# phpman > man > port(4)

[MEM(4)](https://www.chedong.com/phpMan.php/man/MEM/4/markdown)                                Linux Programmer's Manual                               [MEM(4)](https://www.chedong.com/phpMan.php/man/MEM/4/markdown)



## NAME
       mem, kmem, port - system memory, kernel memory and system ports

## DESCRIPTION
       _/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.

       Byte addresses in _/dev/mem_ are interpreted  as  physical  memory  addresses.   References  to
       nonexistent locations cause errors to be returned.

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

       Since Linux 2.6.26, and depending on the architecture, the **CONFIG**___**STRICT**___**DEVMEM**  kernel  con‐
       figuration  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.

       It is typically created by:

           mknod -m 660 /dev/mem c 1 1
           chown root:kmem /dev/mem

       The file _/dev/kmem_ is the same as _/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 **CON**‐‐
       **FIG**___**DEVKMEM** kernel configuration option is enabled.

       It is typically created by:

           mknod -m 640 /dev/kmem c 1 2
           chown root:kmem /dev/kmem

       _/dev/port_ is similar to _/dev/mem_, but the I/O ports are accessed.

       It is typically created by:

           mknod -m 660 /dev/port c 1 4
           chown root:kmem /dev/port

## FILES
       _/dev/mem_
       _/dev/kmem_
       _/dev/port_

## SEE ALSO
       [**chown**(1)](https://www.chedong.com/phpMan.php/man/chown/1/markdown), [**mknod**(1)](https://www.chedong.com/phpMan.php/man/mknod/1/markdown), [**ioperm**(2)](https://www.chedong.com/phpMan.php/man/ioperm/2/markdown)

## COLOPHON
       This page is part of release 5.10 of the Linux  _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/>.



Linux                                        2015-01-02                                       [MEM(4)](https://www.chedong.com/phpMan.php/man/MEM/4/markdown)