Performance
The single biggest performance benefit from the move to 64-bit architecture is the increase in both virtual memory address space and the amount of physical memory that is supported. As detailed in the Windows Server 2003 x64 Editions Deployment Scenarios white paper, the increase in available memory address space and physical memory can make a dramatic improvement in the performance of applications. However, memory address space is only one factor affecting the overall performance of an application, and in many cases is not the limiting factor. Even applications that are not memory-constrained can gain a significant benefit from other efficiencies in the x64 architecture.
A major area of improved efficiency in the x64 architecture is the increased number of registers available. All 32-bit x86 processors are limited to eight 32-bit general-purpose registers, eight floating-point registers, and eight SSE/SSE2 registers. SSE is short for Streaming Single Instruction-Stream, Multiple Data-Stream (SIMD) Extensions, and SSE2 is short for Streaming SIMD Extensions 2.
The x64 architecture uses twice as many general-purpose registers, each a full 64-bits wide, and doubles the number of 128-bit wide SSE/SSE2 registers to 16.
Another performance improvement with the x64 architecture is the gain 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, such as PCI Express and PCI-X 266, to improve 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, especially those that needed to use the /3GB switch. When using the /3GB switch, Windows is forced into a constrained address space, limiting the amount of non-paged pool available. This can cause non-paged pool to be exhausted when there are several I/O requests outstanding.
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