PC/MS DOS
In the DOS era overlays were popular because the operating system and many of the computer systems it ran on lacked virtual memory and had very little RAM by current standards — the original PC had between 16K and 64K depending on configuration. "Several DOS linkers in the 1980s supported in a form nearly identical to that used 25 years earlier on mainframe computers." Binary files containing memory overlays had a defacto standard extension, .OVL. This file type was used among others by WordStar and dBase. The GFA BASIC compiler was able to produce .OVL files.
Read more about this topic: Overlay (programming)
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