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AMD's Radeon HD 7870 GHz Edition


And its $249 sidekick, the Radeon HD 7850, reviewed
— 11:01 PM on March 4, 2012

At long last, AMD has filled the gaping hole in its next-generation Radeon lineup. A new pair of graphics cards has come to slot in right between the high-end Radeon HD 7900 series and the decidedly less high-end 7700 series, completing the Southern Islands trifecta—and offering gamers some fresh meat at $249.

If you were expecting exact replacements for the 6800 series, prepare to be disappointed. The new Radeon HD 7850 is the one priced at $249; its big brother, the Radeon HD 7870 GHz Edition, costs a more burdensome $349. That means today's launch leaves the $199 price point conspicuously devoid of next-generation GPUs. Perhaps that will change in the future—possibly after the arrival of Kepler parts from Nvidia, which are rumored to be coming soon—but for now, we might say the hole in AMD's lineup is only partially plugged.

Also, it turns out AMD has revived that beloved, time-honored tradition known as the soft launch. In the company's words, today marks the lifting of the "preview NDA" (or preview non-disclosure agreement). We can tell you everything we know about these cards and post our performance findings, but actual products aren't due out in volume until two weeks from now, on March 19.

You can look, in other words, but you can't touch.

Nevertheless, the Radeon HD 7850 and Radeon HD 7870 GHz Edition have the potential to be AMD's most compelling next-gen offerings yet. No, really. It's true the flagship Radeon HD 7900 series offerings are the fastest single-GPU cards on the planet right now, but they're also awfully expensive, with prices starting at $449 and ranging all the way up to $600. The Radeon HD 7700 series is less expensive, but overpriced considering the level of performance the cards deliver—so much so that we recommended previous-generation cards in our latest system guide, instead.

With these new arrivals, AMD may finally have next-gen cards that are both affordable and competitive. If the Radeon HD 7850 in particular can prove its mettle against enthusiast classics like the GeForce GTX 560 Ti and Radeon HD 6950, it could be an instant hit.

The GPU—Pitcairn
We were speaking quite literally when we said the Radeon HD 7800 series completes the Southern Islands trifecta. These puppies are driven by a new chip called Pitcairn, the third addition to the 28-nm Southern Islands GPU family. With a die size of 212 mm², Pitcairn is smaller than Tahiti (365 mm²) but a fair bit larger than Cape Verde (123 mm²).

Just like those other two chips, though, Pitcairn is fabbed on a 28-nm process and based on AMD's Graphics Core Next architecture. It also has the same supplemental goodies, like PCI Express 3.0 support, ZeroCore Power, and a hardware video encoding block called VCE.

Scott covered those features, as well as Graphics Core Next, in some depth in his review of the Radeon HD 7970. If you haven't read it already, I'd recommend doing that now. It's okay; I'll wait.

Here's an abstracted overview of Pitcairn's various components:

Being smaller than Tahiti, Pitcairn isn't quite as well-furnished. AMD has cut the number of compute units from 32 to 20, leaving Pitcairn with 1280 stream processors and 80 texture units. (For the record, each compute unit has four texture units and four 16-wide vector units, also known as ALUs or stream processors.)

Two of the 64-bit memory controllers have been lopped off, as well, bringing the total down from six to four. Pitcairn's path to memory is therefore 256 bits wide. However, AMD has endowed Pitcairn with the exact same number of ROP partitions and geometry engines as its larger sibling. Both chips can churn out 32 pixels and rasterize two triangles with each clock cycle.

Here's how they compare, at a glance:

  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
Barts 32 56/28 1120 1 256 1700 255 40 nm
Cayman 32 96/48 1536 2 256 2640 389 40 nm
Cape Verde 16 40/20 640 1 128 1500 123 28 nm
Pitcairn 32 80/40 1280 2 256 2800 212 28 nm
Tahiti 32 128/64 2048 2 384 4310 365 28 nm

Pitcairn is about 17% smaller than Barts, the chip that powers the Radeon HD 6800 series, and 46% smaller than Cayman, the core of the the 6900 series, but it has more transistors than either one. Its per-clock texture filtering and shader resources are somewhere in between, but as we'll see on the next page, its higher clock speeds give it an advantage. Also, keep in mind that improved shader efficiency is one of the hallmarks of AMD's Graphics Core Next architecture.

Comparing Pitcairn to Nvidia's GF114 and GF110 chips (which drive the GeForce GTX 560 Ti and GeForce GTX 570, respectively) is a little trickier, since the GeForces are based on a completely different architecture. Still, Pitcairn looks well-equipped to face them—especially the GF114.

Before we get into our benchmarks, let's take a look at the two cards Pitcairn powers: the Radeon HD 7870 GHz Edition and Radeon HD 7850. (Or, you know, you could just skip ahead to the performance-per-dollar scatter plots on the last page, if you're more comfortable with an incomplete recapitulation of our hard work. Totally up to you.)