Windows Memory (PAE/AWE/3GB)

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PAE (Physical Address Extension)

PAE is the added ability of the IA32 processor to address more than 4GB of physical memory. The following OS's can use PAE to take advantage of physical memory beyond 4GB:

  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Datacentre Server
  • Microsoft Windows Server 2003 Enterprise Edition
  • Microsoft Windows Server 2003 Datacentre Edition

To enable PAE, use the /PAE switch in your server's boot.ini file.

N.B. In Windows Server 2003, PAE is enabled automatically if your server is using hot-add memory devices.In all other cases, you must add the /PAE switch to boot.ini

Typically, a process running under Windows 2000 or Windows 2003 can address up to 2GB of memory address space (3GB if the /3GB switch is used in boot.ini) with some of the memory being physical memory and some being virtual. The more programs (and therefore, more processes) you run, the more memory you commit up to the full 2GB of address space. When this occurs, the paging process increases dramatically and performance may be negatively affected.

Windows 2000 and 2003 memory managers use PAE to provide more physical memory to a program. This reduces the need to swap the memory of the page file and results in increased performance. The program itself is not awareof the memory size, the memory management and allocation of the PAE memory is handled by the memory manager independently of the programs that run.

When the /3GB switch is used is conjunction with the /PAE switch, the OS does not use any memory in excess of 16GB. This is caused by kernel virtual memory space considerations. If the system restarts with the /3GB switch in boot.ini, and the system has more than 16GB available (physical memory), the additional RAM is not used by the OS. Restarting without the /3GB switch enables use of all the physical memory.

AWE (Address Windowing Extensions)

AWE is a set of API's to the memory manager's functionsthat enables programs to address more memory than is available through standard 32-bit addressing. It enables programs to reserve physical memory as non-paged memory to the program's working set of memory. Enables memory intensive programs, such as RDBMS, to reserve large amounts of physical memory for data without having to be paged in and out of a paging for for usage. Data is swapped in and out of of the working set and reserved memory is in excess of the 4GB range. Additionally, the range of memory in excess of 4GB is exposed to the memory manager and the AWE functions by PAE. Without PAE, AWE cannot reserve memory in excess of 4GB.

In Summary:

  • PAE is a function of the Windows 2000 and Windows Server 2003 memory manager that provides more physical memory to a program that requests memory
  • The program is not aware that any of the memory that it uses resides in the range greater than 4GB
  • AWE is an API that enables programs to reserve large chunks of memory
  • Reserved memory is non-pagable and is only accessible to that program
  • You can't allocate more than 4GB of memory to an application on Windows Server 2000 because PAE is not available. Also cannot use the /3Gb switch in the boot.ini file with Windows Server 2000
  • Need to use Windows 2000 Advanced Server or Windows 2000 Datacentre.
  • PAE (36-bit physical addressing mode) allows up to 8GB of physical memory on Windows 2000 Advanced Server and 32GB on Windows 2000 Datacentre
  • Maximum amount of memory supported on Windows Server 2003 is 4GB. Windows Server 2003 Enterprise Edition SP2 supports 64GB of physical RAM and Windows 2003 Datacentre SP2 supports 128GB. You can use the /3GB switch in boot.ini in all versions of Windows Server 2003

Memory Limits for Windows Releases
Operating Systems and PAE support