AMD has unveiled some of the first details of its Carrizo system-on-chip. The processor is the latest iteration of AMD's accelerated processing unit (APU) concept, pairing a CPU with a tightly integrated GPU.

The CPU portion of Carrizo is AMD's latest iteration of its Bulldozer family. This version is called Excavator, with Carrizo having two Excavator modules providing four cores. Currently, not much is known about Excavator, aside from having larger caches: the level 1 data cache has been doubled in size to 32KB per core. Compared to the previous iteration of the design, Steamroller, performance is up about 5 percent at the same clock speed.

The GPU portion has similarly been updated; Carrizo uses 8 cores using the Tonga design, which made its debut in the Radeon R9 285. This is version 1.2 of AMD's Graphics Core Next architecture. It supports the forthcoming DirectX 12 and AMD's own Mantle API, and also heterogeneous system architecture (HSA) 1.0.

HSA is designed to allow developers to more easily mix computation up between different kinds of processor. The common case will be spreading work between a CPU and a GPU (though in principle, HSA could include, say, cryptographic accelerators or other kinds of special-purpose compute unit). Traditional GPU-based computation requires data to be copied back and forth between the CPU and the GPU, making it expensive to split workloads between the two; to run efficiently, a task needs to run almost entirely on the CPU, or almost entirely on the GPU. With HSA, fine-grained interleaving is much more efficient, affording greater flexibility in how tasks are spread between both processing units.

The GPU also has a new codec module, providing support for H.265 decoding in hardware and enough power to transcode nine 1080p streams simultaneously.

The big revelations about Carrizo, however, were around its layout and power efficiency. As with its predecessor, Kaveri, Carrizo is built on a 28nm process. Carrizo uses a lot more transistors than Kaveri, too; 3.1 billion, as compared to 2.4 billion. But the total die size is approximately the same, and the Excavator cores are about 23 percent smaller than Kaveri's Steamroller cores.

With Carrizo AMD has switched from a high performance design to a high density one. As the name implies, this increases the density of the transistors, thereby reducing the die size. Perhaps less expected is the implication for performance: at low power levels, the high density design supports slightly higher frequencies. With larger power budgets the situation is reversed—the high performance design supports higher frequencies in a given power envelope—but for a mobile part such as Carrizo, the high density design has the edge.

These advantages extend to the GPU; for a given power level, the high density design supports a 10 percent higher frequency, and for a given frequency, the high density design supports as much as a 20 percent reduction in power.

Carrizo also includes a new power management feature called Adaptive Voltage and Frequency Scaling. To run stably at a given frequency, a processor needs a certain amount of voltage. The power regulators cannot deliver perfectly smooth voltage; there is noise and occasionally the voltage will drop below the level needed to support a particular frequency. To account for this, excess voltage is used, typically around 10 percent more than that needed to run at a desired frequency. This means that even if there's a brief dip, the chip is still getting enough juice. However, this wastes power; power used is proportional to voltage squared, so a 10 percent voltage excess implies a 20 percent power excess.

With AVFS, Carrizo can eliminate much of that excess power. Through careful monitoring of the power, frequency, temperature, and voltage, Carrizo can near-instantaneously reduce the operating frequency of the chip whenever a brief voltage drop is detected. This means that the chip no longer needs the over-voltage just to handle these momentary glitches. AMD claims that this can provide power savings of up to 10 percent in the GPU, and almost 20 percent in the CPU, providing power reductions above and beyond those from the switch to the high density designs.

Carrizo has some 500 different frequency-sensing modules scattered around the chip, reporting to 10 different AVFS control modules. These allow each individual chip's performance to be measured to determine the necessary voltage at a fine-grained level, so frequency can be pushed as hard as possible at a given voltage/temperature level.

Altogether, AMD reckons that Carrizo will deliver "double digit" increases in both performance and battery life. While we don't expect Carrizo to surpass Intel's Broadwell designs, especially at the high end, the improvements in Carrizo could well make AMD's processors a compelling option in the sub-$500 segment when they eventually reach the market.

Listing image by AMD