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.NET 3.5SP1 64-bit memory model versus 32-bit memory model

As I understand it, the .NET memory model on a 32-bit machine ensures that a 32-bit word writes and reads as atomic operations, but does not provide this guarantee for 64-bit words. I wrote a quick tool to demonstrate this effect on 32-bit Windows XP, and I get the results as described by this memory model.

However, I took the same executable file and ran it on the 64-bit Windows 7 Enterprise OS, and I get completely different results. Both machines are identical to specifications only with different OS installed. I would expect the .NET memory model to guarantee the writing and reading of BOTH of 32-bit and 64-bit words, which will be atomic in a 64-bit OS. I find results that completely contradict the BOTH assumptions. 32-bit reads and writes are not shown as atomic in this OS.

Can someone explain to me why this fails on a 64-bit OS?

Tool Code:

using System;
using System.Threading;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var th = new Thread(new ThreadStart(RunThread));
            var th2 = new Thread(new ThreadStart(RunThread));
            int lastRecordedInt = 0;
            long lastRecordedLong = 0L;
            th.Start();
            th2.Start();
            while (!done)
            {
                int newIntValue = intValue;
                long newLongValue = longValue;
                if (lastRecordedInt > newIntValue) Console.WriteLine("BING(int)! {0} > {1}, {2}", lastRecordedInt, newIntValue, (lastRecordedInt - newIntValue));
                if (lastRecordedLong > newLongValue) Console.WriteLine("BING(long)! {0} > {1}, {2}", lastRecordedLong, newLongValue, (lastRecordedLong - newLongValue));
                lastRecordedInt = newIntValue;
                lastRecordedLong = newLongValue;
            }
            th.Join();
            th2.Join();
            Console.WriteLine("{0} =? {2}, {1} =? {3}", intValue, longValue, Int32.MaxValue / 2, (long)Int32.MaxValue + (Int32.MaxValue / 2));
        }

        private static long longValue = Int32.MaxValue;
        private static int intValue;
        private static bool done = false;

        static void RunThread()
        {
            for (int i = 0; i < Int32.MaxValue / 4; ++i)
            {
                ++longValue;
                ++intValue;
            }
            done = true;
        }
    }
}

Results of a 32-bit version of Windows XP:

Windows XP 32-bit
Intel Core2 Duo P8700 @ 2.53GHz
BING(long)! 2161093208 > 2161092246, 962
BING(long)! 2162448397 > 2161273312, 1175085
BING(long)! 2270110050 > 2270109040, 1010
BING(long)! 2270115061 > 2270110059, 5002
BING(long)! 2558052223 > 2557528157, 524066
BING(long)! 2571660540 > 2571659563, 977
BING(long)! 2646433569 > 2646432557, 1012
BING(long)! 2660841714 > 2660840732, 982
BING(long)! 2661795522 > 2660841715, 953807
BING(long)! 2712855281 > 2712854239, 1042
BING(long)! 2737627472 > 2735210929, 2416543
1025780885 =? 1073741823, 3168207035 =? 3221225470

Note that BING (int) is never written and demonstrates that 32-bit reads / writes are atomic in this 32-bit OS.

Results on a 64-bit version of Windows 7 Enterprise:

Windows 7 Enterprise 64-bit
Intel Core2 Duo P8700 @ 2.53GHz
BING(long)! 2208482159 > 2208121217, 360942
BING(int)! 280292777 > 279704627, 588150
BING(int)! 308158865 > 308131694, 27171
BING(long)! 2549116628 > 2548884894, 231734
BING(int)! 534815527 > 534708027, 107500
BING(int)! 545113548 > 544270063, 843485
BING(long)! 2710030799 > 2709941968, 88831
BING(int)! 668662394 > 667539649, 1122745
1006355562 =? 1073741823, 3154727581 =? 3221225470

, BING (long) AND BING (int)! 32- , 64-?

+5
1

, :

++longValue;
++intValue;

, . Interlocked.Increment, .

+4

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