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CRTDLL.DLL is also known as the C Runtime Library. Prior to Windows 10, it has not been considered a core system component. However, some version of MSVCRT.DLL or CRTDLL.DLL has been installed by Windows for about as long as Windows has existed. It is needed by software that has been built using certain Microsoft compilers. Windows 10 brings the concept of the Universal CRT.
MSVCRT.DLL, MSVCP*.DLL and CRTDLL.DLL (Wikipedia)
How to Use the C Runtime Library (Microsoft)
CRT Library Features (Microsoft)
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