I "believe" that overlays were an old way of doing movie special effects much like animation, where you draw what you want on a clear sheet, then place it overtop of the image. would seem like a good way to do lasers and other lit effects on movies

In computers, an overlay is a primitive (and outdated) method of doing the equivalent of demand paging in user space. Overlays were used heavily for larger programs in msdos due to the 640k memory limitations.

Since the overlay is implemented in user space or x86 real mode, its implementation can't make use of the page fault technique, and instead must break the program into sections which are hopefully self-contained. Each overlay would be loaded into memory, possibly over laying a previous overlay.

Also, since the overlay can't make use of virtual addressing in real mode, overlays either must be implemented either as relocatable code or must be loaded at the same address every time. If it must be loaded at a fixed address, then there will be a fixed set of other overlays it will always need to replace.

However, if the overlay uses relocatable code, or is patched during load, then it can be loaded anywhere. This would allow it to overlay the least-recently used other overlay, or allow the same program to perform better in (for example) 640k, but still work in 384k, just with fewer overlays loaded at once.

Turbo C supported writing programs with overlays. Nethack for Windows 3.1 could be compiled either with some features missing, or with overlays. Wordstar used overlays heavily (even very early versions). That's why your disk would start running the first time you hit some keys -- wordstar was loading the overlay to handle that function.

Overlays were not limited to msdos. For example, UCSD Pascal used overlays to emulate a 128k virtual machine on hardware with only 64k, on hardware that did not have a MMU.

This was brought to you by the Save Our Archaic Technical Terms Society.

In costumes:

A garment, usually lace or a similar material, that is designed to lay on top of another garment. Used to help decorate and bring attention to a particular area. Often a apron or other similar piece of clothing.
Gillette, J. Michael. Theatrical Design and Production. 4th ed. Mountain View: Mayfield, 1999

O`ver*lay" (?), v. t. [imp. & p. p. Overlaid (?); p. pr. & vb. n. Overlaying.]


To lay, or spread, something over or across; hence, to cover; to overwhelm; to press excessively upon.

When any country is overlaid by the multitude which live upon it. Sir W. Raleigh.

As when a cloud his beams doth overlay. Spenser.

Framed of cedar overlaid with gold. Milton.

And overlay With this portentous bridge the dark abyss. Milton.


To smother with a close covering, or by lying upon.

This woman's child died in the night; because she overlaid it. 1 Kings iii. 19.

A heap of ashes that o'erlays your fire. Dryden.

3. Printing

To put an overlay on.


© Webster 1913.

O"ver*lay` (?), n.


A covering.

Sir W. Scott.

2. Printing

A piece of paper pasted upon the tympan sheet to improve the impression by making it stronger at a particular place.


© Webster 1913.

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