.\" 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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.