What is a system call?
Correct answer A. The interface through which a user program requests a service from the operating system kernel
Explanation
A system call is the programmatic entry point into the kernel: a program asks the OS to do something privileged, such as opening a file or creating a process, and the CPU switches to kernel mode to perform it. A device signalling completion is a hardware interrupt. Ordinary function calls stay in user mode, and shell commands are user-level programs that themselves make system calls.
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