Tonga GPU Features

AMD is releasing a new GPU in the $249 price point with updated features and performance.

On December 22, 2011, AMD launched the first 28nm GPU based on an architecture called GCN on the code name Tahiti silicon. That was the release of the Radeon HD 7970 and it was the beginning of an incredibly long adventure for PC enthusiasts and gamers. We eventually saw the HD 7970 GHz Edition and the R9 280/280X releases, all based on essentially identical silicon, keeping a spot in the market for nearly 3 years. Today AMD is launching the Tonga GPU and Radeon R9 285, a new piece of silicon that shares many traits of Tahiti but adds support for some additional features.

Replacing the Radeon R9 280 in the current product stack, the R9 285 will step in at $249, essentially the same price. Buyers will be treated to an updated feature set though including options that were only previously available on the R9 290 and R9 290X (and R7 260X). These include TrueAudio, FreeSync, XDMA CrossFire and PowerTune.

Many people have been calling this architecture GCN 1.1 though AMD internally doesn't have a moniker for it. The move from Tahiti, to Hawaii and now to Tonga, reveals a new design philosophy from AMD, one of smaller and more gradual steps forward as opposed to sudden, massive improvements in specifications. Whether this change was self-imposed or a result of the slowing of process technology advancement is really a matter of opinion.

The New AMD Tonga GPU 

By addressing the $250 market first with Tonga and the Radeon R9 285, AMD is hoping to bring a significant portion of its product line up to par when it comes to the feature set it made available in the Radeon R9 290/290X last fall. Looking at raw specifications of the R9 285 will likely induce a case of deja vu for most PC Perspective readers.

  Radeon R9 280X Radeon R9 280 Radeon R9 285 GeForce GTX 760
GPU Cores 2048 1792 1792 1152
Rated Clock 1000 MHz 933 MHz 918 MHz 980 MHz Base
1033 MHz Boost
Texture Units 128 112 112 96
ROP Units 32 32 32 32
Memory 3GB 3GB 2GB 2GB
Memory Clock 6000 MHz 5000 MHz 5500 MHz 6000 MHz
Memory Interface 384-bit 384-bit 256-bit 256-bit
TDP 250 watts 200 watts 190 watts 170 watts
Peak Compute 3.4 TFLOPS 2.9 TFLOPS 3.29 TFLOPS 2.2 TFLOPS
MSRP Price $289 $239 $249 $239

With 1792 stream processors, 112 texture units and 32 raster operation units, the GPU core specs essentially mirror that of the Radeon R9 280 that this new offering is replacing. The "up to" frequency of the GPU however changes from 933 MHz to 918 MHz, a slightly lower speed however peak theoretical performance goes from 2.96 TFLOPS to 3.29 TFLOPS. Clearly the newer architecture has allowed AMD engineers to spend some time tweaking the organization of the Compute Units in a way to improve overall efficiency.

The TDP of the Radeon R9 285 is 190 watts, 10 watts lower than the R9 280 was rated at when it launched. That might seem pretty minor (and it is) but considering that AMD is able to outperform the R9 280 with Tonga with lower power consumption, while on the same architecture and the same process technology, is a good sign.

AMD updated the instruction set on Tonga as well to improve both parallel processing performance and power efficiency for GPU compute and media processing tasks. While details on the sharing improvements between SIMD lanes is sparse, we can assume that some additional cache was setup to allow better communication between Compute Units.

Easily the biggest change is the move from a 384-bit memory bus and a base 3GB of frame buffer on the R9 280 to a more modest 256-bit memory bus and 2GB of memory on the new Radeon R9 285. AMD has stepped up the memory frequency from 5.0 GHz to 5.5 GHz on the R9 285 though it's clear that raw memory bandwidth is lower on Tonga. To make up for this, AMD has improved compression on frame buffer color data, improving bandwidth efficiency by as much as 40% over Tahiti.

The die size of Tonga compared to Tahiti actually goes up slightly, from 352mm2 to 359mm2. Transistor count is much more impressive though - Tonga clocks in at 5.0B transistors; Tahiti just crossed the 4.3B mark. Even though Tonga has 700M more transistors than Tahiti, AMD was able to maintain a comparative die size with that memory controller bus change and some clever engineering.

The Tonga GPU is improved in other ways as well; it would seem the lack of pressure to get a 20nm GPU out the door has allowed AMD's teams opportunity to tweak GCN. The tessellation system in Tonga can now handle 2-4x the total throughput of Tahiti helping to make up for the performance deficit it has had to Kepler. 

The display controller technology has been updated for all six supported outputs with better filters and scaling support all way up to and down from 4K.

Another change for Tonga is that it supports 4K video encoding/decoding, something that even Hawaii doesn't include. By enabling support for 4K h.264 content in fixed function hardware, AMD claims to have as much as a 47% performance advantage over NVIDIA's GeForce GTX 760 using applications like Cyberlink Media Espresso and Arcsoft Media Converter 8.

The best way to think of the new Radeon R9 285 is that it combines the price point of the R9 280 and the feature set of the R9 290X. That includes improved Mantle support, TrueAudio technology, FreeSync support, the improved PCIe-only XDMA CrossFire technology and updated PowerTune.

While TrueAudio and XDMA support are important for the Radeon R9 285 I think support for FreeSync is the most crucial point here. With AMD attempting to counter NVIDIA's G-Sync technology with Project FreeSync, the biggest hurdle was the lack of compatible GPUs on the market. Until this release, only the R9 290X, R9 290 and R7 260X were going to be able to run FreeSync monitors when they become available. That isn't even close to the majority of the market and NVIDIA can claim G-Sync support for any GPU back to the GTX 650 Ti Boost. AMD needs to refresh the remaining product line to make sure that FreeSync has the best possible chances for adoption upon launch.

Pricing on the R9 285 will start $249, just about $20-30 lower than what the Radeon R9 280 launched at, fitting right between the R9 270X and the R9 280X, for as long as those two parts actually last.

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