Virtual memory in Windows 32-bit operating systemsBefore you try to understand how the 4-GB random access memory (RAM) Tuning feature and the Physical Address Extension (PAE) parameter change memory allocation, it is helpful to review how virtual memory works in Windows 32-bit operating systems. For a detailed description about how virtual memory works in Windows 32-bit operating systems, visit the following Microsoft Web site:
The /3GB parameterOn 32-bit versions of Windows, the /3GB parameter enables 4-gigabyte (GB) random access memory (RAM) Tuning, a feature that enlarges the user-mode virtual address space to 3 GB and restricts the kernel-mode components to the remaining 1 GB.
For a detailed description of the /3GB parameter, visit the following Microsoft Web site:
The /PAE parameterThe /PAE parameter enables Physical Address Extension (PAE). This parameter directs the system to load the PAE version of the Windows kernel. PAE is an addressing strategy that uses a page-translation hierarchy to enable systems that have 32-bit addressing to address more than 4 GB of physical memory.
For a description of the /PAE parameter, subparameters, and operating system support, visit the following Microsoft Web sites:
Note The /3GB parameter could lead to resource starvation of Active Directory when you enable the parameter on an Exchange Server.
The /PAE parameter lets developers perform similar testing of device drivers by forwarding 64-bit addresses to kernel-mode components. This feature is known as Physical Address Extension (PAE), and it may not work on all chip sets. Any addresses that are over 32 bits are guaranteed to work by using the /nolowmem parameter from the Boot.ini file that discards the lower 4 GB of memory.
Important These configurations are not supported on Microsoft Windows 2000 Professional and on Windows 2000 Server. These configurations are only made available for testing. Do not use these parameters in a production environment if you are using Windows 2000 Professional or Windows 2000 Server.
In these products, the 4 GB RAM Tuning feature enables a 3 GB area of user-mode memory for programs to use. This feature can expand the virtual address range for user-mode memory from 0x0000000 through 0xBFFFFFF (the user-mode address range is typically from 0x0000000 through 0x7FFFFFFF). The range of memory that is available for kernel-mode components shrinks from 0x80000000-0xFFFFFFFF to 0xC0000000-0xFFFFFFFF. We do not recommend that you use this feature in Windows Server 2003, Standard Edition in a production environment.
To use the /3GB parameter from the Boot.ini file together with Windows 2000 Professional or with Windows 2000 Server can give the appearance of a 3 GB range of user-mode memory. However, the memory from 0x80000000 through 0xBFFFFFFF is not usable. Because kernel-mode components are now limited to the memory range 0xC0000000 through 0xFFFFFFF, developers can test kernel-mode components.
Article ID: 291988 - Last Review: May 21, 2008 - Revision: 1