System calls are usually invoked by using

Correct answer: a software interrupt.

Explanation: System calls are invoked by executing a trap or software interrupt. This special instruction causes a controlled transfer from user mode to kernel mode, saving the user context and jumping to a predefined kernel entry point so the operating system can perform the requested service safely.

• Mode switch: The trap triggers a switch to privileged (kernel) mode and preserves user context.

• Examples: On x86, legacy systems use an INT instruction as a software interrupt; modern CPUs may provide dedicated trap-like instructions (often called syscall/sysenter) that still perform a controlled transfer to the kernel.

• Why other choices are incorrect:

• Polling is for checking device status repeatedly and does not provide the controlled privilege transition needed for system calls.

• An indirect jump simply transfers control to a computed address within the same privilege level and does not perform the mode switch or security checks required for system calls.

• Privileged instructions are operations that can only be executed in kernel mode. While some architectures expose a specific system-call instruction, it functions as a trap to the kernel rather than a regular privileged instruction executed directly from user mode.