C# Signature:

[DllImport("kernel32.dll", SetLastError=false)]

public static extern void GetSystemInfo(out SYSTEM_INFO Info);

);

public static SystemInfo GetSystemInfo()
{
     var stub = new SystemInfo();
     GetSystemInfo(out stub);
     return stub;
}

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
    }

better c# structure[s]:

[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