AMD Radeon HD 7970 review

AMD might be having a tough time of it on the CPU front, but when it comes to graphics cards it’s just taken a leap ahead of the competition. Its new single-GPU flagship – the Radeon HD 7970 – is the first to use a 28nm manufacturing process, and beats its major rival Nvidia to the punch.

The impact of the new process is significant: the HD 7970 die is 378mm[SUP]2[/SUP] compared to the 389mm[SUP]2[/SUP] size of its predecessor, the Radeon HD 6970, and AMD crams in 4.3 billion transistors – a huge bump up over the 2.6 billion included in last year’s top-end single GPU. The HD 7970 compares favourably to Nvidia’s GeForce GTX 580, too, which includes 3 billion transistors in a 520mm[SUP]2[/SUP] package.

AMD has also given its Very Long Instruction Word 4 (VLIW4) architecture the boot, deeming its bottlenecked parallel performance a hindrance. While VLIW4 cores and their schedulers proved adept at handling groups of identical operations concurrently, they struggled with varied groups of tasks required by more complex applications and games. Some tasks were scheduled and processed promptly but, often, the scheduler couldn’t keep up, with instructions left behind and bottlenecks caused in the GPU.

It’s a big change. VLIW-based architectures, including VLIW4, have been used in AMD graphics cards since the Radeon 9700’s introduction in 2002. Instead, the new card uses multiple instruction multiple data cores (MIMD). These are constructed from several single-instruction multiple data cores (SIMD) grouped together, and are capable of more efficiently handling a more diverse range of tasks, as well as making dynamic changes to the compute schedule – something VLIW4 couldn’t do.

Each MIMD package is made from 64 SIMD cores, and each package has its own L1 cache, with L2 cache and memory controllers shared between several packages. AMD has given these cores their own name, too, with the marketing department swooping into action to dub each unit a Graphics Core Next.

The HD 7970 itself includes 2,048 SIMD cores inside 32 MIMD clusters, which is more than the 1,536 stream processors used in the HD 6970. The core clock is 925MHz, there’s 3GB of GDDR5 RAM running at 1,375MHz, and the memory bus is 384-bits wide. The latter is an improvement on the 256-bit bus of last year’s cards, and on a par with the GTX 580.