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In Windows, PAE or X64 is required to get all 4GB RAM, but that’s not enough.

Before writing this part, I made a quick Google search. Seems that 5 out of 10 would suggest enabling PAE, and the rest go for 64bit system to get around the 4GB barrier. So is PAE or x64 the key for Windows user to get full 4GB support?

What is PAE?

By definition, a 32-bit processor uses 32 bits to refer to the location of each byte of memory.  2³² = 4.2 billion, which means a memory address that’s 32 bits long can only refer to 4.2 billion unique locations (i.e. 4 GB).

Physical Address Extension (PAE) refers to a feature of x86 processors that allows for up to 64 Gigabytes of physical memory to be used in 32-bit systems, given appropriate operating system support. PAE is provided by Intel Pentium Pro and above CPUs (including all later Pentium-series processors except the 400 MHz bus versions on the Pentium M), as well as by some compatible processors such as those from AMD. In 2003, AMD introduced the NX bit as an additional feature of PAE mode in the AMD64 microarchitecture. Windows XP SP2 and later runs in PAE mode by default on systems with NX support in order to make use of the feature. 

 What is 64bit Windows?

64bit, or more exactly, x86-64 or x64, is a 64-bit superset of the x86 instruction set architecture, which is capable of addressing up to 2⁶⁴ bytes (16 exbibytes, or about 18×10¹⁸ bytes). However, most operating systems and applications will not need such a large address space for the foreseeable future. For example, the current Windows implementations for AMD64 are only populating 16 TB, or 44 bits worth.

Will PAE or x64 works for me?

Now the tricky part, the system has 4GB RAM installed, why PAE/x64 is required to utilize it? Since 32bit Windows(with or without PAE) should have no problem in  addressing them.

Let me put the answer here: Yes, PAE/x64 is necessary to get full 4GB support under Windows, but this is not the key point since the limitation is imposed by chipset(hardware) rather than Windows system(software).

As discussed in Part 1, the reserved space is taken by MMIO. So in order to get full 4GB support, rerouting the address space to physical RAM,  rather than MMIO device is required.