only the first 32GiB of physical memory are considered. this can be changed
with pram_pages in paging.cpp. as for virtual memory, the top 1GiB is reserved
for the kernel. the top 1MiB of that is reserved for the kernel stack, with an
unmapped guard page at the top and bottom of the kernel stack.

if any of these facts change, the linker script and paging code should be
checked closely.