V8/usr/man/man4/mem.4

.TH MEM 4 
.SH NAME
mem, kmem, kmemr, kUmem, mtpr \- main memory
.SH DESCRIPTION
.lg
.I 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.
.PP
Byte addresses in
.I mem
are interpreted as physical memory addresses.
References to non-existent locations cause errors to be returned.
.PP
The files
.IR kmem ,
.IR kmemr ,
and
.I kUmem
are the same as 
.I mem
except that kernel virtual memory
rather than physical memory is accessed.
.I 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.
.I KUmem
guarantees that reads and writes will be done
in two-byte quantities;
this is convenient for UNIBUS accesses.
.PP
On the VAX,
.I mtpr
accesses internal processor registers.
Registers are 4 bytes long;
register
.I n
may be read or written at address
.IR n \(**4.
.SH FILES
/dev/mem,
/dev/kmem,
/dev/kmemr,
/dev/kUmem,
/dev/mtpr
.SH BUGS
On PDP11's and VAX's, memory files are accessed one byte
at a time, an inappropiate method for some
device registers.
.PP
Examining and patching device and processor registers is likely
to lead to unexpected results when read-only or write-only
bits are present.
.PP
An attempt to read a nonexistent processor register
returns 0
instead of an error.