pinvoke.net: Edit: kernel32.GetSystemInfo

@msdn=http://msdn.microsoft.com/en-us/library/windows/desktop/ms724381(v=vs.85).aspx
@pinvoke=http://pinvoke.net/$$$.htm

Summary: Retrieves information about the current system

!!!!C# Signature:
[DllImport("kernel32.dll", SetLastError=false)]
public static extern void GetSystemInfo(out SYSTEM_INFO Info);

!!!!User-Defined Types:
        public enum ProcessorArchitecture
        {
            X86 = 0,
            X64 = 9,
            @Arm = -1,
            Itanium = 6,
            Unknown = 0xFFFF,
        }

[StructLayout(LayoutKind.Sequential)]
        public struct SystemInfo
        {
            public ProcessorArchitecture ProcessorArchitecture; // WORD
            public uint PageSize; // DWORD
            public IntPtr MinimumApplicationAddress; // (long)void*
            public IntPtr MaximumApplicationAddress; // (long)void*
            public IntPtr ActiveProcessorMask; // DWORD*
            public uint NumberOfProcessors; // DWORD (WTF)
            public uint ProcessorType; // DWORD
            public uint AllocationGranularity; // DWORD
            public ushort ProcessorLevel; // WORD
            public ushort ProcessorRevision; // WORD
        }

Alternative C# structure[s]: http://msdn.microsoft.com/en-us/library/windows/desktop/ms724958%28v=vs.85%29.aspx

[StructLayout( LayoutKind.Explicit )]
        public struct SYSTEM_INFO_UNION
        {
            [FieldOffset( 0 )]
            public UInt32 OemId;
            [FieldOffset( 0 )]
            public UInt16 ProcessorArchitecture;
            [FieldOffset( 2 )]
            public UInt16 Reserved;
        }

[StructLayout( LayoutKind.Sequential, Pack = 1 )]
        public struct SYSTEM_INFO
        {
            public SYSTEM_INFO_UNION CpuInfo;
            public UInt32 PageSize;
            public UInt32 MinimumApplicationAddress;
            public UInt32 MaximumApplicationAddress;
            public UInt32 ActiveProcessorMask;
            public UInt32 NumberOfProcessors;
            public UInt32 ProcessorType;
            public UInt32 AllocationGranularity;
            public UInt16 ProcessorLevel;
            public UInt16 ProcessorRevision;
        }

!!!!Alternative Managed API:
To determine whether the OS or Process is x64, using .NET 4, see System.Environment (Is64BitProcess, Is64BitOperatingSystem); http://msdn.microsoft.com/en-us/library/z8te35sa.aspx

Sample Code:
        void Test()
        {
        	NativeMethods.SYSTEM_INFO
        	 info;
        	NativeMethods.GetSystemInfo(out info);
        }
Documentation: GetSystemInfo@msdn on MSDN