pointer points below @code{PHYS_BASE}, then dereference it.
An invalid user pointer will cause a ``page fault'' that you can
handle by modifying the code for @func{page_fault} in
pointer points below @code{PHYS_BASE}, then dereference it.
An invalid user pointer will cause a ``page fault'' that you can
handle by modifying the code for @func{page_fault} in