SBus

SBus is a computer bus system that was used in most SPARC-based computers from Sun Microsystems during the 1990s. It was introduced in 1989 to be a high-speed bus counterpart to their high-speed SPARC processors, replacing the earlier (and by this time, outdated) VMEbus that had been used in their earlier Motorola 68030-based systems and early SPARC boxes. When Sun was attempting to "open" the SPARC in the early 1990s, SBus was likewise standardized and became IEEE-1496. In 1997 Sun started to migrate away from SBus onto PCI, and today SBus is no longer used.

SBus is in many ways a "clean" design. It was targetted only to the SPARC so many cross-platform issues were simply not a consideration. SBus was based on a big-endian 32-bit address and data bus, run at 25MHz and thus transfers up to 100Mbyte/second. Devices are each mapped onto a 28-bit address space (16 megabytes), and only eight masters are supported although there are an unlimited number of slaves.

When the 64-bit UltraSPARC was introduced SBus was modified to use clock-doubling and transfer two 32-bit data words per cycle to produce a 200MByte/s 64-bit bus. For contrast, modern 66MHz/64-bit PCI is 528MByte/s.

SBus was a peripheral interconnect only (like PCI). Sun systems used another standardized system as a CPU-memory bus, MBus.

External links:

PCI:SBus Comparison (PDF)



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