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?
Mark Russinovich explains it here: http://blogs.technet.com/b/markrussinovich/archive/2008/11/17/3155406.aspx
It turns out that the difference is centered in a Windows specific feature. For IA64, the top 1TB of the 8TB memory-space is reserved for wow64 mappings. Wow64 is the feature that allows running 32-bit code on a 64-bit system. The reason that X64-64 (a.k.a. amd64) doesn't have it, is that it is ISA-compatable so just need the right libraries to run.
