GF104 - A New Class of Fermi

As previously mentioned, the GeForce GTX 460 isn’t simply a neutered GTX 480. Codenamed GF104, the GPU inside of it has been designed to deliver as much performance in a smaller chip as possible. To accomplish this, NVIDIA integrated a number of changes inside the GPU’s Streaming Multiprocessors (SMs, for short).

To start, NVIDIA increased the number of CUDA cores inside each SM. While the GF100 chip used in the GTX 480/470/465 has 32 CUDA cores per SM, the GF104 has 48, a 50% increase.

With more CUDA Cores per SM, NVIDIA needed to keep the increased CUDA Cores fed with data. To accomplish this, they doubled the number of dispatch units from two in GF100, to four in GF104. As a result, two instructions can be dispatched per warp, for a grand total of four instructions per clock per SM.

Finally, the number of special function units (SFUs) and texture units were doubled from 4 in GF100 to 8 in GF104.

Specifications

Let’s get down to brass tacks. I’m sure you’re wondering about the particulars and how the GTX 460 stacks up to other graphics cards. Below is a chart comparing it to its direct competitor – the Radeon HD 5830 – as well as its bigger, badder brother – the GeForce GTX 480.

GeForce GTX 460 Specifications Comparison
	Radeon HD 5830	GeForce GTX 460 768MB	GeForce GTX 460 1GB	GeForce GTX 480
Graphics Processing Clusters	-	2	2	4
Tessellation Engines	1	7	7	15
CUDA Cores	-	336	336	480
Texture Units	56	56	56	60
ROPs	16	24	32	48
Graphics Core Clock	800 MHz	675 MHz	675 MHz	700 MHz
Stream Processor Clock	800 MHz	1,350 MHz	1,350 MHz	1,400 MHz
Memory Clock	1,000 MHz	900 MHz	900 MHz	924 MHz
Effective Memory Data Rate	4,000 MHz	3,600 MHz	3,600 MHz	3,696 MHz
Video Memory Size	1,024MB GDDR5	768MB GDDR5	1,024MB GDDR5	1,536MB GDDR5
Memory Interface	256-bit	192-bit	256-bit	384-bit
Memory Bandwidth	128 GB/sec	86.4 GB/sec	115.2 GB/sec	177.4 GB/sec
Texture Fill-rate	44.8 Gigatexels/sec	37.8 Gigatexels/sec	37.8 Gigatexels/sec	42 Gigatexels/sec
Max Board Power	175 W	150 W	160 W	250 W

Notes

As you can see, the major difference between the two GTX 460 reference SKUs is the memory. Both feature high quality VRAM running at 900MHz (3.6GHz effective), but the additional amount and bus size lend a significant bandwidth advantage to the 1GB version. Two other, slighter discrepancies can be seen in power consumption and ROP count. It is also worth noting that the 1GB version has 512KB of L2 cache versus the 768MB board’s 384KB.

Tessellation engines – AKA PolyMorph engines for NVIDIA – aren’t brand new to graphics architectures, but have taken center stage along with the debut of DirectX 11. (Although ATI has offered a dedicated tessellation unit in GPUs dating back to the 2900 XT, it was never used in games.) They’re used primarily to accelerate geometry processing for tessellation, of course, which NVIDIA is banking on really taking off in this generation of games.

Comparing raw figures like memory bandwidth and texture fill-rate, it would appear the Radeon HD 5830 has the advantage. However, we’ve found traditional performance metrics like these don’t always prove to be a good indicator of actual gaming performance.

Many of you are no-doubt pleased to see that the GTX 460’s power consumption is significantly less than that of the GTX 480. A whopping reduction of nearly 40% is a welcome improvement, indeed, considering the 480’s operating temperatures peak in excess of 90 degrees C. We’ll go into more detail about that on the next page.