Overlay (programming)

In a general computing sense, overlaying means "the process of transferring a block of program code or other data into main memory, replacing what is already stored".

[3] The following example shows the control statements that instruct the OS/360 Linkage Editor to link an overlay program containing a single region, indented to show structure (segment names are arbitrary): These statements define a tree consisting of the permanently resident segment, called the root, and two overlays A and B which will be loaded following the end of MOD2.

Most importantly, the architect can focus on the problem being solved without the added design difficulty of forcing the processing into steps constrained by the overlay size.

Thus, the designer can use higher-level programming languages that do not allow the programmer much control over size (e.g. Java, C++, Smalltalk).

IBM introduced more general overlay handling[7] in IBSYS/IBJOB, including a tree structure and automatic loading of links as part of CALL processing.

This file type was used among others by WordStar[11] (consisting of the main executable WS.COM and the overlay modules WSMSGS.OVR, WSOVLY1.OVR, MAILMERGE.OVR and SPELSTAR.OVR, where the "fat" overlay files were even binary identical in their ports for CP/M-86 and MS-DOS[12]), dBase,[13] and the Enable DOS office automation software package from Enable Software.