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The kernel makes its services available to the application programs that run on it through a large collection of entry points, known technically as system calls.

From a programmer's viewpoint, these look just like ordinary function calls, although in reality a system call involves a distinct switch in the operating mode of the processor from user space to kernel space. Together, the repertoire of system calls provides a 'Linux virtual machine', which can be thought of as an abstraction of the underlying hardware.

An even less visible function of the kernel, even to programmers, is memory management. Each process runs under the illusion that it has an address space (a valid range of memory addresses) to call its own.