The system call (''syscall'') mechanism is used by applications to access kernel services. The userland (application/non-kernel) program loads certain registers designating the system service (syscall) desired and the arguments to that syscall, then invokes a software interrupt or exception request which transfers control to the operating system kernel. Note that switching to kernel mode requires a change of processor mode (sometimes call the privilege level or exception level), because the kernel has access to instructions, memory structures, memory areas, and devices which userland programs can't access. The syscall numbers may vary between architectures, and the registers as well as the method used to invoke syscalls is are architecture-specific. The processor modes and activity involved in a userland-to-kernel switch also vary by architecture.