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The Common Language Runtime (CLR) is the virtual machine component of Microsoft's .NET initiative. It is Microsoft's implementation of the Common Language Infrastructure (CLI) standard, which defines an execution environment for program code. The CLR runs a form of bytecode called the Microsoft Intermediate Language (MSIL), Microsoft's implementation of the Common Intermediate Language.

Developers using the CLR write code in a high level language such as C# or VB.Net. At compile-time, a .NET compiler converts such code into MSIL (Microsoft Intermediate Language) code. At runtime, the CLR's just-in-time compiler (JIT compiler) converts the MSIL code into code native to the operating system. Alternatively, the MSIL code can be compiled to native code in a separate step prior to runtime. This speeds up all later runs of the software as the MSIL-to-native compilation is no longer necessary.

     Source code                 Bytecode               Native code


                    C# compiler  ___________
     C# ----------------------->|           |
                                |           |
                                |           |
                VB.NET compiler |           |   CLR
     VB.NET ------------------->| MSIL Code |---------> native code
                                |           |
                                |           |
                 other compiler |           |
     Other .NET --------------->|           |
     language                   |___________|


    \________________________________/ \____________________________/
                    |                                |
              compile-time                        runtime

Although some other implementations of the Common Language Infrastructure run on non-Windows operating systems, the CLR runs on Microsoft Windows operating systems.

The virtual machine aspect of the CLR allows programmers to ignore many details of the specific CPU that will execute the program. The CLR also provides other important services, including the following:

• Memory management
• Thread management
• Exception handling
• Garbage collection
• Security

• .NET Framework
• Common Language Infrastructure
• Common Intermediate Language
• Parrot virtual machine
• Java virtual machine
• Portable.NET
• Mono
• Metadata
• Runtime
• Common Type System
• Virtual machine
• C++/CLI

• Overview (Microsoft MSDN)

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