The 32 bit Windows platform has been in existence for nearly 15 years.  While 32 bit Windows is still a very good operating system, 64 bit Windows is a robust platform that offers premier performance that is unparalleled in the mainstream 32-bit environment. The 64-bit systems offer direct access to more virtual and physical memory than 32-bit systems and process more data per clock cycle, enabling more scalable, higher performing computing solutions.  Sixty-four-bit native applications can deliver more data more quickly, so memory-intensive applications can run more quickly and efficiently. Data in memory is accessed thousands of times faster than it is on a disk drive. Applications can preload substantially more data into virtual memory, allowing rapid access by the 64-bit processor.  Features such as Data Execution Prevention (DEP) and PatchGuard improve security by helping to prevent malicious code from executing on a server.

The non-paged pool on a 32 bit node is 256MB and 128GB on 64 bit nodes - 500 times larger.  The x64 architecture enjoys a substantial advantage in overall I/O efficiency and throughput. With support for greater physical memory and memory address space, caches can be substantially larger than in 32-bit Windows, enabling the Windows x64 Editions to fully utilize the improved I/O hardware available and increase overall I/O performance. The larger address space allows more I/O to be in progress simultaneously. Even 32-bit applications can benefit from this improvement.  This is especially beneficial to users that require a lot of SSL sessions, such as online traders, because there is more memory available quickly to open and maintain SSL sessions.

Based on the same code base as Windows Server 32-bit editions, the Windows Server x64 editions offer the time-tested reliability of the Windows Server platform while providing a platform for both your new, high-performance native x64 applications and the legacy 32-bit applications that are still critical today.

All 32-bit applications run in the WOW64 subsystem, with access to 4 GB of virtual memory address space. WOW64 provides a high-performance, highly compatible, and isolated environment for 32-bit applications.   The WOW64 subsystem provides a high-performance 32-bit Windows environment that enables x64 Windows to take full advantage of the more than 10,000 existing 32-bit Windows applications available today. Because of the underlying hardware compatibility of the x64 architecture, 32-bit applications are able to run at full speed in the WOW64 subsystem. Many applications will actually run faster in WOW64 than they would run in 32-bit Windows because of the larger available memory address space and the greater efficiencies of the x64 processor architecture.

Moving to a 64-bit architecture changes the amount of virtual memory that can be directly addressed from 4 GB to 16 terabytes (TB). This 16 TB of memory is divided evenly between user mode processes and kernel mode processes, the same way memory is handled in 32-bit versions of Windows. Native 64-bit applications have 8 TB of available virtual memory address space.

The 32-bit versions of Windows use a flat, 32-bit virtual address space, limiting the amount of virtual memory that can be addressed directly to 4 GB (232). By default, this 4 GB is divided into two equal buckets: 2 GB that can be addressed by a process, and 2 GB that is addressable by the operating system and shared across all processes.

Windows x64 Editions protect against sources of instability due to vulnerabilities that start in the application layer. During the development of Windows x64 Editions, Microsoft carefully analyzed crash dump data and found that many system crashes are caused by improperly written code that hooks or patches into system components. Windows x64 Editions include a technology called Patch Guard, which protects against rogue code that could destabilize a system through patching into system components.

An important new feature in the x64 processor architecture is the Data Execution Protection (DEP) bit that controls which areas of memory can be used to execute code. While AMD and Intel have different names and slightly different implementations for this feature, the result is an enhanced layer of hardware protection against some of the most destructive worms and exploits of the past several years.

A buffer overflow occurs when a data buffer is stuffed with more data than it is designed to handle. (I use buffer overflow as a generic term for exploits that load executable code into areas that are supposed to only contain data, then jump program execution into that code by overloading heaps, stacks, and other memory pools.) For example, if your e-mail client is designed to handle attachments with a maximum of 255-character filenames and you receive a message that has a filename with 256 characters, a buffer overflow can occur. When this happens, adjacent memory space gets overwritten and malicious code can end up being executed with the privileges associated with the original program. The infamous MSBlaster worm was this type of exploit.

Beginning with Windows XP Service Pack 2 (SP2) and continuing with Windows Server 2003 SP1 and Windows XP Professional x64 Edition, Windows uses DEP to prevent malicious code from being able to execute, even when a buffer overrun occurs.

The x64 versions of Windows also support Microsoft’s Patch Guard technology that prevents non-Microsoft originated programs from patching the Windows kernel. This technology, available only on Windows x64 Editions, prevents kernel mode drivers from extending or replacing kernel services including system service dispatch tables, the interrupt descriptor table (IDT), and the global descriptor table (GDT). Third-party software is also prevented from allocating kernel stacks or patching any part of the kernel.

The x64 Editions of Windows are essentially feature comparable with the 32-bit versions. They are able to execute both native 64-bit programs and 32-bit programs efficiently and with high performance, bringing an unprecedented level of compatibility to 64-bit computing.