In 1958, jack St. Clair Kilby and Robert Noyce invented the first integrated circuit. Integrated circuits (called ICs) are circuits consisting of several electronic components like transistors, resistors, and capacitors grown on a single chip of silicon eliminating wired interconnection between components. IC technology was also known as "microelectronics technology because it made it possible to integrate large number of circuit components into very small (less than 5mm square) surface of silicon, know as "chip" [see Figure 1.4(c)]. Initially the integrated circuits contained only about ten to twenty components. This technology was named small-scale integration (SSI). Later with the advancement in technology of manufacturing ICs, it became possible to integrate up to about hundred components on a single chip. This technology was known as medium scale integration (MSI).
Third generation computers were manufactured using ICs. Earlier ones used SSI technology and later ones used MSI technology. ICs were smaller, less expensive to produce, more rugged and reliable, faster in operation, dissipated less heat, and consumed less power than circuits built by wiring electronic components manually. Hence, third-generation computers were more powerful, more reliable, less expensive, smaller, and cooler to operate than second-generation computers.
Parallel advancements in storage technology allowed construction magnetic core based random access memory as well as larger capacity magnetic disks and tapes. Hence, third-generation computers typically had few megabytes (less than 5 Megabytes) of main memory and magnetic disks capable of storing few tens megabytes of data per disk drive.
ON software front, standardization of high-level programming languages, timesharing operating system, unbundling of software from hardware, and creation of an independent software industry happend during third-generation. FORTRAN and COBOL were the most popular high-level programming language in those days. American National Standards Institute (ANSI) standardized them in 1966 and 1968 respectively, and the standardized version were called ANSI FORTAN and ANSI COBOL. The idea was that as long as a programmer follows these standards in program writing, he/she could ran his/her program on any computer with an ANSI FORTAN or ANSI COBOL compiler (see chapter 12 for details). Some more high-level programming languages were introduced during the third-generation period . Notable among these were PL/1, PASCAL , and BASIC.
second-generation computers used batch operating system. In those system, users had to prepare their data and programs and then submit them to a computer centre for processing. The operator at the computer centre collected these user jobs and fed them to a computer in batches at scheduled intervals. The respective users then collected their job's output from the computer centre. The inevitable delay resulting from this batch processing approach was very frustrating to some users, especially programmers, because often they had to wait for days to locate and correct a few program errors. To rectify this situation, John Kemeny and Thomas Kurtz of Dartmouth college introduced the concept of timesharing operating system. Timesharing operating system enables multiple users to directly access and share a computer's resources simultaneously in a manner that each user feels that no one else is using the computer. This is accomplished by using a large number of independent, relatively low-speed, on-line terminals connected to the computer simultaneously. A seprate user each terminal to gain direct axxess to the computer. Timesharing operating system allocates CPU time in such a way that all user programs have a brief share (known as a time slice) of CPU time in turn. Processing speed of CPU allows it to switch from one user job to another in rapid succession and execute a small portion of each job in allocated time slice until the job is completed. Each user gets the illusion that he/she alone is using the computer. Introduction of timesharing concept helped in drastically improving the productivity of programmers and made on-line system feasible, resulting in new on-line applications like airline reservation system, interactive query systems, etc.
Until 1965,computer manufactures sold their hardware along with all associated software without separately charging for software. For example, buyers recived language translators for all languages supported on a computer they purchased. From user's standpoint, software was free. However, the situation chaned in 1969 when IBM and other computer manufacturers began to price their hardware and software products separately. This unbundling of software from hardware gave users an opportunity to invest only in software of their need and value. For example, now buyers could purchase only the language translators they needed and not all language translators supported on the purchased computer. This led to the creation of many new software houses and the beginning of an independent software industry.
Another important concept introduced during third-generation was that of backward compatible family of computers. During this period, IBM introduced its System 360 as a family of computers with backward compatibility as they were different sizes of mainframe systems based on the same machine language. This enabled business to upgrade their computers without incurring costs of replacing peripheral equipment and modifying programs to run on new systems.
Development and introduction of minicomputers also took place during third-generation. Computers built until early 1960s were mainframe systems that only very large companies could afford to purchase and use. Clearly, a need existed for low-cost smaller computer to fill the gaps left by the bigger, faster, and costlier mainframe systems. Several innovators recognized this need formed new firms in 1960s to produce smaller computers. Digital Equipment Corporation (DEC) introduced the first commercially available minicomputer, the PDP-8 (Programmed Data Processor), in 1965. It could easily fit in the corner of a room and did not require attention of a full-time computer operator. It used timesharing operating system and a number of users could access it simultaneously from different locations in the same building. Its cost was about one-fourth the cost of a traditional mainframe system making it possible for smaller companies to afford computers. It confirmed the tremendous demand for small computers for business and scientific applications, and by1971, there were more than 25 computer manufacturers in minicomputer market.
Characteristic features of third-generation computers are as follows:-
They were more powerful than second-generation computers. They were capable of performing about 1 million instructions per second.
They were smaller than second-generation computers requiring smaller space.
They consumed less power and dissipated less heat than second-generation computers. The rooms/areas in which third-generation computers were located still required to be properly air-conditioned.
They were more reliable and less prone to hardware failures than second-generation computers requiring lower maintenance cost.
They had faster and larger primary and secondary storage as compared to second-generation computers.
They were general-purpose machines suitable for both scientific and commercial applications.
Their manufacturing did not require manual assembly of individual components into electronic circuits resulting in reduced human labor and cost involved at assembly stage. Hence, commercial production of these systems was easier and cheaper. However, highly sophisticated technology and expensive setup was required for the manufacture of IC chips.
Standardization of high-level programming languages allowed programs written for one computer to be easily ported to and excuted on another computer.
Timesharing operating system allowed interactive usage and simultaneously use of these systems by multiple users.
Timesharing operating system helped in drastically improving the productivity of programmers cutting down the time and dost of program development by several fold.
Timesharing operating system also made on-line systems feasible resulting in usage of these systems for new on-line applications.
Unbundling of software from hardware gave users of these systems an opportunity to invest only in software of their need and value.
Minicomputers of third-generation made computers affordable even by smaller companies.