Each version of Windows has it's own maximum amount of memory that can be addressed. Microsoft has an article explaining the memory limits of each Operating System here: msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx

For Windows 10, the memory limits are:

64 Bit:
Standard - 128GB
Professional - 512GB
Enterprise - 512GB

32 Bit - 4GB