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). 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

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

www.buildersassociation.com

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