Choosing and Configuring Hardware - Professional SQL Server 2005 Performance Tuning - page 150

0xFFFFFFFF

1 GB
Kernel Mode

0xC00000000

2 GB
Kernel Mode

0xBFFFFFFF

0x80000000
0x7FFFFFFF

3 GB
User Mode

2 GB
User Mode

0x00000000

Without /3 GB With /3 GB

Figure 6-3

PAE just enables Windows to see memory above 4 GB. For applications to use it, they have to be written
specifically to take advantage of it.

Best Practice
Enable PAE if you have more than 4 GB of RAM.

AWE (Address Windowing Extensions)
AWE is a Windows API that allows a 32-bit process to map memory from outside its Virtual Address
Space. It enables a process to use physical memory to store data, but to use the data it has to be mapped
into the process's VAS. Think of it like the page file in windows, only because it's a memory-to-memory
transfer, it's much quicker. It allows a process to use more than 2 GB of memory (or 3 GB if /3GB is on)
but you're still limited by 4 GB that 32-bit can address. For this reason it's most commonly used in com-
bination with PAE on servers with more than 4 GB of memory.

SQL Server has been written to use the Windows AWE API. You enable it by setting the Advanced
Configuration option AWE to 1 using sp_configure . It also requires the Windows Advanced User Right