Computer architecture and organization
Computer Organization and Architecture 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 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 second 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 is the upgraded version of the ENIAC machine . IAS was designed by von Neuman and was designed with 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.
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