PAE and AWE

The amount of RAM that can be accessed by Windows depends on OS version and underlining hardware.
For IA32 hardware, Windows can access beyond 4G RAM using Intel PAE (Physical Address Extension available since Pentium Pro). PAE is an extension to the system level bookeeping that allows a machine (via paging mechanism) to increase the number of address lines from 32 to 36. PAE is enabled in Windows via the /PAE boot option.

AWE (Address Windowing Extension) is a Microsoft specific feature that allows an application to access RAM beyond the 4G linear address space limit. AWE is an API (declard in winbase.h). AWE uses a set of fixed size regions (windows) in an application linear address space and maps them to a larger set of fixed size windows in physical memory.
AWE can operate with or without PAE. Application needs "Lock Page in Memory" privilege to use AWE.
VirtualAlloc() or VirtualAllocEx () - reserve a region in linear address space

AllocateUserPhysicalPages() - allocate pages of physical memory to be mapped to linear memory

MapUserPhysicalPages() or MapUserPhysicalPagesScatter() - map allocated pages of phsyical memory to linear memory

FreeUserPhysicalPages() - release the allocated pages