Most operating systems (OSs) in use are monolithic. In these, all source code is compiled into one logically undivided block of object code, though the source and/or object code may be segmented into different address spaces, unlike in a Single Address Space OS (SASOS). On this page, OSs are arranged in three groups and levels: 1) Top group: types or classes of OSs. 2) Middle group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.