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6.3 COMPUTER SYSTEMS 319

gates that the control section enables or disables according to the program instructions. That is
to say, programming at the machine-language level amounts to wiring with software instead of
hard-wired connections.
Microprocessors have instruction sets ranging from 20 to several hundred instructions,
known asmicroprograms,which are stored in ROM to initiate the microprocessor routines.
The instructions generally consist of a series of arithmetic and logic type operations, and also
include directions for fetching and transferring data. Microprocessors are classified by word size
in bits, such as 1-, 4-, 8-, and 16-bit microprocessors; generally speaking, the larger the word
size, the more powerful the processor. Three popular 8-bit microprocessors (μp) are Intel 8085,
Zilog Z80, and Motorola MC6800; the 16-bit microprocessors dominating the market are Intel
8086 and 80286, Motorola MC68000, and Zilog Z8000; the powerful 32-bit microprocessors at
the very high end of the market started with Intel 80386, Intel 80486, Motorola MC68020, and
National N532032. Still more powerful Intel Pentium processors such as Pentium II and III were
introduced in the 1990’s, and even these are going to be replaced soon by Intel Itanium Processors.
A microprocessor system bus consists of three physical buses: the address bus, the data
bus, and the control bus. The types of circuits connected to microprocessor buses are registers,
accumulators, or buffer circuits between the bus and the external memory or I/O devices.
Multiplexing is usually used, which is to connect each register to a bus on a time-shared basis,
only when it is being read. An operating bus is used to transfer various internal operations
and commands. An interface bus, such as the IEEE-488 (developed in 1975 by the Institute
of Electrical and Electronics Engineers) or the GPIB (general-purpose interface bus) provides the

Data bus

I/O data

buffers

Instruction

registor

Address

generation

Program

address register

Program

storage

Control

Actuator
control

Control

unit

Control Clock

Address bus

Operand bus

Stacks

Accumulator

Temporaryregister Programcounter Statusregister

Arithmetric and

logic unit

Figure 6.3.2Block diagram of typical microprocessor system architecture.