Why does 32-bit Windows impose a 2 GB process address space limit, but 64-bit Windows impose a 4GB limit?

Question

I get that 2^32 = 4,294,967,296 and 2GB is for the OS and 2GB is for the process, but why does a 64 bit OS only give 4GB of address space? Should it not be 2^64 = 18,446,744,073,709,551,615 which when divided evenly amongst the OS and process is a lot more than 4 GB?

Answer 1

If you look at Memory Limits for Windows Releases

The limit for x64 user mode virtual address space is not 4GB but at least 8TB

Some explanation that justifies the 8Tb limit.

By default a 32bit exe is limited to 2GB on 64bit os as well unless it is tweaked with IMAGE_FILE_LARGE_ADDRESS_AWARE

Answer 2

2^32 (4GB) is the maximum amount of address space that can be addressed at one time with 32-bit pointers. So a 32-bit process is necessarily limited to an address space of 4GB, because it uses 32-bit pointers, regardless of OS.

Note that this is purely a restriction on virtual memory that a process can have mapped into its address space at one time.

Answer 3

Your calculations are correct. Running a 64 bit system you don't need to worry about the address space (its much more than your system will support). If the RAM size isn't what it is supposed to be, the problem lays somewhere else. Perhaps your mainboard is limiting or some of the ram sticks are not supported/broken.