[This is preliminary documentation and subject to change.]
Certain data-intensive applications such as database management systems, scientific and engineering software need access to very large caches of data. In the case of very large data sets, restricting the cache to fit within an application's 2 GB of user address space is a severe restriction. In these situations, the cache is too small to properly support the application.
VLM allows applications to directly address memory using 64-bit pointers. Applications that use these extensions can have data caches larger than 2 GB. The larger caches result in better cache-hit rates and increased performance.
Windows NT has until now provided applications with a flat 32-bit virtual address space. The address space is typically split so that 2 GB of virtual address space is directly accessible to applications. The other 2 GB of virtual address space is accessible only to the Windows NT executive software. The Digital Equipment Corporation Alpha processor has a 64-bit flat address range. Under Windows NT on the Alpha, using sign extension, the application appears in the bottom 2 GB of memory, and the operating system in the top 2 GB. VLM virtual space appears in the range in between. Applications have access to 28 GB of VLM virtual space.