In operating systems architecture, a reference monitor is a secure, always-used and fully-testable module that controls all software access to data objects or devices. The reference monitor verifies the nature of the request against a table of allowable access types for each process on the system. For example, Windows 3.x and 9x operating systems were not built with a reference monitor, whereas the Windows NT line (NT/2000/XP), which was designed with an entirely different architecture, does contain one.