3.1 Virtual Address Space on 32-bit Hardware
On 32-bit machines, the lower 2 GB of the virtual address space is user space and the upper 2 GB is reserved as system space by default. If user-mode applications require additional virtual address space, an administrator can reserve 3 GB for user space, thus leaving only 1 GB of system space, by applying the /3GB switch to Boot.ini and then restarting the machine. This switch is also useful during testing to see how a driver performs with limited system address space. The switch is supported on Windows Server 2003 (all editions), Windows XP (all versions), Windows 2000 Advanced Server, and Windows 2000 Datacenter Server.Figure 1 shows how the virtual address space is organized on 32-bit systems. (The figure is not drawn to scale.)
Figure 1. Layout of virtual address space on 32-bit systems
As the figure shows, the user address space occupies the lower range of the virtual address space and the system address space occupies the upper range. By default, both user and system space are 2 GB. When Windows is started with the /3GB switch, the size of the user space increases to 3 GB. The remaining 1 GB of virtual address space contains the system address space. The process page tables and hyperspace remain at the same addresses, but other components do not. When the system address space is limited to 1 GB, the maximum sizes of the paged and nonpaged pools are reduced and less space is available for system page table entries (PTEs) and the file system cache as well.
The file system cache, which is implemented in software, is not the same as the processor cache, which is implemented in hardware. Keep in mind, however, that this limit applies only to the amount of the cache that is visible at any given time. The system uses all the physical memory in the machine for caching. The exact sizes of the paged and nonpaged pools, PTEs, and file system cache vary widely from release to release.
On the Intel Pentium Pro and later processors, which support PAE, Windows can support up to 128 GB of physical memory. PAE provides only additional physical memory and does not increase the size of the virtual address space. When PAE is enabled, the virtual address space remains unchanged at 4 GB and the layout of the 2 GB system space is largely unchanged.
Operating system support for PAE is provided by a special version of the Windows kernel, which is named Ntkrnlpa.exe (single-processor version) or Ntkrpamp.exe (multi-processor version). In this version of the kernel, PTEs and page directory entries (which are involved in mapping virtual memory to physical memory) are 64 bits wide. If the appropriate device and driver support is present, the PAE kernels support direct I/O for the entire physical address space (including any physical memory above 4 GB).
