The file /dev/mem is an interface to the physical memory
  of the computer. Byte offsets in this file are interpreted as physical memory
  addresses. Reading and writing this file is equivalent to reading and writing
  memory itself. An error will be returned if an attempt is made to reference an
  offset outside of /dev/mem.
Kernel virtual memory is accessed via the file
    /dev/kmem in the same manner as
    /dev/mem. Only kernel virtual addresses that are
    currently mapped to memory are allowed.
On the HP300, the last byte of physical memory is always
  0xFFFFFFFF. Therefore, on an HP300 with 8MB of memory,
  physical memory would start at 0xFF800000. On the
  HP300, kernel virtual memory runs from 0 to about
  0x2400000.
The files mem and kmem appeared
  in Version 6 AT&T UNIX.