Elements of a Microcomputer
About the elements of a microcomputer
The primary function of a digital
computer is the manipulation of data. The three elements which are
essential to the fulfillment of this task are the central processing
unit, the memory and the input-output interface. These elements are
collectively known as the computer hardware, and each element exists
physically as one or more integrated circuit chips mounted on a
printed circuit board. Where the central processing unit (CPU)
consists of a single microprocessor, it is usual to regard the system
as a microcomputer. The distinction between microcomputer,
minicomputer and mainframe computer is a very arbitrary division made
according to relative computer power. However, this classification has
become somewhat meaningless, with present-day 'microcomputers' being
more powerful than the mainframe computers of only a few years ago.
The central processing unit (CPU) part
of a computer can be regarded as the brain of the system. The CPU
determines what computational operations are carried out and the
sequence in which they are executed. During such operations, the CPU
makes use of one or more special storage locations within itself known
as registers. Another part of the CPU is the arithmetic and logic unit
(ALU) which is where all arithmetic operations are evaluated. The CPU
operates according to a sequential list of required operations defined
by a computer program, known as the computer software. This program is
held in the second of the three system components known as the
computer memory.
The computer memory also serves several other functions besides this
role of holding the computer program. One of these is to provide
temporary storage locations which the CPU uses to store variables
during execution of the computer program. A further common use of
memory is to store data tables which are used for scaling and variable
conversion purposes during program execution.
Memory can be visualized as a consecutive sequence of boxes in which
various items are stored for a typical memory size of 65 536 storage
units. If this storage mechanism is to be useful, then it is essential
that a means is provided for giving a unique label to each storage
box. This is achieved by labeling the first box as 0, the next one as
1 and so on for the rest of the storage locations. These numbers are
known as the memory addresses. Whilst they can be labeled by decimal
numbers, it is more usual to use hexadecimal notation.
Two main types of computer memory exist and there are important
differences between them. The two kinds are random-access memory (RAM)
and read-only memory (ROM). The CPU can both read from and write to
the former, but it can only read from the latter. The importance of
ROM becomes apparent if the behavior of each kind of memory is
considered when the power supply is turned off. At power-off time, RAM
loses its contents but ROM maintains them, and this is the value of
ROM. Intelligent instruments normally use ROM for storage of the
program and data tables and just have a small amount of RAM which is
used by the CPU for temporary variable storage during program
execution.
The third essential element of a computer system is the input-output
(I/O) interface, which allows the computer to communicate with the
outside world by reading in data values and outputting results after
the appropriate computation has been executed. In the case of a
microcomputer performing a signal processing function within an
intelligent instrument, this means reading in the values obtained from
one or more transducers and outputting a processed value for
presentation at the instrument output. All such external peripherals
are identified by a unique number as for memory addresses.
Communication between these three computer elements is provided by
three electronic highways known as the data bus, the address bus and
the control bus. At each data transfer operation executed by the CPU,
two items of information must be conveyed along the electronic
highway: the item of data being transferred and the address where it
is being sent. Whilst both of these items of information could be
conveyed along a single bus, it is more usual to use two buses which
are called the data bus and the address bus. The timing of data
transfer operations is important, particularly when transfers take
place to peripherals such as disk drives and keyboards where the CPU
often has to wait until the peripheral is free before it can
initialize a data transfer. This timing information is carried by a
third highway known as the control bus.
The latest trend made possible by advances in very large-scale
integration (VLSI) technology is to incorporate all three functions of
central processor unit, memory and I/O within a single chip (known as
the computer on a chip). These chips are already well established in
domestic appliances and vehicle fuel control systems, and their
exploitation within intelligent instruments is likely to grow rapidly.

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