Discussion
Overlays were used in early systems to
Answer: A
Correct answer: run a program larger than available memory by loading parts of it in turn
Overlays allowed a program larger than physical memory to run by keeping only the instructions and data needed at a given moment in memory, and loading a different portion over the same area when a new phase began. A two-pass assembler is the classic example: pass one and pass two are never needed simultaneously, so both occupy the same overlay region in turn, with a small permanently resident driver managing the switching. The critical burden was that overlays had to be designed by the programmer. The program had to be divided by hand into parts that were never needed together, and the loading arranged explicitly. That required detailed knowledge of the program’s structure and was error-prone. Virtual memory made the technique obsolete by achieving the same end automatically: the operating system pages in what is referenced and pages out what is not, with no cooperation from the programmer at all. Overlays survive only in small embedded systems without memory management hardware.
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