PAE, 4GT and AWE

PAE: Physical Address Extension
4GT: 4-gigabyte tuning
AWE: Address Windowing Extensions

• PAE allows the operating system to access and use more than 4 GB of physical memory. PAE also enables several advanced system and processor features, such as hardware-enabled Data Execution Prevention (DEP), Non-Uniform Memory Architecture (NUMA) and the ability to add memory to a system while it is running (hot-add memory), so it can also be used on computers that have less than 4 GB of memory. PAE is supported only on 32-bit versions of Windows; 64-bit versions of Windows do not support PAE.
• 4GT extends the 32-bit user virtual address space from 2 GB to up to 3 GB.
• AWE is a set of APIs that allows a process to allocate non-paged physical memory and then dynamically map portions of this memory into the virtual address space of the process.
• When neither 4GT nor AWE are being used, the amount of physical memory that a single 32-bit process can use is limited by the size of its address space (2 GB). In this case, a PAE-enabled system can still make use of more than 4 GB of RAM to run multiple processes at the same time or to cache file data in memory.
• 4GT can be used with or without PAE. However, some versions of Windows limit the maximum amount of physical memory that can be supported when 4GT is used. On such systems, booting with 4GT enabled causes the operating system to ignore any memory in excess of the limit. For details, see Memory Limits for Windows Releases.
• AWE does not require PAE or 4GT but is often used together with PAE to allocate more than 4 GB of physical memory from a single 32-bit process.