man4/mem.4

.\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
.\"     Fri Apr  2 11:32:09 MET DST 1993
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.\" Modified Sat Jul 24 16:59:10 1993 by Rik Faith (faith@cs.unc.edu)
.TH MEM 4 2015-01-02 "Linux" "Linux Programmer's Manual"
.SH NAME
mem, kmem, port \- system memory, kernel memory and system ports
.SH DESCRIPTION
.IR /dev/mem
is a character device file
that is an image of the main memory of the computer.
It may be used, for example, to examine (and even patch) the system.
.PP
Byte addresses in
.IR /dev/mem
are interpreted as physical memory addresses.
References to nonexistent locations cause errors to be returned.
.PP
Examining and patching is likely to lead to unexpected results
when read-only or write-only bits are present.
.PP
Since Linux 2.6.26, and depending on the architecture, the
.B CONFIG_STRICT_DEVMEM
kernel configuration option limits the areas
which can be accessed through this file.
For example: on x86, RAM access is not allowed but accessing
memory-mapped PCI regions is.
.PP
It is typically created by:
.PP
.in +4n
.EX
mknod \-m 660 /dev/mem c 1 1
chown root:kmem /dev/mem
.EE
.in
.PP
The file
.IR /dev/kmem
is the same as
.IR /dev/mem ,
except that the kernel virtual memory
rather than physical memory is accessed.
Since Linux 2.6.26, this file is available only if the
.B CONFIG_DEVKMEM
kernel configuration option is enabled.
.PP
It is typically created by:
.PP
.in +4n
.EX
mknod \-m 640 /dev/kmem c 1 2
chown root:kmem /dev/kmem
.EE
.in
.PP
.IR /dev/port
is similar to
.IR /dev/mem ,
but the I/O ports are accessed.
.PP
It is typically created by:
.PP
.in +4n
.EX
mknod \-m 660 /dev/port c 1 4
chown root:kmem /dev/port
.EE
.in
.SH FILES
.I /dev/mem
.br
.I /dev/kmem
.br
.I /dev/port
.SH SEE ALSO
.BR chown (1),
.BR mknod (1),
.BR ioperm (2)