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A study of replacement algorithms for a virtual-storage computer

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One of the basic limitations of a digital computer is the size of its available memory.1 In most cases, it is neither feasible nor economical for a user to insist that every problem program fit into memory. The number of words of information in a program often exceeds the number of cells (i.e., word locations) in memory. The only way to solve this problem is to assign more than one program word to a cell. Since a cell can hold only one word at a time, extra words assigned to the cell must be held in external storage. Conventionally, overlay techniques are employed to exchange memory words and external-storage words whenever needed; this, of course, places an additional planning and coding burden on the programmer. For several reasons, it would be advantageous to rid the programmer of this function by providing him with a “virtual” memory larger than his program. An approach that permits him to use a sufficiently large address range can accomplish this objective, assuming that means are provided for automatic execution of the memory-overlay functions.

Note: The Institute of Electrical and Electronics Engineers, Incorporated is distributing this Article with permission of the International Business Machines Corporation (IBM) who is the exclusive owner. The recipient of this Article may not assign, sublicense, lease, rent or otherwise transfer, reproduce, prepare derivative works, publicly display or perform, or distribute the Article.  

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IBM Systems Journal  (Volume:5 ,  Issue: 2 )