(1) See data link layer.

(2) (Dynamic Link Library) An executable program module in Windows that performs one or more functions at runtime. DLLs are not launched by the user; they are called for by an executable program or by other DLLs. The Windows OS contains a huge number of DLLs. With only one instance of the DLL open in memory, its routines can be shared by all running applications (see reentrant code).

DLLs can perform a simple function such as drawing a 3D border around a dialog box, or they can be as large as a full-blown language system, such as one of the Visual Basic runtime modules (see VBRUNxxx.DLL).

Application DLLs
Many programs have a huge cadre of functions that are only used now and then. Breaking up an application into separate DLLs allows routines to remain on the hard disk until required, and application DLLs can also be made sharable. The Unix equivalent of a DLL is a "shared library."

Need a DLL?
If a DLL has become corrupted or is missing, visit www.afreedll.com. DLL downloads are, of course, free, and you may find the DLL you require. See DLL hell.