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AMD Radeon HD 7870 And 7850 Review: Pitcairn Gets Benchmarked

AMD Radeon HD 7870 And 7850 Review: Pitcairn Gets Benchmarked
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There's a big hole in between AMD's £350 Radeon HD 7950 and its £120 Radeon HD 7700. Today, the company introduces Radeon HD 7850 and 7870 to fill that gap, and they push a lot more performance than we expected. But are they really ready for prime time?

There's a big hole in between AMD's £350 Radeon HD 7950 and its £120 Radeon HD 7700. Today, the company introduces Radeon HD 7850 and 7870 to fill that gap, and they push a lot more performance than we expected. But are they really ready for prime time?

If you follow the graphics industry, you probably already knew today’s AMD Radeon HD 7800-series launch was happening. The company told us to expect this back in December when it introduced Radeon HD 7970. It reminded us in January when the Radeon HD 7950 debuted, and again in February during the Radeon HD 7770 and 7750 briefings.

Consequently, we’re also not surprised that, as the Radeon HD 7870 and 7850 became public knowledge, supply of the Radeon HD 6970 and 6950 dried up. After all, it’s hard to recommend last generation’s card, still priced competitively, when it’s no longer for sale. The new models replace those older cards with similar suggested prices of £260 for the 7870 and £185 for the 7850.

Another Paper Preview Launch

Unfortunately, although we’re able to talk about the performance of Radeon HD 7870 and 7850, you won’t be able to buy the card until at least March 19th, AMD tells us. The company says that, because CeBIT and the Game Developers Conference fall so close to the official launch, it’d be difficult to keep information about the cards under wraps. Regardless of the explanation, it sucks to wait another two weeks for street pricing and a better feel for availability.

Cynicism aside, we’re still technology geeks at heart, and we’re desperately curious to know how the 7800-series cards measure up to the Radeon HD 6900s, which earned our affections in the past for their conservative power use, excellent CrossFire scaling, and rich display connectivity compared to Nvidia’s GPUs with two independent pipelines.

This is the third distinct Graphics Core Next-based ASIC from AMD in as many months. Code-named Pitcairn, we finally have a solution in between the flagship Tahiti and the entry-level Cape Verde. The graphics processor is composed of 2.8 billion transistors in a 212-square millimetre die manufactured on TSMC’s 28 nm lithography node. That’s nearly twice as large as the GPU at the heart of Radeon HD 7770, but around two-thirds of Tahiti, which drives the Radeon HD 7970.

Pitcairn hosts 20 compute units, each with four vector units made up of 16 stream processors and a single texture unit. In an uncut GPU, those numbers multiply out to 1280 total shaders and 80 texture units. AMD shears off four compute units from Pitcairn to create the Radeon HD 7850, resulting in 1024 shaders 64 texture units.

The GPU’s back-end is made up of eight render partitions, each with four full-colour ROPs, totalling 32 ROPs. Four 64-bit memory controllers yield an aggregate 256-bit memory bus. AMD doesn’t make any changes to this part of the chip; both the Radeon HD 7870 and 7850 sports identical back-ends.


Radeon HD 7870
Radeon HD 6970
Radeon HD 7850
Radeon HD 6950
Stream processors
12801536
1024
1408
Texture Units
80
966488
Full Colour ROPs
32
3232
32
Graphics (Shdr) Clock
1000 MHz880 MHz860 MHz800 MHz
Texture Fillrate
80 Gtex/s
84.5 Gtex/s55 Gtex/s70.4 Gtex/s
Memory Clock
1200 MHz1375 MHz1200 MHz
1250 MHz
Memory Bus
256-bit256-bit256-bit
256-bit
Memory Bandwidth153.6 GB/s
160 GB/s153.6 GB/s160 GB/s
Graphics RAM
2 GB GDDR5
2 GB GDDR52 GB GDDR51-2 GB GDDR5
Die Size
212 mm2
389 mm2212 mm2389 mm2
Transistors (Billion)
2.8
2.642.82.64
Process Technology
28 nm40 nm
28 nm40 nm
Power Connectors
2 x 6-pin1 x 8-pin, 1 x 6-pin
1 x 6-pin2 x 6-pin
Maximum power (TDP)
175 W
250 W
130 W200 W
Price
£260 MSRP
£250-£310 (EOL)
£185 MSRP£190 (1 GB, EOL)
£200-£220 (2 GB, EOL)

In general, AMD’s Radeon HD 6900 family features more shaders and texture units, while the 7800s operate at higher clock rates. Remember also that both line-ups also employ different architectures optimized for different things. VLIW4 was the product of much evolution, and hence quite mature. Meanwhile, GCN is much newer. We’ve seen it completely tank in DirectX 9-class applications, and then deliver stellar performance in more modern titles.

All of the cards in the above chart feature 256-bit memory interfaces feeding 32 ROPs, but the 6900 series’ higher memory clock rates facilitate a little more bandwidth.

