This powerful computer was part of Sun's first multiprocessor (MP) family of systems. It has four processors working concurrently and ran SunOS/Solaris, Sun's version of UNIX. Such machines were very popular due to their high I/O performance and were typically used as departmental servers. The 4/690 was not just used as a departemental server, it was their top of the line system at the time and was used in both industry and science for all sorts of applications. It was also Sun's last VME bus machine. Note from Alex Bochannek: "The 4/6x0 models were introduced in 1991 as I already said and have some interesting properties. They were Sun's first ever multi-processor and therefor MBus machines and also Sun's last systems to use the VMEbus. Sun used Multibus chassis in their earliest Sun 1's and 2's, then changed to VMEbus for most 2's, 3's, and 4's. All the chassis-based Sun4's had a VMEbus and it wasn't until the 600 series that the motherboard had a seperate bus for the CPU (called MBus and the architecture is called sun4m). Also, the 600 series were the first chassis-based system to utilize the SBus for expansion cards which was introduced earlier with the SPARCstation 1." "The Mbus modules were in use until the Ultras were introduced, so even the 600 series successor, the 2000 used them. The 2000 didn't have a VMEbus though (they used XDBus) and had a different architecture designator (sun4d). A 4/690 could take up to four processors (max of two ROSS SPARC's at 40MHz, or two SuperSPARC's at 40 or 50 MHZ, or four SuperSPARC's at 50MHz). Maximum memory was 640MB with expension boards. Software support started at SunOS 4.1.2 and is just now ending with the removal of VME support in Solaris 2.6. One thing that is kinda interesting about Sun in general is the number of different buses they supported over the years. If you look just at CPU, backplane, and graphics buses, they supported Multibus, VME, P4, SBus, ISA, MBus, XDBus, UPA, and PCI."