Different computer operating systems require different degrees of mode support when terminals are used as computer terminals. The POSIX terminal interface, as provided by Unix and POSIX-compliant operating systems, does not accommodate block-mode terminals at all, and only rarely requires the terminal "itself" to be in line-at-a-time mode, since the operating system is required to provide canonical input mode, where the terminal device driver in the operating system "emulates" local echo in the terminal, and performs line editing functions at the host end. Most usually, and especially so that the host system can support non-canonical input mode, terminals for POSIX-compliant systems are always in character-at-a-time mode. In contrast, IBM 3270 terminals connected to MVS systems are always required to be in block mode.