&gt; Unfortunately, they couldn&#x27;t turn it on universally; there were a lot of programs that &quot;knew&quot; that malloc() would never return a pointer with the top bit set, so they used it as a flag for their own purposes. Such programs would break horribly in 3GB mode when pointers could have the top bit set.<p>Fascinating story. Never knew this. This dedication to backwards-compatibility is something Microsoft was known for. Reminds me of how they took great care when writing Windows to ensure perfect compatibility with old, horrible DOS programs, going so far as re-creating DOS bugs that these broken programs relied on. I suppose they were just accepting reality: if a user upgraded to Windows and some third party program broke, the user would blame Microsoft, not the incompetent application developer.