An Interview With Nvidia’s Nick Stam & Sridhar Ramaswamy NICK STAM joined Nvidia in early 2005 as a technical marketing director. Prior to that, he worked for Ziff Davis as lab director and technical director of PC Magazine for nearly 14 years. SRIDHAR RAMASWAMY joined Nvidia as a senior marketing manager in October, 2007, and is currently responsible for technical marketing of the Tegra product line. Sridhar has more than 10 years’ experience in the PC semiconductors industry. BY
BARRY BRENESAL
CPU
Let’s talk a bit about the background of the Tegra line, and the move from Tegra 1 to Tegra 2.
NS
Tegra 1 was our first attempt at getting into the mobile device market. It was very good, and in some ways ahead of its time. You saw it in the Microsoft Zune HD and used by Samsung and a few others, too. But the problem was its operating system. It just didn’t have a stable, well-developed architecture from an OS perspective, so we really didn’t have a strong software foothold. For Tegra 2 we worked closely with Google, and many hardware vendors took early versions. There was a lot of effort put in, and in late 2010 we began to announce OEM partnerships. Now, with the combination of Tegra 2 and the Android OS, we have an environment that works well. It’s the first dual-core, ARM-based system-on-a-chip to come to market, with products that started showing up in the early part of the year. The first phones and tablets emerged in quantity in February and March, including the Motorola Xoom tablet and the Atrix 4G and LG Optimus 2X superphones, etc. Others have appeared since then.
CPU
Returning to the differences between the two Tegra chipsets, what are those at the architecture level?
SR
The Tegra 2 CPU is a Cortex A9, while the Tegra 1 was a Cortex
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A8. Each core runs at 1GHz, with its own L1 cache and a shared L2 cache. We have dedicated cores for video encoding/ decoding and image processing. In addition, a small ARM7 shadow processor takes care of all system management when the main processor needs to be shut down to avoid consuming power for a relatively small task. In addition to the CPU, we have the GeForce architecture-based GPU in there.
NS
It depends on how you look at graphics architecture, but these are four floating-point units for pixel and four for vertex processing: we call that eight cores. The graphics side of the chip is more of a classic architecture that’s been heavily modified for mobile usage. Thus it has many caches and prevents going off-chip as much as it can, since that consumes more power.
CPU
Power is an issue that’s come up in a number of contexts. Part of the single-core mystique
is that a faster chip will outperform a slower dual-core at a single task. The Tegra 2 is clocked at 1GHz.
NS
But they can be clocked down based on the task that is running for lower battery consumption. What’s more, if the task is multithreaded, then you can run both cores at a lower clock rate and attain higher performance while consuming less power than a fasterclocked single core.
SR
T h e re a re m a n y l e ve l s o f power management built on the chipset. At the system level, if the CPU’s not being used, the frequency is brought all the way down to zero, and it’s power-gated. When blocks within a core are not being used, we turn off those cores. We have even finer-grained control; if we see registers in data paths that are not being used often, then we clock-gate those.