Sun4d is a computer architecture introduced by Sun Microsystems in 1992. It is a development of the earlier Sun-4 architecture, using the XDBus system bus, SuperSPARC processors, and SBus I/O cards. The XDBus was the result of a collaboration between Sun and Xerox; its name comes from an earlier Xerox project, the Xerox Dragon. These were Sun's largest machines to date, and their first attempt at making a mainframe-class server.