AMD ATI Radeon HD 4870 1024MB review

AMD's Radeon series 4800 chipset

First a little 101 on what the RV770 actually is all about. As you guys know by now ATI's Radeon HD 4850/4870 are both using the same GPU (graphics processor). The codename for these chips is RV770. AMD put nearly a billion transistors into that GPU, which is now built upon a 55nm (260 mm2 Die size) production. The chip literally is 16 mm wide and high. Which for AMD still is quite large, for a 55nm product. The number of transistors for a midrange product like this is extreme and typically it's best to directly relate that to the number of shader processors to get a better understanding. But first let's look at some nice examples of Die sizes of current architectures.

The Radeon 4850/4870 series graphics processors have 800 scalar processors (320 on the HD 3800 series) and now have a significant forty texture units (was 16 in last-gen architecture).  The stream/compute/shader processors (can we please just name them all shader processors?) definitely had a good number of changes; if you are into this geek talk, you'll spot 10 SIMD clusters each carrying 80 32-bit Shader processors (this accumulates to 800). If I remember correctly, one SIMD unit can handle double precision.

Much like we recently noticed in the NVIDIA GTX 200 architecture, the 80 scalar stream processors per SIMD unit have 16KB of local data cache/buffer that is shared among the shader processors. Next to the hefty shader processor increase you probably already notice the massive amount of texture units. In the last generation product we noticed 16 units, the 4800 series has 40 units.

When you do some quick math, that's 2.5x the number of shader processors over the last-gen product, and 2.5x the number of texture units. That's a pretty grand change folks. Since the GPU has 800 shader processors it can produce the raw power of 1000 to 1200 GFlops in simple precision. It's a bit lame and inaccurate to do but divide the number of ATI's scalar shader processors with the number 5 and you'll roughly equal the performance to NVIDIA's stream processor. You could (in an abstract way) say that the 4800 series have 160 Shader units, if that helps you compare it towards NVIDIA's scaling. Again there's nothing scientific or objective about that explanation.

Effectively combined with the clock speed and memory this product can poop out 1000/1200 GigaFLOPs of performance. Depending on how that is measured of course. But still, with an entry product at 199 USD for the 4850 and 299 USD for the 4870 that's just an awful lot of computing power. Let's compile a chart:

Now other than the 1024 MB GDDR5 memory packed on the card we are testing today, there is no difference otherwise. The clocks remain the same for the core/shader domain and memory frequencies. What the extra 512MB memory will do is give us a little more headroom in heavy texturized high quality environments and in the higher anti-aliasing modes preventing the frame from swapping forwards and backwards into the frame buffer.