only the first 32GiB of physical memory are considered. this can be changed
with pram_pages in paging.cpp. vram layout is as follows:

0x0000.0000.0000 - 0x0000.0000.0fff: always unmapped
0x0000.0000.1000 - 0x003f.ffff.ffff: available to user process
0x0040.0000.0000 - 0x007f.ffff.ffff: each 16MB:
       0x00.0000 -        0x00.0fff: always unmapped
       0x00.1000 -        0xff.ffff: available for user thread stack
0x0080.0000.0000 - 0xffff.bfff.ffff: always unmapped
0xffff.c000.0000 - 0xffff.ffdf.ffff: available to kernel
0xffff.ffe0.0000 - 0xffff.ffe0.0fff: always unmapped
0xffff.ffe0.1000 - 0xffff.ffef.efff: interrupt stack
0xffff.ffef.f000 - 0xffff.fff0.0fff: always unmapped
0xffff.fff0.1000 - 0xffff.ffff.efff: syscall stack / kernel init stack
0xffff.ffff.f000 - 0xffff.ffff.ffff: always unmapped