The user mode has access to limited system resources. It cannot change modes freely within the user mode. The user program, however, can make a supervisor call using the Software Interrupt (SWI) instruction. SWI is usually called "supervisor call" because it is most frequently used to call the supervisor while in user mode. System mode is similar to user mode but is typically used by operating systems that need access to the system resources. In other words, it has privileged access. The system mode is also used during nested interrupt handling.