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A supervisor call is an instruction sent to a computer's processor that directs it to transfer computer control to the operating system's supervisor program. Supervisor calls are requests for an operating system service from the operating system itself or another running application. These requests are made through macros or language functions.
Supervisor calls may also be referred to as system calls.
Supervisor calls are instructions in application programs that switch a computer to the supervisor state. This permits an operating system to interrupt the normal flow of processing and process the supervisor call, which requests services related to system internals such as the process involving main memory access, the process involving network hardware access or any other lower level system process.
Supervisor calls provide interfaces between the operating system and the system processes. The majority of the operations that interact with the system require permissions that are not available to user-level processes. There are a number of macros that simplify the supervisor call procedures.
Popular system calls present in Unix- and POSIX-compatible operating systems are open, write, read and close. Every new operating system houses hundreds of system calls.
System calls are grouped into five categories: