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AMD's Radeon HD 7790 graphics card reviewed

Old ingredients, new recipe

Some of us were expecting AMD to unleash next-generation Radeons this spring, but it was not to be. We learned last month that the company intends to keep its Radeon HD 7000 series around through much of 2013. A completely new product series is in the works, but it's not due out until very late this year—likely just before Christmas.

As we hung our heads listening to the news, we learned that AMD's plans didn't preclude new releases long before the holidays. In fact, we were told that the Radeon HD 7000 series would soon be expanded with fresh cards featuring new silicon. We'd soon have some previously unseen hardware to sink our teeth into.

True to its word, AMD has now introduced the Radeon HD 7790, a $149 graphics card powered by a new GPU called Bonaire. This addition offers an interesting middle ground between the Radeon HD 7770 and the Radeon HD 7850, not to mention a potentially compelling alternative to Nvidia's GeForce GTX 650 Ti.

The new Bonaire GPU
Buckle up, folks, because AMD's code names get a little bumpy here. Bonaire is officially part of the Sea Islands product family. Sea Islands no longer implies a next-gen graphics architecture as it once did, however; in AMD's words, the name now encompasses "all products we're producing in 2013." Bonaire, despite being a completely new ASIC, is actually based on the exact same Graphics Core Next graphics architecture as the Radeon HD 7000 series (which was itself code-named Southern Islands).

Bonaire also happens to be the name of an island in the planet's northern hemisphere. And Northern Islands code name refers to the Radeon HD 6000 series. But I digress.

All this code-name mumbo jumbo aside, Bonaire is an exciting addition to AMD's GPU lineup. While it features the same 128-bit memory interface and ROP arrangement as Cape Verde, the chip that powers the Radeon HD 7770, it has four more compute units and one additional geometry engine. That means the ALU count has gone up from 640 to 896, the number of textures filtered per clock has increased from 40 to 56, and the number of triangles rasterized per clock cycle has risen from one to two.

width (bits)
process node
Cape Verde 16 40/20 640 1 128 1500 123 28 nm
Bonaire 16 56/28 896 2 128 2080 160 28 nm
Pitcairn 32 80/40 1280 2 256 2800 212 28 nm
GF114 32 64/64 384 2 256 1950 360 40 nm
GK104 32 128/128 1536 4 256 3500 294 28 nm
GK106 24 80/80 960 3 192 2540 214 28 nm

Translation: Bonaire is rigged to offer higher floating-point math performance, more texturing capability, and better tessellation performance than Cape Verde. Also, as you'll see on the next page, AMD equips Bonaire with substantially faster GDDR5 RAM, which gives it a bandwidth advantage despite its identical memory controller setup.

In addition to the different unit mix, Bonaire has learned a trick from Trinity and Richland, AMD's mainstream APUs. That trick takes the form of a new Dynamic Power Management (DPM) microcontroller, which enables Bonaire to switch between voltage levels much quicker than Cape Verde or other members of the Southern Islands family. Behold:

The diagram above shows the different DPM states available to Bonaire. You can click the buttons under the image to switch between the first diagram, which shows Bonaire's capabilities, and the second diagram, which shows the states available to a "Boost"-equipped version of Tahiti, as found in the Radeon HD 7970 GHz Edtion.

In Tahiti, there are four discrete DPM states, each with its own voltage and clock speed. The GPU can switch between clock speeds very rapidly—in as little as 5-10 ms—but voltage changes require "several hundred milliseconds." In order to stay within its power and thermal limits at the High and Boost states, the chip attempts to reduce its clock speed without lowering the voltage level. AMD call these reductions "inferred" states. They enable the GPU to respond quickly to load increases in order to prevent power consumption from going over the limit. If lowering the clock speed isn't enough, then the chip falls back to a lower discrete state, which involves a voltage cut—and therefore takes longer than a simple clock-speed adjustment.

That's not a bad approach. However, it means the GPU may often find itself with more voltage than it needs to operate at a given clock speed. As a result, power consumption may be higher than it should be, while the core clock speed (and thus performance) might be lower than it needs to be.

How does Bonaire improve on this formula? Well, it has a total of eight discrete DPM states, each with a different clock speed and voltage. Bonaire can switch between those states as quickly as every 10 milliseconds, which removes the need for the "inferred" states seen in Tahiti—that is, clock speed reductions without corresponding voltage cuts. This means the GPU can very quickly select the optimal clock speed and voltage combination to offer the best performance at the predefined power envelope.

Although it lacks support for the Boost power state, the Cape Verde chip in the Radeon HD 7770 otherwise behaves much like Tahiti, whose DNA it shares. Thus, the additional power states in Bonaire give the Radeon HD 7790 an advantage in power efficiency over the 7770.