What is a system call()
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A request by the users to access kernel functions
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A request by kernel to the device
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A request by the device to kernel
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None
A system call is a mechanism for programs to request services from the kernel. It's the interface between user space and kernel space - userspace programs invoke system calls to access protected kernel functions like file I/O, process management, etc.
A system call is the mechanism by which a user-mode process requests a service from the operating system kernel — e.g., to open a file, allocate memory, or create a process — crossing the user/kernel boundary in a controlled way. It's initiated by user programs, not by the kernel or by devices, so 'a request by kernel to the device' and 'a request by the device to kernel' both describe something else entirely (interrupts/DMA, not system calls).