Kernel

The kernel is the core part of an operating system. It has complete control over the hardware and sits between applications and the physical machine, deciding which program runs on the CPU, how memory is shared, and how data moves to and from disks, networks and other devices.

Applications ask the kernel for these services through system calls. Monolithic kernels, such as Linux, run most services, including device drivers, inside the kernel for speed; microkernels keep the kernel small and run other services as separate programs for stability. Windows and macOS use hybrid kernels that mix both approaches.