NVIDIA have recently released 2 whitepapers detailing the pros and design aspects of their Quad-Core CPU, Kal-El, or Tegra 3. In the whitepaper they mention that Tegra 3 actually has a 5th core, that’s more-or-less a companion core for the CPU. This companion core powers the device when it is in sleep mode or when performing background tasks that don’t need a heap of processing power to run (such as syncing emails, Twitter, Facebook, etc). This enables the CPU to continue processing whilst sucking less life out of the battery.
The core is clocked at a maximum of 500MHz so that it doesn’t run crazy and eliminate the point of power saving, though NVIDIA didn’t cut corners, instead they built the core on the same Cortex A9 architecture as the 4 main cores.
Somehow I think unofficial mods are going to overclock this core to make Tegra 3 more of a processing beast than it already will be — that is, once it is actually inside devices that consumers can buy. Bring on the Penta-Core CPUs!