The HD7900 series features the new Tahiti core which is the successor to the Caymen core of the HD6900 family. The Caymen core was based on a 40nm manufacturing process so it is fair to say that perhaps the biggest improvement to the Tahiti core is the reduction to 28nm process. This has enabled AMD to cram 4.31 billion (4,312,711,873) transistors into the new HD7970 core as opposed to the 2.64 billion on the HD6970 Caymen XT.
Comprising of what AMD calls 'Graphics Core Next (GCN)' archetecture which is optimised for heterogeneous computing, the new Tahiti core has upto 32 CU's (compute units) with dual geometry engines along with 8 back-ends comprising of 32 colour ROPs per clock and 128 Z/Stencil ROPs per clock. Each CU has a Vector unit and Scaler Co-Processor and can execute instructions from multiple kernels.
The VLIW4 SIMD has evolved to the GCN quad SIMD which equates to similar computational power yet grants better performance, threading and simplification. The GCN also has a new Cache hierarchy with 16KB instruction Cache and32KB Scaler Cache shared between 4 CU's. The L2 Cache is accessible by each CU with it's own separate process to ensure coherency. These changes increase the GFLOPs by 140%.
Tesselation on the Tahiti core is also enhanced by increased Vertex re-use off-core which theoretically at least, means the 7900 series has 4 times the throughput of the 69XX series. The are improved ansioscopic filtering algorithms too which come at no performance cost allegedly. These new algorithm improvement will allow the use of Forward Rendering with 1000's of potential light sources and will also mean MSAA can be rendered easily.
Looking at the schematics above, we can identify the bulk of the GPU is taken up with the 32 CU's. Above these are the geometry engines. To the left and right are the 8 Render back-ends, 4 each side. There's 768KB of L2 cache along with 6x64bit dual channel memory controllers making a 384-bit GDDR5 memory interface capable of delivering up to 264GB/sec. All of technology coupled together is transferred through a fully compatible PCI Express 3.0 x16 bus interface. Have no fear though because the cards are backwards compatible with PCIe 2.0.
Comparing the HD7970 against its competitor, the GTX680, the AMD card has the advantage of more stream processors but because it is lower clocked, the GTX680 has a higher texture fillrate. Memory size goes to the AMD card while memory speed favours the NVIDIA counterpart. The pendulum swings back to AMD card when total bandwidth is considered though with the AMD card gaining a 37% advantage. The NVIDIA card however consumes 20% less power (load) than the HD7970 so while the NVIDIA care may present a higher initial outlay, over time it may prove to be the cheaper card, depending on how long you plan to keep it.
Let's move on to the features of both cards...
