The New Nation - Internet Edition

Peter Glaskowsky

Intel announced on Monday that it will be presenting a paper at Siggraph 2008 about its "many-core" Larrabee architecture, which will be the basis of future Intel graphics processors.

The paper is a pretty thorough summary of Intel's motives for developing Larrabee and the major features of the new architecture. Basically, Larrabee is about using many simple x86 cores-- more than you'd see in the central processor (CPU) of the system-- to implement a graphics processor (GPU). This concept has received a lot of attention since Intel first started talking about it last year.

The paper also answers perhaps the biggest unanswered question about Larrabee-- what are the cores, and how can Intel put "many" of them on a chip when desktop CPUs are still moving from 2 to 4 cores?

Intel describes the Larrabee cores as "derived from the Pentium processor," but I think perhaps this is an oversimplification. The design shown in the paper is only vaguely Pentium-like, with one execution unit for scalar (single-operation) instructions and one for vector (multiple-operation) instructions.

That's the basic answer: Larrabee cores just have less going on. A quad-core desktop processor might have six or more execution units, and a lot of special logic to let it reorder instructions and execute code past conditional branches just in case it can guess the direction of the branch correctly. This complexity is necessary to maximize performance in a lot of desktop software, but it's not needed for linear, predictable code-- which is what we usually find in 3D-rendering software. But the vector unit in Larrabee is much more powerful than anything in older Intel processors-- or even in the current Core 2 chips-- because 3D rendering needs to do a lot of vector processing. The vector unit can perform 16 single-precision floating-point operations in parallel from a single instruction, which works out to 512 bits wide-- great for graphics, though it would be overkill for a general-purpose processor, which is why the vector units in mainstream CPUs are 128 or 256 bits wide at most.