Editing System call (section)

== Privileges ==
The [[Instruction set|architecture]] of most modern processors, with the exception of some embedded systems, involves a [[CPU modes|security model]]. For example, the ''[[ring (computer security)|rings]]'' model specifies multiple privilege levels under which software may be executed: a program is usually limited to its own [[address space]] so that it cannot access or modify other running programs or the operating system itself, and is usually prevented from directly manipulating hardware devices (e.g. the [[frame buffer]] or [[Computer network|network]] devices).

However, many applications need access to these components, so system calls are made available by the operating system to provide well-defined, safe implementations for such operations. The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often initiated via [[interrupt]]s. An interrupt automatically puts the CPU into some elevated privilege level and then passes control to the kernel, which determines whether the calling program should be granted the requested service. If the service is granted, the kernel executes a specific set of instructions over which the calling program has no direct control, returns the privilege level to that of the calling program, and then returns control to the calling program.