See data link layer
ibrary) An executable program module in Windows that performs one or more functions at runtime. There are a huge number of DLLs that make up the Windows OS. A DLL is not launched by the user; it is called for by an executable program or by another DLL. Only one instance of the DLL is required in memory as its routines can be shared by all running applications (see re-entrant 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
DLLs Can Cause Bloat
In order to limit the total number of files in the Windows repository, a Windows DLL often contains many routines. Even if an application requires only one of them, the entire DLL must be called into memory. It may be difficult, too time consuming or downright impossible for application developers to write their own equivalent routines, and they therefore use the Windows DLLs that Microsoft makes available.
For Applications Too
Some applications have a huge cadre of functions, some of which are rarely used. Therefore, developers can break their application into several DLLs, and only those functions called for by the user need be brought into memory. Application DLLs can also be sharable.
The Unix equivalent of a DLL is a "shared library." Prior to Mac OS X, which is Unix based, the DLL equivalents in the Mac were called "extensions."
Need a DLL?
To replace a corrupted DLL or find a missing one, visit www.aFreeDLL.com. See DLL hell