MEM(4)                                                     MEM(4)

          mem, kmem, kmemr, kUmem, mtpr - main memory

          Mem is a special file that is an image of the main memory of
          the computer.  It may be used, for example, to examine (and
          even to patch) the system.

          Byte addresses in mem are interpreted as physical memory
          addresses.  References to non-existent locations cause
          errors to be returned.

          The files kmem, kmemr, and kUmem are the same as mem except
          that kernel virtual memory rather than physical memory is
          accessed.  Kmemr is read-only, and excludes system buffer
          pools and device addresses; it is intended as a ``safe'' and
          ``secure'' mechanism for accessing system tables.  KUmem
          guarantees that reads and writes will be done in two-byte
          quantities; this is convenient for UNIBUS accesses.

          On the VAX, mtpr accesses internal processor registers.
          Registers are 4 bytes long; register n may be read or writ-
          ten at address n*4.

          /dev/mem, /dev/kmem, /dev/kmemr, /dev/kUmem, /dev/mtpr

          On PDP11's and VAX's, memory files are accessed one byte at
          a time, an inappropiate method for some device registers.

          Examining and patching device and processor registers is
          likely to lead to unexpected results when read-only or
          write-only bits are present.

          An attempt to read a nonexistent processor register returns
          0 instead of an error.