COA-Meaning of Computer Organisation and Architecture

Computer Organisations and Computer Architecture are not the same terms .In literature of Computer, Computer scientists often make a distinction between these two terms.Now let us define computer organisation and Architecture.

Computer Architecture:

Computer Architecture refers to those attributes of a system that are visible to a programmer, or in other words, those attributes that have a direct impact on a logical execution of a program.

From technical point of view, Computer Architecture deals with ISA.ISA here stands for "Instruction Set Architecture".Instruction Set Architecture is defined as the abstract image of computing system that is seen by a machine language (or assembly language) programmer.

That is ISA(Computer Architecture) includes following attributes/concepts:

Instruction Set, Addressing Modes(techniques for addressing memory), Data Formats, Processor Register, Number of bits used to represent data types (e.g., number, characters etc.,). 

Thus, in short, we can say that Computer Architecture is concerned with the structure and behaviour of computer as seen by the user.

Computer Organisation:

Computer Organisation refers to the operational units and their interconnection that realize the architectural specifications.In other words, Computer Organisation is concerned with the way the hardware components  operate and the way they are connected together to form the computer system.

From technical point of view, we can say that, it deals with HSA.HSA here stands for "Hardware System Architecture". 

HSA states that Organisation of computer include following attributes/concepts:

Hardware details transparent to programmer such as Control Signals, Interface between Computer and Peripherals , and the memory technology used etc.

The distinction between Computer Organisation and Computer Architecture is clear from the following example:

As an example:

It is an Architectural design issue that - whether a computer will have a Multiply instruction.

And it is an organisation issue that - whether that instruction(MultiplyInstruction) will be implemented by special hardware of Multiply unit unit or by a mechanism that make repeated use of add unit of the system. 

Historically, and still today the distinction between Architecture and Organisation is very important.

Many computer manufacturers offer a family of computer models, all with the same Architecture but with differences in Organisation like some computer models have more cache, some have less, speed of some computer is fast, while some have slow, but architecture that is, computer have one cache, one hard disk etc are same, in all models.

Furthermore, a particular Architecture may spam many years and encompass a number of different computer models, but its organisation changing with the changing technology.

A prominent example of both these phenomenon is "Von Neumann Architecture", because on the basis of this Architecture variety of models come in the market.

Why Study Computer Organisation and Architecture?

The IEEE/ACM(Institute of Electrical and Electronics Engineering/Association for Computer Machinery) computer curricular 2001[JTF 001], prepared by Joint Task Force on computing currica of the IEEE computer society and ACM , lists computer Organisation and Architecture as one of the core subjects that should be in curriculum of all students in computer science and computer engineering. The report says the following:

The computer lies at the heart of computing.Without it most of the computing disciplines today would be branch of theoretical mathematics. To be a professional in any field of computing, today, one should not regard the computer as just black box that executes program by magic. All students of computing should require some understanding and appreciation of computer system's functional components, their characteristics, their performance, and their interactions. These are practical  implications as well.Student also need to understand Organisation and Architecture of Computer in order to structure a program so that it runs more efficiently on a real machine. And In selecting a system to use, they should also be able to understand the trade off among various components, such as CPU clock speed vs memory size.

[CLE M00] also gives the following examples-as reasons for studying computer Architecture:

• Suppose a graduate enters the industry and is asked to select the most cost-effective computer for use throughout a large organisation.To make such decision, an understanding of the implications of spending more for various alternatives, such as larger cache or a higher processor clock rate, is essential, which one gain through understanding of Organisation and Architecture of Computer.

• Many processor are not used in PCs or servers but in embedded systems(An Embedded system is a dedicated computer system designed for one or two specific functions.The Embedded  System is unlike the general purpose computer, which is engineered to manage a wide range of processing task.)A designer may program a processor in 'C' that is embedded in some real time or larger system, such as intelligent automobile electronics controllers.Debugging the system may require the use of logic analyser that displays the relationship between interrupt request from engine sensors and machine level code.An understanding of Computer Organisation and Architecture help in taking decision in them.

• Concepts used in computer organisation and Architecture find application in other courses also.In particular, the way in which the computer provides architectural support for programming language and operating system facilities reinforces concepts from those areas.