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Review: AMD Radeon HD 7870 and HD 7850 graphics cards

by Tarinder Sandhu on 5 March 2012, 05:00 4.0


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Pitcairn pixellised

Editor's note: the Radeon HD 7870 and HD 7850 have merely been announced today, coinciding with public viewings at the CeBIT and GDC shows. Retail availability is slated for week commencing March 19, according to AMD.

A hole needs fillin'

What better way to start off the first full week of March than with two more graphics-card announcements from AMD. You'll know that the Austin outfit has been busy pumping out Radeon HD 7000-series GPUs from just before Christmas last year. First came the daddy Radeon HD 7970 card, announced on December 22. Five weeks later its little brother, the Radeon HD 7950, arrived on the scene. Doing nicely at the high-end of the enthusiast market, these £350-plus cards are wish-list material for many.

Fast forward to mid-February and AMD chose to skip the next logical cards in the sequence. Rather, it focussed on the mainstream market with Radeon HD 7770/50, but such is the competition in this well-contested £85-£125 space, that these new GPUs are more hmmm than wow. Price-equivalent last-generation cards from both AMD and NVIDIA are simply better for gaming.

But launching Radeon HD 7000-series cards in this anachronistic manner means AMD has to fill a rather large hole with yet another Radeon HD 7000-series GPU line. You see, the £125-£325 segment is being supported by last year's Radeon HD 6970/50 - two cards that are going to see the GPU glue factory pretty soon.

Step forward 'Pitcairn,' tasked to provide compelling GPUs in an area which AMD and NVIDIA already have a bunch of tasty products. Pitcairn, the codename for the latest upper-mainstream silicon, is based on the same GCN compute architecture as the other HD 7K GPUs, meaning it's hewn from 28nm technology. To see how it slots into the stack, head on down to the Trusty Table.

Trusty Table™

GPU Radeon HD 7970
Radeon HD 7950
Radeon HD 7870
Radeon HD 7850
Radeon HD 6970
Radeon HD 6870
Codename Tahiti XT Tahiti Pro Pitcairn XT Pitcairn Pro Cayman XT Barts XT
DX API 11.1 11.1 11.1 11.1 11 11
Process 28nm 28nm 28nm 28nm 40nm 40nm
Transistors 4.3bn 4.3bn 2.8bn 2.8bn 2.64bn 1.70bn
Die Size 352mm² 352mm² 212mm² 212mm² 389mm² 225mm²
Processors 2,048 1,792 1,280 1,024 1,536 1,120
Default memory 3GB 3GB 2GB 2GB 2GB 1GB
Compute units 32 28 20 16 24 16
Texture Units 128 112 80 64 96 56
ROP Units 32 32 32 32 32 32
GPU Clock (MHz) 925 800 1,000 860 880 900
Shader Clock (MHz) 925 800 1,000 860 880 900
GFLOPS 3,789 2,867 2,560 1,761 2,703 2,016
Memory Clock (MHz) 5,500 5,000 4,800 4,800 5,500 4,200
Memory Bus (bits) 384 384 256 256 256 256
Max bandwidth (GB/s) 264 240 153.6 153.6 176 134.4
Power Connectors 8+6 6+6 6+6 6-pin 8+6 6+6
TDP (watts) 250 200 175 130 250 150
GFLOPS per watt 15.15 14.34 14.63 13.55 10.81 13.44
CrossFire Support 4-way 4-way 2-way 2-way 4-way 2-way
Release MSRP $549 $449 $349 $249 $369 $239

Pitcairn dissected

OK, AMD is initially launching two PCIe 3.0-certified Pitcairn GPUs into the fold. The names come as no surprise given AMD recent nomenclature convention. Pitcairn Pro is to be known as Radeon HD 7850 and Pitcairn XT, the faster model, as Radeon HD 7870. Perusal of the specifications only tells us theoretical performance that doesn't account of advances in architectures, but it's still an exercise worth persevering in.

