Explainations of the Windows 4GB Limit, PAE, AWE and Large Page Support

posted 13 Jan 2012, 02:05 by Tristan Self
Microsoft has produced some good documents about each of these topics, see these links for details:
Windows 4GB Limit
On a 4GB 32-bit machine running Windows 2003 standard the OS (kernel) takes 2GB leaving 2GB for the applications. By use of the /3GB switch, it reduces the OS (kernel) memory to 1GB leaving 3GB for the applications.
PAE - Physical Address Extensions
Basically allows a 32bit Windows 2003 (Enterprise or Datacenter) to address more than 4GB of RAM, however each process is limited to 4GB each. So if you had a 32-bit server with 64GB of RAM and Windows 2003 Enterprise you could use more than 4GB of RAM, but your applications would be still be limited to the 4GB virtual address space each.
AWE - Address Windowing Extensions
If an application supports it this allows an application to use more than the 4GB 32-bit virtual address space limit. For example a Windows 2003 Enterprise server on a 32-bit server with 64GB of RAM, has the PAE enabled, then runs say SQL 2005 Enterprise server. Normally a SQL 2005 instance would be limited to use only 4GB of RAM, but with AWE turned on it can use the remaining 60GB (total of 64GB) of RAM by the AWE mapping these address spaces onto 4GB blocks. I think.
Large Page Support