A system call is a request made by a program to the:

Question

BlackTom AI · Accepted Answer

System calls are the interface by which a user-level program requests services from the operating system kernel.

1. Relevant knowledge points and concepts
- User space vs kernel space: applications run in user space with limited privileges; the kernel runs in kernel (supervisor) mode with full access to hardware and protected resources.
- System call: a controlled entry point from user space into the kernel that lets programs request OS-managed services (file I/O, process control, memory management, device I/O).
- Mechanism: a software trap or interrupt switches the CPU to kernel mode so the kernel can execute the requested privileged operation.

2. Step-by-step reasoning process
- A program needs an OS service (e.g., open a file).
- It calls a library wrapper (e.g., fopen) which prepares arguments and issues a system call instruction (software interrupt/trap).
- The trap causes a context switch to kernel mode and transfers control to the kernel’s system call handler.
- The kernel validates parameters, performs the requested privileged operation (access file system, schedule I/O), and returns results/errors.
- Control is returned to the user program, which continues in user mode.

3. Intermediate steps (mechanism detail)
- User calls library function → library executes special CPU instruction (e.g., syscall, int 0x80) → CPU switches to kernel mode → kernel dispatches to correct system call routine → kernel executes operation → result placed in registers/stack → CPU switches back to user mode and returns.

4. Why the correct answer is correct
- Only the kernel has authority to perform protected operations (hardware access, process management, filesystem changes). System calls are specifically the mechanism for programs to request these kernel services, so the correct recipient is the kernel.

5. Why other options are incorrect
- b. User: “User” is a privilege level, not an entity that provides OS services; user-level code cannot perform privileged operations directly.
- c. Another application program: Inter-process calls exist, but they are not “system calls” — they are IPC or API calls between programs.
- d. CPU: The CPU executes instructions and handles mode switches but does not implement OS services; it facilitates the trap to the kernel.

6. Conclusion
The correct answer is a. Kernel — this matches the provided answer.

A system call is a request made by a program to the:Single choice

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