system 7 (codenamed "big bang" and sometimes retrospectively called mac os 7) is a graphical user interface-based operating system for macintosh computers and is part of the classic mac os series of operating systems. it was introduced on may 13, 1991, by apple computer, inc. it succeeded system 6, and was the main macintosh operating system until it was succeeded by mac os 8 in 1997. features added with the system 7 release included virtual memory, personal file sharing, quicktime, quickdraw 3d, and an improved user interface.