A Secret Weapon Exposed: Specs
As is the case with Intel’s Core 2 Duo and its shared L2 cache, it’d be ideal if both of the 4870 X2’s GPUs could both access the 2 GB frame buffer. Despite the persistent rumors surrounding this new card, those capabilities simply aren’t possible at this time. However, the two GPUs do have a new side-port interface between them able to contribute additional bandwidth should the PCI Express 2.0 bus become saturated.
AMD confirms for us that this communication pathway isn’t currently being utilized and that it will really only need to do so in the next few months by way of software. According to representatives at AMD, today’s applications do well enough with the throughput available through PCI Express, meaning the side-port connection wouldn’t really add much to the card’s performance picture anyway. For now, the Radeon HD 4870 X2 enjoys enough of a speed-up thanks to the addition of a PCI Express 2.0 bridge linking its two GPUs. In fact, we wouldn’t be surprised if the side-port feature never really takes off in this generation of hardware.
| GPU | HD 4870 | HD 4870 X2 | GTX 280 |
|---|---|---|---|
| GPU Frequency | 750 MHz | 750 MHz | 602 MHz |
| ALU Frequency | 750 MHz | 750 MHz | 1296 MHz |
| Memory Frequency | 900 MHz | 900 MHz | 1107 MHz |
| Width of the Bus Memory | 256 bits | 2 x 256 bits | 512 bits |
| Memory Type | GDDR5 | GDDR5 | GDDR3 |
| Memory Quantity | 512 MB | 2 x 1 GB | 1 GB |
| Number of ALUs | 800 | 1600 | 240 |
| Number of texturing units | 40 | 80 | 80 |
| Number of ROP | 16 | 32 | 32 |
| Shading Power | 1.2 TFlops | 2.4 TFlops | 933 GFlops |
| Memory Bandwidth | 115.2 GB/s | 115.2 GB/s (x2) | 141.7 GB/s |
| Number of transistors | 956 million | 2 x 956 million | 1.4 billion |
| Process Technology | 55 nm | 55 nm | 65 nm |
| Surface die | 260 mm² | 2 x 260 mm² | 576 mm² |
| Generation | 2008 | 2008 | 2008 |
| DirectX Supported | 10.1 | 10.1 | 10 |
AMD has quadrupled the amount of memory on the Radeon HD 4870 X2 versus its existing Radeon HD 4870, although we are expecting to see a 1 GB version of the company’s single-GPU desktop board soon. For all intents and purposes, the X2 behaves like a card equipped with 1 GB of available memory, just like the GeForce GTX 280. Another point point to mention: memory frequency. Whereas AMD had to reduce the speed of the memory on its Radeon HD 3870 X2, the 4870 X2 runs at the same frequency as single-chip 4870 cards. The GPUs haven’t slowed down at all, either. That’s good news, especially given the big board’s power consumption, which we suspected might limit its potential headroom. AMD is clearly gunning for the performance title on this one.