AMD Radeon HD 7850 & HD 7870 Launch Review



Product: AMD Radeon HD 7800 Series
Company: AMD
Author: James Prior
Editor: Charles Oliver
Date: March 4th, 2012

Pitcairn

The Pitcairn die is on the small side of the halfway point between Cape Verde and Tahiti, using 2.8Bn transistors to deliver 1280 stream processors. Using Graphics Core Next (GCN) architecture compute units as building blocks, this gives 20 compute units and a corresponding 80 texture units. The 256-bit memory bus is complemented by 32 ROPs and 2GB of GDDR5 VRAM.

GCN - Pitcairn

The full specifications list provided by AMD offers a couple of intriguing points. The 2.8Bn transistor count is higher than Cypress (HD 5800), at 2.8Bn and Cayman (HD 6900) at 2.64Bn, but the stream processor count is down, 1280 plays 1536 and 1600. At first you might think that this implies that the stream cores are bigger to perform more work with less, and while that's partially true that's neglecting all the extra cache added to the design and the optimizations AMD made designing GCN, to be more efficient and have higher utilization. Any performance differences between Cypress, Cayman and Pitcairn may be down to more than clock speed and stream core count.

Pitcairn

Tahiti's dual graphics engine and 2 prim/clk rate is preserved, as is hardware double precision rate, although designed to run at 1/16 of single precision rate. Pitcairn XT is not far short of Tahiti Pro's compute rate - 2.56TFLOPs plays 2.87TFLOPS, and interestingly the pixel fillrate of Pitcairn XT is higher than that of Tahiti. Texturing is obviously lower, thanks to fewer texture units, with Pitcairn XT around 12% slower in pure texture fillrate than Tahiti Pro.

Pitcairn Pro, the HD 7850, is reduced in three ways, compared to Pitcairn XT. The core engine clock is decreased, from 1GHZ to 860MHz. The number of stream cores drops to 1024, a reduction of 4 compute units, bringing the CU count to 16 and the corresponding texture unit count to 64. Texturing rate is reduced by 32%, and raw compute power by 45%, but pixel fillrate is only down 16%.

The memory interface, as previously noted, is 256-bit and uses GDDR5. Interestingly, AMD specified 1.2GHz GDDR5, giving an effective rate of 4.8Gbps and 153.6GB/s bandwidth. This is intriguing as many suspected Cayman's use of 1375MHz memory was to alleviate the memory bottleneck Cypress suffered from. With the same memory bus width, speed and bandwidth as Cypress, it will be interesting to see if memory speed impacts gaming performance linearly or not - is the design memory bandwidth limited or not. With both XT and Pro designs using the same memory configuration, either they are not particularly bandwidth limited except for corner cases, or they are bandwidth limited enough that even specifying 6gbps memory wouldn't offer a worthwhile perf/w increase.

Like the Tahiti designs, AMD specified a blower fan mounted at the rear and a large aluminum heat array using vapor chamber technology to cool the card. The plastic shroud is detachable for easy cleaning, secured by six teeny little Phillips head screws on the sides of the card. The cards have a full slot rear exhaust and a tiny set of internal vents back into the case. Interestingly, our review sample 7850 has a different cooler design from that indicated in the press pictures. We can only talk about what we've got, so it's a blower fan design as reference as far as we know.

AMD Radeon HD 7870 1GHZ Edition
AMD Radeon HD 7850

The PCI-Express power inputs are side mounted at the rear of the card, one 6pin 75W input needed for the HD 7850 and two 6-pin 75W inputs for the HD 7870. Maybe a future PCIe and ATX specification will allow for a single 8-pin PCIe 150w connector, but for right now that's how card power inputs scale up. The board power ratings are quite noteworthy, with the HD 7850 claiming 130W typical board power, hence the single power connector, and 150W TDP max, although using PowerTune + 20% might nudge you over that limit. The HD 7870 is rated at 180W, a little under the typical gaming power given for the 5870 1GB. The maximum PowerTune + 20% should get you to 216W, around the limit of PCIe slot plus two PCIe 6pin inputs.

Display Outputs

The AMD Radeon HD 7800 series provides four outputs: two mini-DP 1.2 outputs plus a full-size HDMI 1.4a/Fast HDMI output, and a dual-link DVI output. Expect at least one passive adapter to be included in the box, although Add-In Board (AIB) partners will likely differentiate their products using different connectivity options.