This MSDN article lists the virtual address space limits for current versions of Windows.
I notice that for 32 bit systems the address space is divided into 2 GB user and 2 GB kernel, which makes sense. Likewise on amd64 systems the (presumably 44 bit) address space is divided into 8 TB user space and 8 TB kernel space.
But ia64 has 7 TB user space. Is this an arbitrary limit or is it because the system uses 9 TB for the kernel space or why is this?