It is commonly claimed that 32-bit processors and operating systems are limited to 4 GB of RAM,[1][2] and that this is a primary cause of the "3 GB barrier." This is not a true limit of these processors. Almost all modern x86 processors (from the 1995 Pentium Pro onward) can in fact already address up to 64 GB RAM via physical address extension (PAE).[3] PAE is a modification of the protected mode address translation scheme. It allows virtual or linear addresses to be translated to 36-bit physical addresses, instead of the 32-bit addresses available without PAE.[4] The CPU pinouts likewise provide 36 bits of physical address to the motherboard. [4]
Many x86 operating systems, including any version of Linux with a PAE kernel and some versions of Windows Server, support the use of PAE to address up to 64 GB of RAM on an x86 system.[5][6][7]
Use of PAE to address RAM above the 4 GB point is key to breaking the "3 GB barrier." There are, however, factors that limit this ability, and lead to the "3 GB barrier" under certain circumstances, even though the processor fully supports PAE.
