ATI Radeon 4870 X2 Performance Preview

The ATI Radeon 4870 X2

The not so secret Radeon HD 4870 X2

Normally companies like to keep their upcoming plans pretty quiet. After all, you don’t want to tip off your competitor(s) about your future products. Divulging these details could also affect sales of your existing products.

Apparently someone forgot to pass that tip along to ATI – the company has made no secret of their graphics plans for this generation, particularly at the high-end of the market, where the development of the Radeon HD 4870 X2 was openly acknowledged before, during, and after the Radeon 4850 and 4870 launch.

Today’s arrival of the 4870 X2 is the latest piece in ATI’s new strategy of focusing on smaller, more efficient midrange GPU designs, and then scaling that technology up and down the rest of their graphics lineup. This new strategy was a direct result of lessons learned during the launch of their previous two next generation architectures (R520 and R600), which suffered through numerous delays. The Radeon HD 4850 and 4870 were the midrange parts, while today’s Radeon HD 4870 X2 is the high-end derivative based on the Radeon 4870. ATI is also putting the finishing touches on a Radeon HD 4850 X2 GPU that will be launching sometime later this month.

Check out the new black PCB

So what exactly is a Radeon HD 4870 X2? As its name implies, ATI merely takes two Radeon HD 4870 GPUs and slaps them on the same PCB. With two RV770 chips on one card, the 4870 X2 packs twice the punch of a conventional Radeon 4870. The card boasts 1600 shaders and 2.4 teraFLOPS of computing horsepower. Clock speeds are 100% identical to the Radeon HD 4870, but ATI spices up the package by doubling up on the memory: 1GB of 3.6GHz GDDR5 memory is devoted to each R700 chip (2GB total for the entire card) instead of the 512MB on the 4870 card we tested two months ago. Let’s take a look at the board’s specs:

Radeon 4800 Series Comparison
	Radeon 4870 X2	Radeon 4870	Radeon 4850
# of Transistors	1.5 billion	965 million	965 million
Manufacturing Process	55-nm	55-nm	55-nm
# of Stream Processors	1600	800	800
Texture Units	80	40	40
ROPs	32	16	16
Core Clock Speed	750MHz	750MHz	625MHz
Memory Clock	3.6GHz GDDR5	3.6GHz GDDR5	2.0GHz GDDR3
MADD rate	2.4 TeraFLOPS	1.2 TeraFLOPS	1.0 TeraFLOPS
Interface	PCIe 2.0	PCIe 2.0	PCIe 2.0
DirectX Support	10.1	10.1	10.1
Tessellation Unit	Yes	Yes	Yes
UVD	Yes	Yes	Yes
PowerPlay	Yes	Yes	Yes

Officially the Radeon 4870 X2 retails for $549, but ATI has told us that they expect the board to sell for $499 very shortly. Considering that the 512MB 4870 card can often be found selling online for around $260 after rebate, we think this pricing is pretty fair for what you get. After all, the board does ship with 2GB of memory. While it’s not depicted in the table above, the Radeon 4850 X2 shares the same clocks as the regular Radeon 4850 and it will sell for $399. We’ve been told that ATI’s partners will ship this board with either 1GB of GDDR3 (512MB per GPU) or 2GB (1GB of memory per GPU) of GDDR3 memory. Not all of ATI’s board partners will ship 4850 X2 cards however.

The 4870 X2 (bottom) and a Radeon 4870 (top)

GeForce GTX 280 poses with 4870 X2

Why 2GB of memory?

With most graphics cards running fine with 512MB of memory, you may wonder why ATI decided to ship the 4870 X2 with 2GB of GDDR5 memory – that’s more RAM than many PCs!

The reason is simple: 8x (or better) AA.

The type of gamer who is going to buy one of these cards isn’t going to run it at 1600x1200 with 4xAA. With two GPUs running at over 700MHz, you’d barely be tapping the capabilities of the 4870 X2 and would be better off saving your money and getting a Radeon 4850 or 4870 card instead.

This card is for the gamer who wants to turn it all on for maximum eye candy, including 8xAA. As we’re about to show you in the benchmarks, under the right conditions the added memory supported by the Radeon 4870 X2 reaps significant performance benefits over the 512MB Radeon 4870.

By adding the additional memory, it also future proofs the card for the next generation of DX10 and DX10.1 games.

Radeon 3870 X2 (top) and 4870 X2 (bottom)

Improving on the 3870 X2

The GPU powering the Radeon 4870 X2 is known as R700. ATI has made many improvements with this GPU in comparison to their previous R680 dual-GPU architecture found in the 3870 X2.

For starters, the PCIe 1.1 interconnect bridge originally used on the 3870 X2 has been replaced with a PCIe 2.0 bridge chip for the 4870 X2. If you recall ATI had wanted to integrate this chip on the 3870 X2, but it wasn’t ready in time for the 3870 X2 launch. With the PCIe 2.0 bridge chip, bandwidth doubles to 5GB/sec in each direction. The CrossFire interconnect cable linking both cards then provides an additional 900MB/sec of bandwidth in each direction between the chips.

The final piece to the puzzle is ATI’s CrossFire Sideport block. Sideport provides a dedicated link (GPU-to-GPU) combining both GPUs to each other. Thanks to the addition of Sideport, an additional bidirectional 5GB/sec of bandwidth is provided between the graphics processors. This brings the total combined bandwidth between the two GPUs to 21.8GB/sec. This is a significant improvement over the 3870 X2’s 6.8GB/sec.

However, before you get too worked up on that number, a quick reality check. While all this looks impressive on paper, keep in mind that the 3870 X2 got by just fine with its 6.8GB/sec: in our benchmarks neither the PCIe 1.1 bridge chip nor the lack of Sideport prevented it from scaling just as well as running two independent Radeon 3870 cards in CrossFire. In fact, thanks to its higher core clock and lower latency GDDR3, it scaled slightly better than two 3870 cards running CrossFire.