[1] The problem mainly affects 32-bit versions of operating systems like Microsoft Windows and Linux, although some variants of the latter can overcome this barrier.
In a computer with a 32-bit architecture, the memory address stored in one of the CPU registers will be limited to this number, thus the number of possible memory locations that can be addressed is limited to exactly 2 gibibytes, or roughly 2GB.
While Linux, FreeBSD, and most Unix-like operating systems support PAE so long as the hardware does,[4][5] Windows needs this boot option enabled manually because many device drivers are incompatible with PAE.
Once enabled, executables can have the "large address aware" flag set to increase their memory limit to 3 GB.
However, they can use the "large address aware" flag as well, except that it doesn't require the /3GB switch and increases the limit to 4 GB.