Overlay (programming): Difference between revisions

Revision as of 13:00, 20 May 2022

Revision as of 09:32, 6 June 2022

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==Usage==

Constructing an overlay program involves manually dividing a program into self-contained [[object code]] blocks called '''overlays''' or '''links''', generally laid out in a [[tree structure]].{{efn|In [[OS/360 and successors]], there may be multiple ''regions''{{efn|This has nothing to do with the term ''region'' in [[OS/360 and successors#MVT|MVT]] storage management.}} each containing a complete overlay tree.}} ''Sibling'' segments, those at the same depth level, share the same memory, called ''overlay region''{{efn|The nomenclature varies depending on the system, e.g., in [[OS/360]] ''region'' refers to an entire overlay tree.}} or ''destination region''. An overlay manager, either part of the [[operating system]] or part of the overlay program, loads the required overlay from [[Auxiliary memory|external memory]] into its destination region when it is needed; this may be automatic or via explicit code. Often [[Linker (computing)|linker]]s provide support for overlays.<ref name="GNULink"/>

==Example==

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==Historical use==

IBM introduced the concept of a ''chain job'' <ref>{{cite manual

| title = IBM 7090/7094 Programming Systems -– FORTRAN II Programming

| id = C28-6054-4

| date = August 1963

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IBM introduced more general overlay handling<ref>{{cite manual

| title = IBM 7090/7094 Programming Systems -– IBJOB Processor -– Overlay feature of IBLDR

| id = C28-6331

| year = 1963