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MBus is a computer bus designed and implemented by Sun Microsystems for communication between high-speed computer system components, such as the central processing unit, motherboard and main memory. Contrast this with SBus, used in the same machines to connect add-on cards to the motherboard.
MBus was first used in the initial SPARC-based multiprocessing system, the SPARCserver-690, and found use in several SPARC-based computer workstations (such as the SPARCstation series) beginning in the late 1980s. The bus permitted the integration of several microprocessors on a single motherboard, in a multiprocessing configuration with up to 8 CPUs with the CPUs were packaged in detachable MBus modules (see, for example, the SPARCstation 10 and SPARCstation 20). Single-processor systems were also sold that used the MBus protocol internally, but had CPUs that were permanently attached to the motherboard to lower manufacturing costs.
MBus specified a 64-bit datapath, which used 36-bit physical addressing, giving an addess-space of 64 GByte. The transfer-rate is 80 MByte/s (320 MByte/s peak). Bus-controlling is done by an Arbiter. Also an interrupt-, reset- and timeout-logic is specified.
Several related buses were also developed, such as XBus, a packet-switched bus corresponding to the circuit-switched MBus, with identical electrical characteristics and physical form factor but an incompatible signalling protocol, and KBus, a high-speed interconnection system for linking multiple MBuses, used in Solbourne Series 6 and Series 7 computer systems.
Among the computer system manufacturers who produced computer systems using the MBus were Sun Microsystems, Ross Technologies, Hyundai/Axil, Fujitsu, Solbourne, Tatung, GCS, Auspex, ITRI, ICL, Cray, Amdahl, Themis and DTK.
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