System 7 (codenamed Big Bang and sometimes called Mac OS 7) is a version of the Mac OS, the operating system of the Macintosh computer, introduced in 1991. It succeeded System 6, and was the main Mac operating system until it was succeeded by Mac OS 8 in 1997. Features added with the System 7 release included cooperative multitasking, virtual memory, personal file sharing, an improved user interface look, Quick Time, and Quick Draw 3 D.