Physical Address Extension

PAE, Physical Address Extension, is a processor feature, first introduced with Intel 32-bit processor Pentium Pro. It extends physical addressing from 4GB to 64GB space. AMD later employed it as the fundamental component in AMD64, and extended to 1TB and 256TB. PAE does not change the IA-32 architecture, 2 or 3GB virtual user space could be utilized by each process without changed. But Processor's paging unit has been extended to page each 32-bit process to any region bound by the actual physical memory, not only 4GB space any more. Technologies such as AWE found in Windows provide an indirect way enabling each user process to utilize even more physical memory, through a fixed size region reserved in virtual space. Because PAE is a necessary component of AMD64 architecture, so each 64-bit process does not have a 4GB space limitation, and does not need technologies like AWE to address more physical memory, but generally, user space in each 64-bit process are 2GB by default. NX is a feature relating with PAE, so Operating System with support of NX must first implement PAE feature on its system level, but does not need to support space above 4GB, take 32-bit client versions of Windows for example. So the remapped space above 4GB would be dropped on this kind of Operating System, and famous 3 GB Barrier problem would happen. Most other desktop Operating Systems have already deployed PAE-enabled kernel for their IA-32 images, such as Ubuntu and OpenSuse. top and free are the mostly used commands to check whether PAE has enabled or not.