+/* Base address of the 1:1 physical-to-virtual mapping. Physical
+ memory is mapped starting at this virtual address. Thus,
+ physical address 0 is accessible at PHYS_BASE, physical
+ address address 0x1234 at (uint8_t *) PHYS_BASE + 0x1234, and
+ so on.
+
+ This address also marks the end of user programs' address
+ space. Up to this point in memory, user programs are allowed
+ to map whatever they like. At this point and above, the
+ virtual address space belongs to the kernel. */