11-22-2015
As an addendum to bakunin's post as he has pointed to the history of _memory_management_ of the Intel 8088 and above it might be of interest to have a browse of, note not just swap files but '*.TMP' and '*.OVL' files too:-
(Intel Architecture.)
Bottom 64KB usage, where interrupt vectors reside inside the bottom 1024 bytes.
ROM at the top 64KB of 1MB address space, (shadowed by RAM).
Conventional Memory.
HMA, High Memory Area.
UMB, Upper Memory Blocks.
EMS, Expanded Memory Specification. LIM 4.x - Lotus, Intel, Microsoft Specifiaction 4.x
XMS, Extended Memory Specification.
DOS Extenders.
The '.OVL' file extension.
The use of '.TMP' files.
These are all related...
And as a subdivision:-
VCPI, Virtual Control Program Interface.
DPMI, Dos Protected Mode Interface - Superceded VCPI.
Unreal Mode of running.
There is much more and all this is of historic interest but shows how we have gotten to where we are now, especially in the Intel architecture arena...
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LEARN ABOUT CENTOS
shm_overview
SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
shm_open(3) Create and open a new object, or open an existing object. This is analogous to open(2). The call returns a file descriptor
for use by the other interfaces listed below.
ftruncate(2) Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)
mmap(2) Map the shared memory object into the virtual address space of the calling process.
munmap(2) Unmap the shared memory object from the virtual address space of the calling process.
shm_unlink(3) Remove a shared memory object name.
close(2) Close the file descriptor allocated by shm_open(3) when it is no longer needed.
fstat(2) Obtain a stat structure that describes the shared memory object. Among the information returned by this call are the
object's size (st_size), permissions (st_mode), owner (st_uid), and group (st_gid).
fchown(2) To change the ownership of a shared memory object.
fchmod(2) To change the permissions of a shared memory object.
Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
Persistence
POSIX shared memory objects have kernel persistence: a shared memory object will exist until the system is shut down, or until all pro-
cesses have unmapped the object and it has been deleted with shm_unlink(3)
Linking
Programs using the POSIX shared memory API must be compiled with cc -lrt to link against the real-time library, librt.
Accessing shared memory objects via the file system
On Linux, shared memory objects are created in a (tmpfs) virtual file system, normally mounted under /dev/shm. Since kernel 2.6.19, Linux
supports the use of access control lists (ACLs) to control the permissions of objects in the virtual file system.
CONFORMING TO
POSIX.1-2001.
NOTES
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
System V shared memory (shmget(2), shmop(2), etc.) is an older shared memory API. POSIX shared memory provides a simpler, and better
designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V
shared memory.
SEE ALSO
fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_over-
view(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-10 SHM_OVERVIEW(7)