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For example, the Nvidia Tegra 4 is described as having 4 + 1 CPU cores. Is this simply a conventional way of saying that it has 5 cores, or does that phrasing imply something different?

http://www.nvidia.com/object/tegra-4-processor.html

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It's an extra core used when the higher power CPU (4 cores) isn't necessary.

PCMag

Nvidia has settled on a name for the unique processor architecture used for its quad-core Tegra 3 chip – 4-PLUS-1. The "PLUS-1" refers to the mobile processor's fifth core, a "battery-saver" core that runs on very low power and executes tasks at low frequency when the chip's more powerful and power-hungry ARM Cortex-A9 cores aren't needed.

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It might be worthwhile to note that the "Companion Core" is the same microarchitecture as the main cores, so not only is the interface for system software the same (as with ARM's big.LITTLE) but application software optimization is largely portable (unlike big.LITTLE). This design increases the cost of manufacturing (combining aspects of high-performance and low-power processes) but may have reduced time-to-market and ARM licensing costs (compared to big.LITTLE). –  Paul A. Clayton Jan 6 at 20:25

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