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AMD's Radeon HD 4870 graphics processor


...and a deeper look at the 4850, too
— 5:30 AM on June 25, 2008

Not much good has happened for either party since AMD purchased ATI. New chips from both sides of the fence have been late, run hot, and underperformed compared to the competition. Meanwhile, the combined company has posted staggering financial losses, causing many folks to wonder whether AMD could continue to hold up its end of the bargain as junior partner in the PC market's twin duopolies, for CPUs and graphics chips.

AMD certainly has its fair share of well-wishers, as underdogs often do. And a great many of them have been waiting with anticipation—you can almost hear them vibrating with excitement—for the Radeon HD 4800 series. The buzz has been building for weeks now. For the first time in quite a while, AMD would seem to have an unequivocal winner on its hands in this new GPU.

Our first peek at Radeon HD 4850 performance surely did nothing to quell the excitement. As I said then, the Radeon HD 4850 kicks more ass than a pair of donkeys in an MMA cage match. But that was only half of the story. What the Radeon HD 4870 tells us is that those donkeys are all out of bubble gum.

Uhm, or something like that. Keep reading to see what the Radeon HD 4800 series is all about.


The RV770 GPU
Work on the chip code-named RV770 began two and a half years ago. AMD's design teams were, unusually, dispersed across six offices around the globe. Their common goal was to take the core elements of the underperforming R600 graphics processor and turn them into a much more efficient GPU. To make that happen, the engineers worked carefully on reducing the size of the various logic blocks on the chip without cutting out functionality. More efficient use of chip area allowed them to pack in more of everything, raising the peak capacity of the GPU in many ways. At the same time, they focused on making sure the GPU could more fully realize its potential by keeping key resources well fed and better managing the flow of data through the chip.

The fruit of their labors is a graphics processor whose elements look familiar, but whose performance and efficiency are revelations. Let's have a look at a 10,000-foot overview of the chip, and then we'll consider what makes it different.


A block diagram of the RV770 GPU. Source: AMD.

Some portions of the diagram above are too small to make out at first glance, I know. We'll be looking at them in more detail in the following pages. The first thing you'll want to notice here, though, is the number of processors in the shader array, which is something of a surprise compared to early rumors. The RV770 has 10 SIMD cores, as you can see, and each them contains 16 stream processor units. You may not be able to see it above, but each of those SP units is a superscalar processing block comprised of five ALUs. Add it all up, and the RV770 has a grand total of 800 ALUs onboard, which AMD advertises as 800 "stream processors." Whatever you call them, that's a tremendous amount of computing power—well beyond the 320 SPs in the RV670 GPU powering the Radeon HD 3800 series. In fact, this is the first teraflop-capable GPU, with a theoretical peak of a cool one teraflops in the Radeon HD 4850 and up to 1.2 teraflops in the Radeon HD 4870. Nvidia's much larger GeForce GTX 280 falls just shy of the teraflop mark.

The blue blocks to the right of the SIMDs are texture units. The RV770's texture units are now aligned with SIMDs, so that adding more shader power equates to adding more texturing power, as is the case with Nvidia's recent GPUs. Accordingly, the RV770 has 10 texture units, capable of addressing and filtering up to 40 texels per clock, more than double the capacity of the RV670.

Across the bottom of the diagram, you can see the GPU's four render back-ends, each of which is associated with a 64-bit memory interface. Like a bad tattoo, the four back-ends and 256 bits of total memory connectivity are telltale class indicators: this is decidedly a mid-range GPU. Yet the individual render back-ends on RV770 are vastly more powerful than their predecessors, and the memory controllers have one heck of a trick up their sleeves in the form of support for GDDR5 memory, which enables substantially more bandwidth over every pin.

Despite all of the changes, the RV770 shares the same basic feature set with the RV670 that came before it, including support for Microsoft's DirectX 10.1 standard. The big news items this time around are (sometimes major) refinements, including formidable increases in texturing capacity, shader power, and memory bandwidth, along with efficiency improvements throughout the design.