To be more precise, they changed the 4GB physical memory limit to a physical address space limit. MS never intended even XP SP1 to officially support PAE, but t

To be more precise, they changed the 4GB physical memory limit to a physical address space limit. MS never intended even XP SP1 to officially support PAE, but the code happened to treat the 4GB limit as a physical memory limit. When XP SP2 added NX support which required PAE, driver compatibility forced MS to add the physical address space limit.