Question

System calls are usually invoked by using which one of the following means?

• A. A privileged instruction
• B. A jump instruction
• C. Software interrupt
• D. Hardware interrupt

Hint:

System calls rely on software-generated interrupts or traps, not hardware interrupts.

Answer 1

The Correct Option is C

User programs run at a restricted privilege level and can't directly execute kernel-only instructions or jump straight into kernel code, that would defeat the whole purpose of protection between user programs and the OS.
What they can do is execute a special trap/software-interrupt instruction, which is specifically designed to hand control to the kernel at a safe, predefined entry point, switching the CPU into kernel mode in a controlled way.
Hardware interrupts, by contrast, come from external devices, not from the user program requesting a service.
So the correct answer is software interrupt.

Answer 2

Another angle is to ask who initiates each of these mechanisms, and whether that matches how a program requests OS services:

1. Privileged instruction: Can only be initiated by code already running in kernel mode, a user program has no legal way to execute one directly, so this can't be how user code asks for a service.
2. Jump instruction: Initiated by the running program itself, but it only moves execution within the program's own privilege level, it can't hand control to the kernel.
3. Software interrupt: Deliberately initiated by the running user program itself, precisely to ask the operating system for a service, and it's built specifically to switch privilege levels safely when executed.
4. Hardware interrupt: Initiated by an external device asynchronously, not by the user program wanting a service at a specific point in its code.

Only the software interrupt is both initiated by the requesting program and capable of safely reaching kernel code.

Therefore, the correct answer is Software interrupt.