32-bit OS supports 4 GB memory or above

1) Why can 32-bit systems use more than 4 GB of memory?
As long as the CPU, motherboard, and operating system support and enable PAE (physical address extension), the physical memory address bus is 36 bits.
2 ^ 36= 68719476736 = 64 GB
In addition,Current x64Instead of using a 64-bit address bus48 digitsSo the maximum memory size is 2 ^ 48.
So, the current32-bit systems are not pure "32-bit"; x64 is not pure "64-bit ".
2) Why does Windows 2003 32-bit support up to GB of memory?
20090417 amendment by fujianabc:
32-bit CPU since P4 and K8, the physical address is 40 bits, so 128 GB is supported.
For details, see this post.
3) Why does the motherboard support 4 GB of memory and 4 GB of memory have been installed, but 32-bit Windows only support 3.1 GB?
There are two reasons:
3. a. Restricted by Windows licenses.
The Windows Kernel checks the license of the current operating system at system startup to determine the maximum number of physical memory that is currently allowed.
For example, the Windows XP/Vista license is limited to 4 GB.
3. Limits on the resources of B, motherboard, and various PCI devices above.
The motherboard must support more than 4 GB of memory and must support the memory Replay Function.