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An excerpt of Wikipedia’s article on Physical Address Extension:
x86 processor hardware-architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This, theoretically, increases maximum physical memory size from 4 GB to 64 GB.
Along with an image explaining the mechanism:
But I can’t see how the address space is expanded from 4GB to 64GB. And 4 * 512 * 512 * 4K still equals 4GB, isn’t it?
x86 processors running in 32-bit mode uses page translations for memory addresses. This means that there is a mapping layer between the address used by the code (both kernel and user mode) and the actual physical memory. E.g. in Windows all processes map the image of the .exe file to the same address.
The mapping layer between the virtual and physical addresses can normally only map 4GB of memory. With PAE enabled, the 32 bit virtual addresses are mapped 36 bit physical addresses. Still, a single process cannot access more than 4GB at a single time. That’s what you see in the image you’ve pasted, the 32-bit address space of one process. You can also see that the PTE (Page Table Entry) containing the physical address is 64 bit wide.
A PAE aware application can swap in and out different parts of memory into the visible address space to make use of more than 4GB of RAM, but it can only see 4GB at any single point in time.