Can be used in perf/latency measurement.

1Userspaceuser application
2UserspaceGNU C library (glibc)
3KernelspaceSystem Call Interface
4KernelspaceSubsystems: virtual filesystem, memory management, process management
5KernelspaceArchitecture Dependent Code, device drivers
6HardwarePhysical devices


To execute a system call, a program must execute a special Trap instruction. This instruction simultaneously jumps into the kernel and raises the privilege level to kernel mode; once in the kernel, the system can now perform whatever privileged operations are needed (if allowed), and thus do the required work for the calling process.
When finished, the OS calls a special return-from-trap instruction, which, as you might expect, returns into the calling user program while simultaneously reducing the privilege level back to user mode.

To specify the exact system call, a system-call number is usually assigned to each system call. The user code is thus responsible for placing the desired system-call number in a register or at a specified location on the stack; the OS, when handling the system call inside the trap handler, examines this number, ensures it is valid, and, if it is, executes the corresponding code. This level of indirection serves as a form of protection; user code cannot specify an exact address to jump to, but rather must request a particular service via number

Being able to execute the instruction to tell the hardware where the trap tables are is a very powerful capability. Thus, as you might have guessed, it is also a privileged operation.