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AMD's Radeon HD 7970 graphics processor


We've spent the holidays on the Southern Islands
— 11:47 PM on January 2, 2012

Oh, man. Just a few days before Christmas, AMD uncorked a massive jug of holiday cheer in the form of the Radeon HD 7970 graphics card. Sloshing around inside? The world's first GPU produced on an 28-nm manufacturing process. This incredibly fine new production process has allowed AMD to cram more transistors—and thus more graphics horsepower of virtually every sort—into this puppy than any graphics chip to come before. While many kids were looking forward to the latest Xbox 360 game under the tree on Christmas morning, the Radeon HD 7970 delivers nearly fifteen times the texel filtering speed of Microsoft's venerable game console, to name one key graphics rate. I don't want to dwell on it, but this new Radeon is nearly an order of magnitude more powerful than an Xbox 360 in nearly every respect that matters.

Ok, so I kinda do want to dwell on it, but we need to move on, just like the former ATI has done since creating the Xbox 360's GPU.

This new Radeon's true competitors, of course, are the other PC graphics processors on the market, and it has nearly all of them beaten on paper. The chip behind the action is known as "Tahiti," part of AMD's "Southern Islands" lineup of next-gen GPUs. As a brand-new design, Tahiti is, of course, infused with all of the latest features—and a few new marketing buzzwords, too. The highlights alone are breathtaking: 2048 shader ALUs, a 384-bit memory interface, PCI Express 3.0, support for DirectX 11.1, and a hardware video encoding engine. Tahiti features the "Graphics Core Next" (note to Rory Read: time to stop letting engineers name these things) shader architecture that promises more efficient scheduling and thus higher delivered throughput, especially for non-graphics applications.


A vague functional block diagram of the Tahiti GPU. Source: AMD.

If the prior paragraph wasn't sufficient to impress you, perhaps the block diagram above will do the trick. One of the themes of modern GPUs is massive parallelism, and nowhere is that parallelism more massive than in Tahiti. Honestly, the collection of Chiclets above leaves much to be desired as a functional representation of a GPU, especially the magic cloudy bits that represent the shader cores. Still, the basic outlines of the thing are obvious, if you've looked over such diagrams in the past. Across the bottom are six memory controllers, each with a pair of 32-bit memory channels. Running up and down the center are the shader or compute units, of which there are 32. Flanking the CUs are eight ROP partitions, each with four color and 16 Z/stencil ROP units. The purple bits represent cache and buffers of various types, which are a substantial presence in Tahiti's floorplan.

We will get into these things in more detail shortly, but first, let's take a quick look at how Tahiti stacks up, in a general sense, versus the DirectX 11 GPUs presently on the market.


The Tahiti GPU between my thumb and forefinger

ROP
pixels/
clock
Texels
filtered/
clock
(int/fp16)
Shader
ALUs
Rasterized
triangles/
clock
Memory
interface
width (bits)
Estimated
transistor
count
(Millions)
Die size
(mm²)
Fabrication
process node
GF114 32 64/64 384 2 256 1950 360 40 nm
GF110 48 64/64 512 4 384 3000 520 40 nm
Cypress 32 80/40 1600 1 256 2150 334 40 nm
Barts 32 56/28 1120 1 256 1700 255 40 nm
Cayman 32 96/48 1536 2 256 2640 389 40 nm
Tahiti 32 128/64 2048 2 384 4310 365 28 nm

The most immediate comparison we'll want to make is between Tahiti and the chip it succeeds, the Cayman GPU that powers the Radeon HD 6900 series. At 4.3 billion, its transistor count doesn't quite double Cayman's, but Tahiti is easily the most complex GPU ever. Tahiti improves a bunch of key graphics resources by at least a third over Cayman, including texture filtering capacity, memory interface width, and number of shader ALUs. Even so, Tahiti is a smaller chip than Cayman, and it carries on AMD's recent practice of building "mid-sized" chips to serve the upper portions of the market. As you can see, Nvidia's GF110 still dwarfs Tahiti, although Tahiti crams in more transistors courtesy of TSMC's 28-nm fabrication process.

Of course, Tahiti is just the first of a series of GPUs, and it will contribute its DNA to at least two smaller chips still in the works. One, code-named "Pitcairn," will supplant Barts and drive the Radeon HD 7800 series of graphics cards in a more affordable (think $250 or less) portion of the market. Below that, another chip, known as "Cape Verde," will at last relieve the Juniper GPU of its duties, which have included both the Radeon HD 5700 series and the re-branded 6700 series. Although we believe both of these new chips are imminent, we don't yet know exactly when AMD plans to introduce them. Probably before they arrive, AMD will unleash at least one additional card based on Tahiti, the more affordable Radeon HD 7950.

There is one other code name in this collection of Southern Islands. At its press event for the 7970, AMD simply showed the outline of a pair of islands along with the words, "Coming soon." The rest isn't too hard to parse out, since the contours of the islands pictured match those of New Zealand—which also happens to be the name of the rumored upcoming dual-GPU video card based on a pair of Tahiti chips. New Zealand will probably end up being called the Radeon HD 7990 and serving the very high end of the market by being really, truly, obnoxiously, almost disturbingly powerful. We're curious to see whether New Zealand will be as difficult to find in stock at Newegg as Antilles, also known as the Radeon HD 6990. Maybe, you know, the larger land mass will help folks locate it more consistently.

Absent any additional code names, we're left to speculate that AMD may rely on older chips to serve the lower reaches of the market. The recent introduction of the mobile Radeon HD 7000M series, based entirely on Cypress derivatives, suggests that's the plan, at least for a while.