Assembly Level Documentation?

The CUDA page mentioned a “low level assembly language layer and driver interface” for language development, which I interpret as availability of GPU(or at least virtual machine level) instruction set documentation for compiler research and development.
Where is it?

Of course, targeting the C+extensions language is one approach of development, but I was hoping for a more direct layer…

yes, this would be very nice, especially as the C compiler hardly allows to limit the amount of registers used