File extension DLL is used by Microsoft Windows to store Dynamic-Link Library. DLL files contain executable code in "Portable Executable" format. This code can be designed to run on either 16, 32 or 64 bit systems. 16-bit systems are long time obsolete. 64-bit systems are still uncommon while 32-bit systems are mainstream. Attempting to run code on wrong type of system will cause a failure. My websites: file extension dll