The Texture Units
As we said earlier, Larrabee doesn’t look much like a GPU, but while Intel was able to eliminate the setup engine and ROP by implementing those functions directly at the level of the Larrabee cores, the same isn’t true of the texture units. These units handle a very specific job that can be done much more efficiently in a dedicated unit. Texture decompression (DXTC), for example, is very simple to do in hardware, which is what guaranteed its success, but it requires enormous resources to do it on the software side. Intel calculated that performing texture operations on the processors would be between 12 and 40 times slower than on dedicated units, depending on the configuration (such as filtering quality and compressed or uncompressed texture format).
The texture units are fairly classic, and Intel doesn’t really provide a lot of detail about them, except that they will support all standard Direct3D 10 operations and compression modes. In fact, that’s the only point that might limit Larrabee’s ability to evolve. As we’ll see later, software entirely handles Larrabee’s rendering pipeline and so could address the future functionalities of Microsoft's application programming interface (API) via a simple update of the 3D engine executed by Larrabee’s cores. On the other hand, the texture units will be stuck at a certain level of functionality, which will determine that of the chip as a whole.
One interesting specificity of these texture units is that they handle translation of virtual addresses into physical addresses, which, in more concrete terms, means that there’s no longer a need to load an entire texture with its mipmap stack into local memory, as only the portions needed for display will be stored in the form of pages of a few kilobytes in size. If a page isn’t available in memory (a page fault), the texture unit notifies the processor, which then calls for it. This mechanism would make it very simple for programmers who aren’t afraid to get their hands dirty to implement, for example, an algorithm like id Software’s MegaTexturing.
