Filter
Top Products
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 MUL TIBUS 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.
9.1
MULTI
BUS
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.
9-1