Differences in Addressable Memory

Differences in Addressable Memory

The first thing most developers notice is that 64-bit processors provide a huge leap in the amount of physical and virtual memory that can be addressed.

• 32-bit applications on 32-bit platforms can address up to 2 GB.
• 32-bit applications built with the /LARGEADDRESSAWARE:YES linker flag on 32-bit Windows XP or Windows Server 2003 with the special /3gb boot option can address up to 3 GB. This constrains the kernel to only 1 GB which may cause some drivers and/or services to fail.

• 32-bit applications built with the /LARGEADDRESSAWARE:YES linker flag on the 32-bit editions of Windows Vista, Windows Server 2008, and Windows 7 can address memory up to the number specified by the boot configuration data (BCD) element IncreaseUserVa. IncreaseUserVa can have a value ranging from 2048, the default, to 3072 (which matches the amount of memory configured by the /3gb boot option on Windows XP). The remainder of 4 GB is allocated to the kernel and can result in failing driver and service configurations.

  For more information about BCD, see Boot Configuration Data on MSDN.

• 32-bit applications on 64-bit platforms can address up to 2 GB, or up to 4 GB with the /LARGEADDRESSAWARE:YES linker flag.
• 64-bit applications use 43 bits for addressing, which provides 8 TB of virtual address for applications and 8 TB reserved for the kernel.

Beyond just memory, 64-bit applications that use memory-mapped file I/O benefit greatly from the increased virtual address space. The 64-bit architecture also has improved floating-point performance and faster passing of parameters. Sixty-four-bit processors have double the number of registers, of both general purpose and streaming SIMD extensions (SSE) types, as well as support for SSE and SSE2 instruction sets; many 64-bit processors even support SSE3 instruction sets.

Specifying Large-Address-Aware When Building

It is a good practice to specify large-address-aware when building 32-bit applications, by using the linker flag /LARGEADDRESSAWARE, even if the application is not intended for a 64-bit platform, because of the advantages that are gained at no cost. As explained earlier, enabling this flag for a build allows a 32-bit program to access more memory with special boot options on a 32-bit OS or on a 64-bit OS. However, developers must be careful that pointer assumptions are not made, such as assuming that the high-bit is never set in a 32-bit pointer. In general, enabling the /LARGEADDRESSAWARE flag is a good practice.

Thirty-two-bit applications that are large-address-aware can determine at run time how much total virtual address space is available to them with the current OS configuration by callingGlobalMemoryStatusEx. The ullTotalVirtual result will range from 2147352576 bytes (2 GB) to 4294836224 bytes (4 GB). Values that are larger than 3221094400 (3 GB) can only be obtained on 64-bit editions of Windows. For example, if IncreaseUserVa has a value of 2560, the result is ullTotalVirtual with a value of 2684223488 bytes.