Intel manual Chapter, Multi BUS I and Intel386 DX Microprocessor

CHAPTER 9

MULTI BUS I AND Intel386 DX MICROPROCESSOR

Previous chapters have presented single-bus systems in which a single Tnte1386 DX microprocessor connects to memory, I/O, and coprocessors. This chapter introduces the system bus, which connects several single-bus systems to create a powerful multiprocess- ing system. Two examples of multiprocessing system buses are the Intel MULTIBUS I, discussed in this chapter, and the Intel MULTIBUS II, discussed in Chapter 10.

A system bus connects several processing subsystems (each of which can include a local bus and private resources) and the resources that are shared between the processing subsystems. Because all the processing subsystems perform operations simultaneously on their respective local buses, such a multiprocessing system results in a significant increase in throughput over a single-bus system.

Another advantage of using a system bus is that the system can be expanded modularly. The system bus establishes the standard interface through which additionaJ processing subsystems communicate with one another. Through this interface, components from different vendors can be integrated.

A central concern of any multiprocessing system is dividing resources between the sys- tem bus and the individual local buses; that is, determining which resources to share between all processors and which to keep for only one processor's use. These cho~ces affect system reliability, integrity, throughput, and performance. The deciding factors are often the requirements of the particular target system.

Because local resources are isolated from failures occurring in other parts of the system, they enhance the overall reliability of the system. Also, because the processor does not have to contend with other processors for access to its local resources, bus cycles are performed quickly. However, local resources add to the system cost because each resource must be duplicated for each subsystem that requires it.

Resources used by more than one processing subsystem but not used frequently by any subsystem should be placed on the system bus. The system can minimize the idle time of such resources. However, this advantage must be weighed against the disadvantage of increased access time when more than one processor must use a system resource.

9.1MULTIBUS I (IEEE 796)

The Intel MULTIBUS I (IEEE 796 Standard) is a proven, industry-standard, 16-bit multiprocessing system bus. A wide variety of MULTIBUS I compatible I/O subsystems, memory boards, general purpose processing boards, and dedicated function boards are available from Intel. Designers who choose the MULTIBUS I protocols in their system bus have a ready supply of system components available for use in their products.

MULTIBUS I protocols are described in detail in the Intel MULTIBUS® I Architecture

Reference Book.

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