Dynamic-Linked Libraries (DLLs)
In the Windows and protected mode environments, dynamic-link libraries (DLLs) permit several applications to share code and resources.
A DLL is an executable module (extension .DLL) that contains code or resources that are used by other DLLs or applications.
DLLs provide the ability for multiple programs ("clients") to share a single copy of a routine they have in common. The .DLL file must be present when the client program runs.
The Borland Pascal concept most comparable to a DLL is a. However,
routines in units are linked into your executable at link time ("statically
linked"), whereas DLL routines reside in a seperate file and are made available at run time ("dynamically linked").
When the program is loaded into memory, the Windows or DOS protected mode program loader dynamically links the procedure and function calls in the program to their entry points in the DLLs used by the program.
A Borland Pascal application can use DLLs that were not written in Borland Pascal. Also, programs written in other languages can use DLLs written in Borland Pascal.
DLLs that are compiled for Windows can also be used in DOS protected mode if the DLLs use only the Windows functions defined in WinAPI.unit. This subset of the Windows API is emulated by the DOS protected mode Run-Time Manager, allowing one DLL file to run in Windows or in DOS without compiling.