[Windows RT General] SixSixSevenSeven:
No, the 5th core is not an actual 5th core. The idea is you have 4 full blown cores at 1.2, 1.4 or 1.6ghz depending on the tegra model (and then the tegra can overclock automatically to 1.3, 1.5 or 1.7), thats quite power hungry really. But as CPU usage falls the tegra shuts a few cores off, if the system cant benefit from all 4 cores being active it will drop to 3, then 2 and then 1. Sometimes even that 1 core running at 1.2ghz is compartively power hungry, so the tegra shuts the final core down and fires up the companion core which I think runs around the 700MHz range, its slow at any rate, its also built optimised purely for power consumption over performance. Idea is you can go from a full quad core chip when you need the performance but then when the device is idling you can switch over to the companion core and shut the main 4 all off and save alot of power.

NVidia claim that the companion core combined with the hardware video acceleration of the tegra should be able to play HD videos on its own. That doesnt really seem to happen outside of the lab. But when you lock the screen on your android device it often jumps into companion core mode, you can browse around the android home screen and use a few lightweight apps on the companion core no problem, and when it does begin to struggle the tegra just has to skip over to its main core and gradually bring the other 3 main cores online as it needs them.

It never has the companion and main cores on in a state able to be used by the operating system simultaneously though.

Samsungs so called octa-core chips also do the same. They arent really octa core chips, in reality they are a quad core cortex A15 chip and a quad core lower clock speed cortex A9 chip (possibly even A7) on the same piece of silicon, when CPU load is high it runs as a quad core A15, when it doesnt need so much performance it shuts down the A15 and swaps for the A9, the 2 CPU's are near separate and at any one time the chip is only running as a single quad core processor not an octacore. Similar to the companion core design this can lead to a massive boost in battery life. In both A15 and A9 modes the processor is capable of shutting down individual cores as need be.

Tegra may well be the chip in all main tablets, but when microsoft first started working on windows RT there were meant to be qualcomm snapdragon, NVidia tegra and texas instruments OMAP devices all coming to market so of course microsoft at the time needed RT to run on all 3. The original plan was that there would be56 3rd party manufacturers manufacturing RT tablets, 2 per chip vendor except TI. Originally qualcomm partnered with HP and Samsung, NVidia with Lenovo and Asus and Toshiba with TI In the end TI dropped out and shortly after downscaled OMAP production (I think it has completely stopped with the exception of existing contracts now, or at least chips intended for tablet usage have been, they had a few industrial chips under the OMAP branding that might still be available, their ARM based microcontroller and DSP lines are still going fine), TI took Toshiba with them. Of course by the time TI dropped out there were already running builds of RT. HP dropped out and were replaced by dell. Acer were slated to be joining the program but didn't, when MS unveiled the surface that killed it for acer.

Another limitation is that Windows RT is essentially just an ARM port of windows 8, windows 8 and the NT kernel in general didnt already have support for the companion core or similar tech, it would be pointless adding it to the base NT kernel as hardly any devices use it and it would probably lead to issues introducing it only for tegra.