Computer Architecture and Organization

Introduction

Computer Architecture and Organization is the study of internal working, structuring and implementation of a computer system. Architecture in computer system, same as anywhere else, refers to the externally visual attributes of the system. Externally visual attributes, here in computer science, mean the way a system is visible to the logic of programs (not the human eyes!). Organization of computer system is the way of practical implementation which results in realization of architectural specifications of a computer system. In more general language, Architecture of computer system can be considered as a catalog of tools available for any operator using the system, while Organization will be the way the system is structured so that all those cataloged tools can be used, and that in an efficient fashion.

How it came along

History of computer systems, in strict sense of name, will date back to as back as the basic need for computation among humans. We, however, are more concerned with architecture and organisation of Electronic computer systems only as 'the computing systems' before this had very vague (or atleast different!) representation of these terms in their construction.

The beginning

The first among the electronic computers was The ENIAC, designed by John Mauchly and J. Presper Eckert. This, although a great achievement altogether, was not of much importance on front of standards of Architecture and organisation. The programming of this giant machine required manual change of circuitry by expert individuals by changing connecting wires and lots of switches; It sure was a tedious task. Besides ENIAC was not a digital machine. It worked on decimal systems much similar to the way we, humans, do in our normal lives.

Von Neumann architecture

A major break through came with the draft of second electronic

computer, EDVAC. This computer was proposed by John von 

Neumann and others in 1945. It used stored program model for 

computers, wherein all instructions were also to be stored in 

memory along being data to be processed thereby removing the 

need for change in hardware structure to change the program. The 

architecture of this computer described the digital system to be 

divided into a Processing Unit consisting of an Arithmetic and 

Logic Unit and Processor registers, Control Unit consisting of a 

Program Counter and an Instruction Register, Memory Unit and 

Input/Output mechanisms. This basic structure of computer system 

has since then served as the basic idea for a computer system. The 

trend continues even today with few changes in the design. This 

architecture however is more popular for implementation in IAS 

computer, (as Neumann, later, shifted to this project). We will see 

the architecture of IAS computer in detail at a later stage.

Rapid restructuring of Organisation

As all this was going on an major advancement in field of 

electronics was achieved at Bell labs as William Shockley invented 


transistor. Transistors were devices comparable in purpose to an 

vacuum tube, but amazingly small, efficient and reliable. 

Transistors revolutionized the organisation of a normal computer 

system. The systems grew smaller, less power consuming, less heat 

generating, more reliable and much more efficient. This generation 

of computers using transistors as basic components is commonly 

known as secong generation of computers. Transistors, however, 

were just a beginning soon a new phase took over. Integrated 

Circuits were developed which could contain more than one 


transistors on a single chip. This further reduced size, power 

consumption and heat generation. This led to development of third 

generation of computers. After this generation, however, there is 

no consensus on how generations changed as the number of transistors 

on a single IC kept increasing and thereby name of technologies 

involved kept changing from MSI to LSI to VLSI to ULSI but the 

basic structure of IC based computer was maintained. Although 

now a whole computer was available on single machine, thanks to 


VLSI techniques. Today the number of transistors on a chip 

doubles every 18 months and so!

IAS Computer

IAS computer the upgraded version of the ENIAC machine . IAS 

was designed by von neuman and was designed with the the 

concept of stored-program, which allowed the machine operator to 

store the program along with its input and output into some 

memory location, but in ENIAC the program had to be manually 

entered.

Memory Organisation

RAM, ROM computer

Input/Output Unit

Control Unit

Instruction set is defined as group of instruction that a processor 

can execute to perform different operations It can be classified on 

the basis of complexity and number of instruction used