Storage allocation has always been an important consideration in computer programming due to the high cost of main memory and the relative abundance and lower cost of secondary storage. Program code and data required for execution of a process must reside in main memory to be executed, but main memory may not be large enough to accomodate the needs of an entire process. Early computer programmers divided programs into sections that were transferred into main memory for a period of processing time. As the program proceeded, new sections moved into main memory and replaced sections that were not needed at that time. In this early era of computing, the programmer was responsible for devising this overlay system.
As higher level languages became popular for writing more complex programs and the programmer became less familiar with the machine, the efficiency of complex programs suffered from poor overlay systems. The problem of storage allocation became more complex.
Two theories for solving the problem of ineffiecient memory management emerged -- static and dynamic allocation. Static allocation assumes that the availability of memory resources and the memory reference string of a program can be predicted. Dynamic allocation relies on memory usage increasing and decreasing with actual program needs, not on predicting memory needs.
Program objectives and machine advancements in the '60s made the predictions required for static allocation difficult, if not impossible. Therefore, the dynamic allocation solution was generally accepted, but opinions about implementation were still divided. One group believed the programmer should continue to be responsible for storage allocation, which would be accomplished by system calls to allocate or deallocate memory. The second group supported automatic storage allocation performed by the operating system, because of increasing complexity of storage allocation and emerging importance of multiprogramming. In 1961, two groups proposed a one-level memory store. One proposal called for a very large main memory to alleviate any need for storage allocation. This solution was not possible due to very high cost. The second proposal is known as virtual memory . [D1]