about the card, which sports two Radeon HD 4870 graphics processors and twin banks of 1GB GDDR5 memory—one for each GPU. This is the same basic sort of layout we saw in the Radeon HD 3870 X2 before it.

We do know a little more about the 4870 X2 now, though. Some of that knowledge is encapsulated in this nifty logical block diagram of the 4870 X2 provided by AMD:


A nifty logical block diagram of the 4870 X2. Source: AMD.

This diagram provides confirmation for what we'd suspected: that the X2 provides additional bandwidth between its two RV770 graphics chips by means of a dedicated "sideport" connection. You may have noticed that the sideport connection offers 5GB/s in each direction, very much like a PCIe 2.0 x16 link. AMD says the sideport is electrically similar to PCI Express but is simpler because it's only intended as a peer-to-peer link between GPUs. This link augments the bandwidth already available via the X2's CrossFire bridge interface (CFBI in the diagram), which is only used to pass final frames from one GPU to the next for compositing, and its PCI Express lanes. The sideport connection should help improve performance in cases where multi-GPU applications have typically had performance scaling problems, such as when texture synchronization between the GPUs becomes a problem.

Whether the sideport connection will be a big help is an open question, though. The PCI Express switch on the X2 already manages traffic for 48 lanes worth of Gen 2 PCIe connectivity, including 16 lanes to each GPU and 16 lanes to the rest of the system. On top of that, AMD says the bridge chip supports a broadcast write function that can relay data to both of the X2's graphics processors simultaneously, conserving bandwidth.


Another thing we now know about the 4870 X2 is its price. AMD is aiming for the $549 mark, which is just a tad shy of what'd you'd pay for two Radeon HD 4870 cards. Since the X2 comes with 1GB of memory per GPU rather than 512MB like most current Radeon HD 4870 cards, it's really not a bad deal, as far as embarrassingly expensive video cards go.

And here's a bit of a shock: the card pictured above comes from the folks at Palit, who have been hawking GeForce cards on these shores for a little while now. They are an equal opportunity eye candy vendor these days, which leads to another interesting development...


Yes, folks, the atomic frog has joined the red team, and he's apparently kinda cheesed off about something. Watch out, or he'll unleash his robotic bagpipes on your ass. They are full of liquid metal awesome.

Speaking of awesomeness, Radeon board vendors have another surprise in store for us soon, as well: Radeon HD 4850 X2 cards. These things will feature two Radeon HD 4850 GPUs, each with 512MB of GDDR3 memory. We haven't yet procured one of these things, but I'd expect them to perform more or less like a pair of Radeon HD 4850s in CrossFire, which is pretty darned fast. The price? $399, or less than the going rate for a GeForce GTX 280. Now, that's just not even fair.

AMD says we can expect 4850 X2s anytime now, between the middle of August and the third week of the month.