From the outset, Mac OS X and macOS have been designed around a relatively small kernel which is given additional capabilities by kernel extensions. The kernel itself runs at a highly privileged level, giving it most direct access to resources such as the processor, memory and hardware devices, often known as Ring 0. Kernel extensions operate at a close level of privilege (Ring 1) so that they too can make hardware such as ethernet and Thunderbolt ports work, and they’re loaded once the kernel itself is running, before the rest of macOS. Big Sur’s kernel has just over 550 standard kernel extensions which extend it to make everything else work.

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