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{{Use dmy dates|date=July 2019|cs1-dates=y}}
In a general computing sense, '''overlaying''' means "the process of transferring a [[Block (data storage)|block]] of program code or other data into [[main memory]], replacing what is already stored".<ref name="OED"/> Overlaying is a [[computer programming|programming]] method that allows programs to be larger than the computer's [[main memory]].<ref name="Commodore"/> An [[embedded system]] would normally use overlays because of the limitation of [[physical memory]], which is [[internal memory]] for a [[system-on-chip]], and the lack of [[virtual memory]] facilities.
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| section = Chapter 12 The Chain Job
| section-url = http://bitsavers.org/pdf/ibm/7090/C28-6054-4_7090_FORTRANII.pdf#page=34
| pages =
| url = http://bitsavers.org/pdf/ibm/7090/C28-6054-4_7090_FORTRANII.pdf
| work = Systems Reference Library
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In the [[home computer]] 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 [[IBM PC]] had between 16K and 64K, depending on configuration. Overlays were a popular technique in [[Commodore BASIC]] to load graphics screens. In order to detect when an overlay was already loaded, a [[flag (computing)|flag variable]] could be used.<ref name="Commodore"/>
"Several DOS linkers in the 1980s supported [overlays] in a form nearly identical to that used 25 years earlier on mainframe computers."<ref name="Levine"/> [[Binary file]]s containing memory overlays had a de facto standard extension, '''.OVL'''. This file type was used among others by [[WordStar]], [[dBase]], and the ''Enable'' DOS office automation software package from [[Enable Software, Inc.]]
== See also ==
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