Operating systems have developed through a number of distinct phases or generations, which corresponds roughly to the decades. Which are described below.
i. The 1940’s first generations
The earliest electronic digital computers had no operating systems. Machines of the time were so primitive that programs were often entered one bit at time on rows of mechanical switches. Programming languages were unknown (not even assembly languages). Operating systems were unheard of.
ii. The 1950’s second generations
By the early 1950’s, the routine had improved somewhat with the introduction of punch cards. The general motors’ research laboratories implemented the first operating system in early 1950’s for their IBM701. The system of 50’s generally ran one job at a time. These were called single-stream batch processing systems because programs and data were submitted in groups or batches.
iii. The 1960's Third Generation.
The Systems of the 1960's were batch processing system but they were able to take better advantage of the computer resources by running several jobs at once. So operating system designers developed the concept of multiprogramming in which several jobs are in main memory at once, a processor is switched from job to job as needed to keep several jobs advancing while keeping the peripheral devices in used.
For example, on the system with no multiprogramming, when the current job paused to wait for other I/O operation to complete, the CPU simply sat idle until the I/O finished. The solution for this problem that evolved was to partition memory into several pieces, with a different job in each partition. While one job was waiting for I/O to complete, another job could be using the CPU.
Another major feature in third generation operating system was the technique called spooling. In spooling, a high-speed device like a disk interposed between a running program and low-speed device involved with the program in Input/output. Instead of writing directly to a printer, for example outputs are written to the disk. Programs can run to completion faster, and other programs can be initiated sooner when the printer becomes available, the outputs may be printed.
Another feature present in this generation was time-sharing technique, in which each user has an on-line terminal. Because the user is present and interacting with the computer, the computer system must respond quickly to the user requests, otherwise user productivity could suffer. Time searing systems were developed to multiprogramming large number of simultaneous interactive users.
iv. Fourth Generation
With the development of LSI (Large Scale Integration) circuit, chips, operating system entered in the personal computer and the workstation age. Microprocessor technology evolved to the point that it became possible to build desktop computers as powerful as the mainframes of the 1970's. Two operating systems have dominated the personal computer scene Ms-Dos written by Microsoft Inc. for the IBM PC and other machines using the Intel 8080 CPU and its successors and UNIX. Which is dominant on the large personal computers using the Motorola 6899 CPU family.
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