While based entirely on the aforementioned GCN architecture, the two Pitcairns aren't silicon-disabled GPUs from the Tahiti line. Rather, Pitcairn has its own 212mm² die that fits midway between the HD 79x0's 355mm² and HD 77x0's 123mm². Pitcairn, then, has been specifically designed for the upper-mid-range segment.

A shrink in process, from 40nm to 28nm, means that Pitcairn fits in more transistors - read performance-enhancing silicon - than last year's Radeon HD 6970/50 (aka Cayman) into a die which is significantly smaller. Presuming rather too much, this makes the new GPU cheaper to produce. Indeed, the die is about the same size as a Radeon HD 6870/50's, to give you a better comparison.

What AMD has really done is keep the die size fairly constant between the HD 6870/50 and 7870/50, which makes sense, but has added all the goodness of a 28nm process and GCN architecture into the mix. But, and it's an important one, AMD has jacked-up the release MSRP to levels that equate with the faster HD 6970/50, rather than the lower launch price of HD 6870/50. Placing another wrinkle into this tenuous model-to-model comparison, both Pitcairn cards have a 2,048MB framebuffer as standard, thus making apples-to-apples comparos against HD 6870/50 somewhat moot. Got all that positioning? No? Good. Let's now drill deeper.

Radeon HD 7870 2GB

Due to debut at $349 (£260), HD 7870 will, for a while at least, be fighting it out against last-gen HD 6970 and NVIDIA's GeForce GTX 570 cards. It needs to be fast, clearly, yet AMD has genuinely stripped the architecture down from even the Radeon HD 7950. Heck, the 1,280 stream processors are barely an improvement over cheap-as-chips HD 6870, while the memory speed of 4,800MHz is, really, pretty slow for a high-end card. Que pasa?

AMD understands that the snips chops are pretty severe for a $349 card, and this is why it cranks up the core frequency to 1,000MHz, driving up the pure computational GFLOPS throughput to acceptable levels. This is a GPU that uses core frequency more than parallel processing ability to achieve hoped-for performance.

We had expected HD 7870 to roll in with a couple more compute units (1,408 stream processors) and the 256-bit-wide GDDR5 memory clocked in at 5,000MHz-plus. AMD is putting heck of a lot of faith in the efficiency of the GCN architecture to deliver adequate gaming results for a £260 card, we feel, and it's just as well the Pitcairn GPUs keep a full complement of 32 ROPs; anything lower and the cards' potential would surely be restricted.

Radeon HD 7850 2GB

Sharing the same die is the Pitcairn Pro/HD 7850 2GB. Available in a couple of weeks with a $249 (£185) retail price and to compete against NVIDIA's GeForce GTX 560 Ti and Ti 448, as well as AMD's own Radeon HD 6950, AMD rings in the changes by deactivating four Compute Units (240 processors), resulting in 1,024 cores, and further reduces core frequency from 1,000MHz to 860MHz. Everything else, as far as we can tell, is left the same as on HD 7870.

A one-two punch of lower clocks and fewer shaders provides enough differentiation for the two Pitcairn parts. Again, the numbers in the table don't paint it in a refulgent light, as the Radeon HD 6870, now available for £125, beats it in terms of stream processors, GFLOPS throughput and gets fairly close with respect to memory bandwidth.

One upside of the 28nm process is power-draw that's rather yummy. Just like its HD 6850 forebear, the cheaper Pitcairn card makes do with a single 6-pin PCIe connector. AMD quotes a typical power-draw of 130W - 45W less than HD 7870 - and we envisage partners constructing near-silent solutions in the coming weeks.

Technical summary

The release of two Pitcairn models marks a complete transition to the Radeon HD 7000-series for AMD graphics cards costing >£85. Radeon HD 7870 and HD 7850 don't look overly impressive on paper, really, so let's evaluate the faith AMD has placed in the all-new GCN architecture with lots of benchmarks.