The numbers game

FiringSquad: Where does the 2-terabit number that’s been floating around come from?

ATI: The 2-terabit (256GB/sec) number comes from within the EDRAM, that’s the kind of bandwidth inside that RAM, inside the chip, the daughter die. But between the parent and daughter die there’s a 236Gbit connection on a bus that’s running in excess of 2GHz. It has more than one bit obviously between them.

Also we’re the memory controller for the system, so we have bandwidth between the CPU and the graphics engine, we have bandwidth between the memory, the DDR3 memory, which is also, well the DDR3 is the system memory so all these numbers are sometimes close to each other so things start to blur together

FiringSquad: And then 22.4GB/sec to the system memory?

ATI: 25GB/sec to the system memory, and 22GB/sec between the CPU and the GPU. Which, or in this case we’re more than the GPU, we’re the system memory controller too, so the bus between the CPU and GPU is 22GB. It’s really 11 in both directions, so 11 input and 11 output.

FiringSquad: And this is a 128-bit memory interface or 256-bit?

ATI: 128-bit, 700MHz interface.

FiringSquad: What types of operations do the EDRAMs 192 processors perform?

ATI: Well they do z-compares, they do alpha blends, they do blends of samples to make a pixel. That kind of thing. They do stencil operations also. And this is the first time memory has access to something like this, right in the memory, so it never leaves the memory die. The memory and the logic is all built into one die. And it’s also a power savings by the way.

One of the big uses of power is actually driving I/O pins. In this case, you never have to go off chip so everything is just internal there. So power is important you know, of course, it’s not quite like the handheld or mobile space but it’s still important and you want to reduce it as much as possible because we are going pretty fast. You know we have a lot of logic going pretty fast in memory, a lot of CPU logic going fast, so you want to reduce power wherever you can.