Interestingly, the Radeon HD 6950 only proffers about 10% fewer shaders than the Radeon HD 6970. In contrast, the Radeon HD 7850 comes armed with a roughly 25% deficit compared to Radeon HD 7870. AMD seems to have decided that it needed to spread its mid-range cards further apart from each other. I’ve often wondered if the Radeon HD 6950 cannibalized Radeon HD 6970 sales because they were so similar (surely it didn’t help that 6950s were getting updated via firmware to look even more like 6970s) That could be what AMD is trying to avoid this time around.

Both of the Radeon HD 7800s employ core clocks in the 1 GHz neighbourhood: the Radeon HD 7870 runs at 1 GHz exactly, while the 7850 operates at 860 MHz.  As with its other GCN-based products, AMD says these two boards are extremely overclockable—a claim we, of course, intend to test in our own lab. Although it’s tempting to look at raw specifications and draw comparisons, you really can’t, on account of the dissimilar architectures. We can, however, point out that all of AMD’s GCN-based cards put a special emphasis on maintaining exceptional performance at lower maximum power. The Radeon HD 7750 doesn’t even need a dedicated PCIe power connector, and yet its performance is often comparable to the Radeon HD 5770/6770. With maximum TDPs at 175 and 130 W, the Radeon HD 7870 and 7850 sport power ceilings that are 75 and 70 W lower than their predecessors, respectively. The Radeon HD 7870 requires two six-pin PCIe power connectors, and the Radeon HD 7850 only needs one. Lower power means less heat. Less heat translates to more conservative cooling. And that leaves the door open for gaming enthusiasts to enjoy quieter systems that go easier on the power bill. These cards might not even necessitate a power supply upgrade, if your machine is reasonably modern.

A Sizable Gap

We’re curious about the space between AMD’s £185 Radeon HD 7850 and its £120 Radeon HD 7770. Like it or not, the Radeon HD 6800s are gone for good. AMD isn’t taking orders on the 6870, 6850, or 6790. The company won’t comment on future products, but we find it hard to believe it’d abandon the price point currently inhabited by the GeForce GTX 560, 560 Ti, and Radeon HD 6870. That leaves two likely possibilities for the future: a cheaper 1 GB version of the Radeon HD 7850, or perhaps a crippled version of the 7850 that might populate a lower position in the hierarchy.

Radeon HD 7870 and 7850

Officially, AMD gives its Radeon HD 7870 the same GHz Edition suffix as the Radeon HD 7770, indicating only that the core breaks the completely arbitrary gigahertz barrier.

As far as appearances go, the Radeon HD 7870 is quite similar to the Radeon HD 6870. It’s 9.5” long with two six-pin auxiliary power inputs. The biggest functional difference between old and new is the loss of a DVI display connector. As with the flagship Radeon HD 7970, AMD says it got rid of the second DVI output to improve exhaust airflow. Two mini-DisplayPort connectors and a single HDMI port remain intact. Of course, the card’s style is a little different, adhering to the tapered end we’ve seen from other 7000-series models.

Keep in mind that the Radeon HD 7870 actually replaces the 6970, though, and not the 6870. The higher-end 6970 is an inch longer, and it requires eight- and six-pin power inputs to feed an additional 75 W.

We like the 7870’s more compact dimensions, but enthusiasts will be disappointed to learn about two notable omissions: a BIOS selector switch and dual CrossFire connectors. Unlike the Radeon HD 6900s, which scaled up to four cards in a CrossFire array, the Radeon HD 7870 and 7850 can only be used in pairs. Moreover, the lack of a backup firmware makes flashing the BIOS riskier.

The reference Radeon HD 7850 seems identical to the 7870 with one exception: a single six-pin power input (rather than two), reflecting the 7850’s lower 130 W typical board power rating.

But the reference card isn’t necessarily representative of what you’ll see at retail, and some of the models AMD showed us are actually quite a bit smaller. 

Almost all of the partner boards share the same combination of one DVI-D output, a single HDMI connector, and twin mini-DisplayPorts. Based on the pictures AMD provided, the lone exception is XFX’s model with two DVI-D outputs.

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  • 1 Hide
    jamesedgeuk2000 , 5 March 2012 16:31
    Quote:
    The Radeon HD 7800s don’t do particularly well in World of Warcraft, finishing behind their respective predecessors, albeit just slightly. More importantly, they succumb to the comparable GeForce cards they’ve been matching pace with up until now.


    WoW favours Nvidia hardware over ATi, its always been this way.
  • 1 Hide
    HEXiT , 5 March 2012 22:10
    still no absolute reason to change from my 5870... come on amd thats 2 generations of cards and neither have given a significant enough increase to warrant a replacement.
  • 1 Hide
    Tab54o , 5 March 2012 23:27
    jamesedgeuk2000WoW favours Nvidia hardware over ATi, its always been this way.


    Yeah but the question is why would anyone be buyng one of these cards to play Wow in the first place?
  • 0 Hide
    wild9 , 6 March 2012 18:18
    fufussMore importantly, they succumb to the comparable GeForce cards they’ve been matching pace with up until now.

    That's the problem with annoying bots like fufuss.. they can't see benchmark graphs.