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System call (syscall): “A well-defined protocol for user programs to request the kernel to perform privileged operations, including device I/O.” (SysPrfBGrg 2021)
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, accessing a hard disk drive or accessing the device's camera), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.
In most systems, system calls can only be made from userspace processes, while in some systems, OS/360 and successors for example, privileged system code also issues system calls.
For embedded systems, system calls typically do not change the privilege mode of the CPU.
“System call is the mechanism used by an application program to request service from the operating system or more specifically, the operating system kernel. System calls often use a special CPU instruction which causes the processor to transfer control to more privileged code, as previously specified by the more privileged code. This allows the more privileged code to specify where it will be entered as well as important processor state at the time of entry. When the system call is invoked, the program which invoked it is interrupted, and information needed to continue its execution later is saved. The processor then begins executing the higher privileged code, which, by examining processor state set by the less privileged code and/or its stack, determines what is being requested. When it is finished, it returns to the program, restoring the saved state, and the program continues executing.”
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