AMD Radeon HD 7870 & 7850 Launch Review - PAGE 4

We can really see the Southern Islands family resemblance in the HD 7800 series. Both models share the same red and black color scheme and uses a similar rear mounted blower style fan as their older siblings. Along with the aesthetics, the HD 7800 cards also use the same rounded design of the back end of the shroud, designed to improve ventilation when the cards are being used in CrossFireX configurations. As far as the dimensions go, both the HD 7870 and HD 7850 are 9.5" in length, which means they should fit into most cases on the market.

The "Pitcairn" GPU is the third iteration in the Southern Island series, and is designed to offer better performance-per-dollar than its higher-end counterparts. Examining the core internally, Pitcairn has a total of 2.8 billion transistors and like the other cards in the series is built on a  28nm node. The die size however is different; at just 212mm², Pitcairn is over half the size of the Tahiti GPU and smaller than the 255mm² Barts GPU.

Let’s start with the HD 7870. Internally this card is based on the Pitcairn XT design which sports 20 compute units, 1280 streaming processors, 32 ROPS and 80 texture units. The default GPU clock speed is set at 1000MHz, which makes this the second graphics card released from AMD to have a default clock speed of 1GHz. At this speed, the GPU has a pixel fillrate of 32GPixel/s, a texture fillrate of 80GTexel/s and compute performance at 2.65TFLOPs. Additionally, the HD 7870 runs on a 256-bit memory interface and has a 2GB GDDR5 frame buffer set at 1200MHz (4.8Gbps effective).

Like its older sibling, the HD 7850 is based on the GCN design, but it uses the slightly scaled back specifications of the Pitcairn Pro GPU. This gives the HD 7850 a total of 16 compute units instead of 20, which makes for 1024 streaming processors. The rest of the specifications include 32 ROPs, 64 texture units and a GPU engine clock that is set at 860MHz, giving it a compute performance of 1.76TFLOPS. Turning to the memory, the HD 7850 has the same interface, 2GB frame buffer and clocks speeds, making it identical down to the total memory bandwidth.

Attached to the PCB of both cards is a single CrossFireX connector and a PCIe Generation 3.0 x16 lane.  By using the PCIe 3.0 interface, the board has double the maximum data rate over Gen 2.0, giving the card up to 32 GB/s of bi-directional bandwidth on an x16 connector. It is going to be hard for a single graphics card to saturate the PCIe Gen 3 interface with so much bandwidth, so the benefit will most likely only be noticeable with scaling multiple graphics cards together in CrossFireX configuration. We are a bit disappointed to see the HD 7800 series only supports 2-way CrossFire support, but this tends to be the preferred configuration for most CrossFire users. It would have been amazing to see exactly what three or four of these cards would be capable of though!

Since the HD 7870 and HD 7850 have the same feature-set as other cards in the Southern Island family, they are equipped with PowerTune technology. To recap, PowerTune is basically a power management system that maximizes the performance of the board via dynamic power adjustment. It does this by increasing the GPU clock speed in real time when the GPU detects power headroom, and throttling the clocks when a certain power limit is exceeded. This allows the board to adjust the clock speeds on a microsecond level.

Breaking it down by board, the HD 7870 has a typical board power rating of 175W, while the HD 7850 has a rating of only 130W. Considering these cards could potentially perform at the same level as last generation's high-end hardware, the power ratings for both boards is extremely good. This is in keeping with the better performance-per-watt we have been seeing from the 28nm node, but it should also give both cards the ability to scale beyond 1GHz.

The HD 7870 and HD 7850 also support "Zero Core Power". Traditionally, anyone using multiple GPUs in a single system had to deal with a high power idle state, simply because each card was still actively drawing system power; each graphics card could produce 30+ watts of power even when the system wasn't under load. With "Zero Core Power", the extra graphics cards in a CrossFireX system are disabled, shutting down the fans and capping any voltage from going to the core. Since PowerTune works on a microsecond level, "Zero Core Power" will not interfere with gaming as all the GPUs can become active again in just microseconds.

The video outputs are consistent with the rest of the series. In total there are two Mini-DP connectors, a single HDMI 1.4a connector and a Dual-Link DVI connector. Both cards also uses the same non-stacked DVI design as the HD 7970, which improves both the acoustics and exhaust rate giving airflow a clearer path to travel out of the heatsink. Like many other recent AMD cards, the HD 7800 series comes bundled with a HDMI to DVI dongle, and mini-DP to DVI dongle that allow the card to support up to three DVI connections out of the box.

All the outputs include the same design and features as those found on the HD 7900 series. This gives the Pitcairn cards' DP 1.2 ports the ability to support up to three monitors per port (via MST Hub) as well as AMD's HD3D technology. The middle HDMI 1.4a connector also supports 3GHz speeds with frame packing. Essentially this allows the connection to run the frames faster, which is going to make viewing images and playing games smoother across the board. The HDMI and DP ports can also be paired together (with a HDMI to DVI adapter) to support HD3D Surround.