Q: Is the main memory designed to speed up memory access as well?
A: Splash, the Gamecube's main memory, uses 1T-SRAM from MoSys. It's a very specialized chip.
Q: 1T-SRAM?
A: That's one-transistor SRAM. Memory latency [the time between memory request and reception of data] has become a major bottleneck in data processing, and 1T-SRAM solves that bottleneck. It's both low-cost and low-latency.
Q: What made you decide to use that memory?
A: That was just another technology Takeda found after looking around. Actually, I believe the RDRAM from Rambus that's becoming standard in PCs today was first used mass-market by Takeda for the Nintendo 64. After using the N64 and studying what kind of memory is best suited to the needs of a game machine, we decided that 1T-SRAM is the best choice.
Q: How is 1T-SRAM better suited for consoles?
A: There's a major difference in random-access time between RDRAM and 1T-SRAM. RDRAM's most distinctive feature is that the first read from memory is very latent, but if you read in a lot of consecutive data at once, the latency gradually shrinks down. The problem is, games access memory very haphazardly, reading little fragments here and there again and again. The merits of RDRAM aren't geared towards this type of usage. A typical memory wait is a few dozen nanoseconds, and on a 480MHz processor or a 160MHz video chip that's a few dozen cycles wasted waiting for memory.
Q: So memory latency can downgrade console performance.
A: Exactly. CPU pipelines have made huge advances, and chips are clocked to run faster and faster, so now memory is the main factor which dictates how fast the entire machine runs. With 1T-SRAM, though, it uses SRAM so reading and writing data is a lot faster generally. It's about ten times faster than using DRAM, so the processor only has to wait a tenth of the time... or, really, not at all, usually. 1T-SRAM is absolutely perfect for game systems. You could almost say the Gamecube doesn't have a main memory -- it just has a giant level-3 cache.
Q: The whole memory's a cache?
A: Right. The memory in Flipper is 1T-SRAM as well, to make the memory system go as fast as possible. Games access random parts of memory for textures all the time, so this gives us a major edge over the DRAM used in the PS2. The 1T-SRAM is probably the most distinctive feature of the Gamecube, actually.
Q: There's also something called A Memory in the specs.
A: That's short for auxiliary memory; it's a regular synchronized DRAM. We couldn't use more than 24MB of 1T-SRAM from a board-area perspective, so we put this extra RAM in as a kind of temporary storage area. We used to call it "audio memory", but then the sound guys would want it all for themselves, so we changed it to "A Memory" (laughs).
Q: What sort of things would the A memory be used for?
A: A sample buffer, an animation bufer, a disc cache; pretty much anything.
Q: You've devoted a lot of work to RAM here.
A: We have, and we're getting just as much out of it too. Especially in graphics. If it wasn't for 1T-SRAM we'd never be able to test out the idea of virtual texture RAM. We'd just have to use textures without any tuning, processing speed would've gone down, and the whole machine would have been slower. We couldn't have tried it without these very high speed, very low latency chips. Thanks to that, you can get fabulous performance without devoting so much time to optimization. It makes development far more efficient.
, so I'll just create a new one instead. Now to the issue: