ds in commercially available machines by the early 1960s, with access times of 2 or 3 microseconds. These machines were very expensive to purchase or to rent and were especially expensive to operate because of the cost of expanding programming. Such computers were typically found in large computer centers operated by industry, government, and private laboratories staffed with many programmers and support personnel. This situation led to modes of operation enabling the sharing of the high capability available; one such mode is batch processing, in which problems are prepared and then held ready for computation on a relatively inexpensive storage medium, such as magnetic drums, magnetic-disk packs, or magnetic tapes. When the computer finishes with a problem, it typically "dumps" the whole problem program and results on one of these peripheral storage units and takes in a new problem. Another mode of use for fast, powerful machines is called time-sharing. In time-sharing the computer processes many waiting jobs in such rapid succession that each job progresses as quickly as if the other jobs did not exist, thus keeping each customer satisfied. Such operating modes require elaborate "executive" programs to attend to the administration of the various tasks. Advances in the 1960sIn the 1960s efforts to design and develop the fastest possible computers with the greatest capacity reached a turning point with the completion of the LARC machine for Livermore Radiation Laboratories of the University of California by the Sperry-Rand Corporation, and the Stretch computer by IBM. The LARC had a core memory of 98,000 words and multiplied in 10 microseconds. Stretch was provided with several ranks of memory having slower access for the ranks of greater capacity, the fastest access time being less than 1 microsecond and the total capacity about 100 million words.During this period the major computer manufacturers began to offer a range of computer ca...
