Dynamic-link library
Microsoft's implementation of the shared library concept in Windows and OS/2
Dynamic-link library (DLL) is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. These libraries usually have the file extension DLL, OCX (for libraries containing ActiveX controls), or DRV (for legacy system drivers).
The file formats for DLLs are the same as for Windows EXE files – that is, Portable Executable (PE) for 32-bit and 64-bit Windows, and New Executable (NE) for 16-bit Windows. As with EXEs, DLLs can contain code, data, and resources, in any combination.
Data files with the same file format as a DLL, but with different file extensions and possibly containing only resource sections, can be called resource DLLs. Examples of such DLLs include icon libraries, sometimes having the extension ICL, and font files, having the extensions FON and FOT.
Background
The first versions of Microsoft Windows ran programs together in a single address space. Every program was meant to co-operate by yielding the CPU to other programs so that the graphical user interface (GUI) could multitask and be maximally responsive. All operating-system level operations were provided by the underlying operating system: MS-DOS. All higher-level services were provided by Windows Libraries "Dynamic Link Library". The Drawing API, Graphics Device Interface (GDI), was implemented in a DLL called GDI.EXE, the user interface in USER.EXE. These extra layers on top of DOS had to be shared across all running Windows programs, not just to enable Windows to work in a machine with less than a megabyte of RAM, but to enable the programs to co-operate with each other. The code in GDI needed to translate drawing commands to operations on specific devices.
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This entry incorporates text from “Dynamic-link library” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.