Computer architecture covers the design of system software,
such as the operating system (the program that controls the computer), as
well as referring to the combination of hardware and basic software that
links the machines on a computer network. Computer architecture refers to
an entire structure and to the details needed to make it functional. Thus,
computer architecture covers computer systems, microprocessors, circuits,
and system programs. Typically the term does not refer to application
programs, such as spreadsheets or word processing, which are required to
perform a task but not to make the system run.
In designing a computer system, architects consider five major
elements that make up the system’s hardware: the arithmetic/logic unit,
control unit, memory, input, and output. The arithmetic/logic unit performs
arithmetic and compares numerical values. The control unit directs the
operation of the computer by taking the user instructions and transforming
them into electrical signals that the computer’s circuitry can understand.
The combination of the arithmetic/logic unit and the control unit is called
the central processing unit (CPU).
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The memory stores instructions and
data. The input and output sections allow the computer to receive and
send data, respectively.
Different hardware architectures are required because of the
specialized needs of systems and users. One user may need a system to
display graphics extremely fast, while another system may have to be
optimized for searching a database or conserving battery power in a laptop
computer.
In addition to the hardware design, the architects must consider
what software programs will operate the system. Software, such as
programming languages and operating systems, makes the details of the
hardware architecture invisible to the user. For example, computers that use
the C programming language or a UNIX operating system may appear the
same from the user’s viewpoint, although they use different hardware
architectures.
When a computer carries out an instruction, it proceeds
through five steps. First, the control unit retrieves the instruction from
memory—for example, an instruction to add two numbers. Second, the
control unit decodes the instructions into electronic signals that control the
computer. Third, the control unit fetches the data (the two numbers).
Fourth, the arithmetic/logic unit performs the specific operation (the
addition of the two numbers).
Fifth, the control unit saves the result (the
sum of the two numbers).
Early computers used only simple instructions because the
cost of electronics capable of carrying out complex instructions was high.
As this cost decreased in the 1960s, more complicated instructions
became possible. Complex instructions can save time because they make
it unnecessary for the computer to retrieve additional instructions. For
example, if seven operations are combined in one instruction, then six of
the steps that fetch instructions are eliminated and the computer spends
less time processing that operation. Computers that combine several
instructions into a single operation are called complex instruction set
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computers.
However, most programs do not often use complex
instructions, but have mostly simple instructions. When these simple
instructions are run on CISC architectures they slow down processing
because each instruction—whether simple or complex—takes longer to
decode in a CISC design. An alternative strategy is to return to designs
that use only simple, single-operation instruction sets and make the most
frequently used operations faster in order to increase overall performance.
Computers that follow this design are called reduced instruction set
computers.
RISC designs are especially fast at the numerical computations
required in science, graphics, and engineering applications. DISC designs
are commonly used for nonnumeric computations because they provide
special instruction sets for handling character data, such as text in a word
processing program. Specialized CISC architectures, called digital signal
processors, exist to accelerate processing of digitized audio and video
signals.
The CPU of a computer is connected to memory and to the
outside world by means of either an open or a closed architecture. An
open architecture can be expanded after the system has been built, usually
by adding extra circuitry, such as a new microprocessor computer chip
connected to the main system. The specifications of the circuitry are made
public, allowing other companies to manufacture these expansion
products.
Closed architectures are usually employed in specialized
computers that will not require expansion—for example, computers that
control microwave ovens. Some computer manufacturers have used closed
architectures so that their customers can purchase expansion circuitry only
from them. This allows the manufacturer to charge more and reduces the
options for the consumer. This sounds good because you make tons of
money this way!
COMPUTER ARCHITECTURE
Bibliography
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