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Overlays are a programming technique for dealing with reduced availability of main memory - either because of a limited address space (as in the PDP-11, towards the end of its life), or because the machine just did not have much (very common in the early days of computers).

The program to be 'overlaid' was split up into tranches, called 'overlays', of which only one (or a limited number) were directly available to the CPU at a time. When the program needed to transfer (via either a jump, or a subroutine call) to an overlay that was not currently accessible, progress had to stop momentarily while that overlay was made accessible (perhaps by reading it in from secondary storage).

Overlays perform the same basic function as the later virtual memory ('jam three gallons into a one-gallon bucket'), but do so explicitly, not automatically and invisibly.

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