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Terminology: CPU or APU?

If you are already familiar with the desktop processors with integrated graphics inside offered by Intel and AMD, you know that both these makers try to position their processors as far apart from one another as possible to make you believe that it would be incorrect to compare them side by side. AMD is the one that makes the most stir around it by positioning their products as APUs and not as regular CPUs. So, what’s the difference?

The APU abbreviation stands for Accelerated Processing Unit. The detailed explanation reveals that from a hardware standpoint it is a hybrid device that has traditional general-purpose computational cores combined with a graphics core in the same semiconductor die. In other words, it is the same CPU, but with integrated graphics core inside. However, there is some difference on the software level. The graphics core in the APU should have universal architecture represented by an array of streaming processors that can not only synthesize a 3D image, but also complete some computational tasks.

In other words, the APU offers something more flexible than a simple combination of graphics and computational resources inside a single semiconductor die. The main idea is to create a symbiosis of these diverse parts, when some calculations may also be performed by the graphic core. Although, as it is often the case in similar situations, there should be proper support on the software level.

AMD Fusion processors known as Llano are in full sync with this definition: they are the APU. They have integrated Radeon HD graphics cores inside them, which among other things support ATI Stream technology and OpenCL 1.1 software interface, which make it possible for the graphics core to perform calculations, too. There are a number of applications that can theoretically benefit from being executed by the Radeon HD streaming processors. These applications use cryptographic algorithms, 3D image rendering and image, video and audio post-processing. However, things are much more complicated in reality. Implementation difficulties and doubtful performance gain still prevent this concept from becoming increasingly popular. Therefore, in most cases the APU may be regarded just a regular CPU with the integrated graphics core.

Intel, on the contrary, sticks to more conservative terminology. They continue using the traditional “CPU” term in reference to their Sandy Bridge processors with the integrated HD Graphics core. And, in fact, this is quite justified, because Intel’s graphics doesn’t support software OpenCL 1.1 interface (only the next generation Ivy Bridge processors will be compatible with it). So, at this time Intel doesn’t have different parts of their processors working on the same computational tasks.

However, there is one important exception. Namely, the graphics cores in Intel processors have a special Quick Sync unit that should provide hardware acceleration for video stream decoding process. Of course, just like with OpenCL, it needs to be supported on the software level. However, it is indeed capable of increasing the HD video decoding speed nearly by 10x. So, in the end, it would be fair to say that Sandy Bridge is a hybrid processor to some extent.

Is it fair to compare the AMD FPU and Intel CPU against one another? Theoretically, we can’t establish identical equality between an APU and a CPU with a built-in graphics accelerator, but in reality we have two different names for the same exact thing. AMD Llano processors can accelerate parallel calculations, while Intel Sandy Bridge can use the graphics resources only during video transcoding, but in reality both of these are barely in use. So, from a practical prospective, any of the processors discussed in this article is a combination of a regular CPU and a graphics card inside a single chip.

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