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market for the genre. Somewhere in the range of 200 to 400 of these machines were sold at a price of around $400,000 each. The IBM 650 was vacuum tube based and it made use of a mechanical drum for its main memory. This memory was comprised of a metal drum approximately 6 inches in diameter and 16 inches long that rotated at something greater than 10,000 revolutions per minute. A series of read and write heads were positioned along the rotational axis of the drum, allowing them to magnetically record signals on the surface of the drum that represented numbers encoded in a digital form and then to subsequently read them back from the surface.

During the mainframe epoch, the problems to be addressed by computers were typically divided into two categories: business problems and scientific problems. Keeping the books for a multi-billion dollar corporation is a pretty clear example of a business problem. Providing local forecasts of severe weather is obviously a scientific problem, along with simulating the physical results of a large-scale chain reaction such as that produced by a thermonuclear device. In the early days, these problems were addressed by somewhat different sets of equipment. Business problems required lots of data storage capacity and the ability to stream input and output data to and from this storage. Magnetic tapes were the preeminent mechanisms providing this facility in the earliest systems. Scientific problems required lots of main memory and a significant amount of intermediate capacity for fast input/output storage. The main memory demanded direct access by the central processing unit of the computer; such memory was extremely expensive at the time. Fast intermediate storage was provided by disk drives; they were higher speed and higher capacity derivatives of the drum memory used for the earliest mainframes.

In the business world, such as in banking, or insurance, the necessity of linking computers together to exchange information came early; ad hoc computer-to-computer links started to develop that in time would give birth to the first private networks. Some of those networks are still active today, for example for inter-bank transactions.

Mainframe computers were big and expensive. They required specially prepared environments, giving rise to the term and concept of a computer room. Such facilities had raised floors, allowing cabling among the many boxes comprising the computer to be run as necessary. Raised flooring also allowed for air handling such that chilled air, or water, to be directed through the various racks of electronic gear in order to dissipate the large amounts of heat given off as a byproduct of the inefficient circuitry of the age. Because of their expense, mainframes were of necessity multi-user. Of course, this had the beneficial effect of causing computer designers to be cognizant of the probability that all of these users were going to offer the possibility of creating problems for all the other users. Consequently, computer architectures had to provide mechanisms for keeping the work done by the computer for each user separate from all other work. Also driven by size, power and expense constraints, the problems to be addressed by mainframe computers tended to require strong economic justification. As a result, the problems addressed tended to be big and complex.

As we’ll see in some later chapters, a central feature of cognitive abilities, whether it is of people or of computers, is connection of the sensori-motor experience to the computing mechanism. The successive epochs of computer evolution can be characterized by the sensori-motor linkage between the computer and the person. Indeed, it is clear that a significant aspect of computer evolution has been to more closely mimic the direct sensori-motor experience of the human species. Consider the manner in which the earliest computers, the mainframes, were connected to their human users, both for control (input) and results presentation (output).

At the beginning of the mainframe era, the dominant mechanism for control input was the punched paper card, and in some instances the punched paper tape. A punch card was made of

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