The Computer and the Brain
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One of the fundamental choices in the design of a computer or data processing system is concerned with the medium for the storage of information. Once the system requirements impose the need for information storage that exceeds a certain level, typically several hundred bits, a "hierarchy" principle of memory sets in. As expressed by Von Neumann, the system may call for an overall storage of N words at an access time t. For economic or other reasons it may be more practical to provide for some smaller quantity of storage N, with an access time t1= t and obtain the total storage N at some longer access time depending on the system needs. Extending this reasoning then to the more general case there would be a sequence of capacities N12k-1k with access times t12k-1k so that N1 words are required for each access time t1. Then each value of i would represent one level in the hierarchy of memories, and the hierarchy has k such levels. In many modern computers there are also quasi-levels that don't fall into Von Neumann's ordered sequence of levels. Certain bodies of the information storage may have special characteristics or requirements that call for different storage media that would yield greater capacities at shorter access times or vice versa that may not fall strictly into the ordered sequence. In making the choice of memories for a given system the characteristics for each level should be considered. The starting point should be the set of system requirements. These would be examined to determine the different levels that may be accommodated to satisfy the requirements. A set of requirements would then be generated for each level. An initial choice might then be made for each level. Since the choice of one level could strongly influence another they should be examined together and appropriate changes made. It should be borne in mind that several levels of the hierarchy may exist within one medium, for example the complete tracks as compared to the short recirculating loops on a magnetic drum would represent different levels. References to specific types of memories in this discussion such as core or drum will usually be in the generic sense. For example, the family of core memories includes twistors, thin films and multi-aperture cores, as well as the conventional torroids. Also, the comments on drum memories will usually apply to the disc, as well as the cylindrical configuration.