source: rtems-docs/filesystem/system_init.rst

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 1988, 2002 On-Line Applications Research Corporation (OAR)

System Initialization
*********************

After the RTEMS initialization is performed, the application's initialization
will be performed. Part of initialization is a call to
``rtems_filesystem_initialize()``. This routine will mount the 'In Memory File
System' as the base filesystem.  Mounting the base filesystem consists of the
following:

- Initialization of mount table chain control structure

- Allocation of a ``jnode`` structure that will server as the root node of the
  'In Memory Filesystem'

- Initialization of the allocated ``jnode`` with the appropriate OPS, directory
  handlers and pathconf limits and options.

- Allocation of a memory region for filesystem specific global management
  variables

- Creation of first mount table entry for the base filesystem

- Initialization of the first mount table chain entry to indicate that the
  mount point is NULL and the mounted filesystem is the base file system

After the base filesystem has been mounted, the following operations are
performed under its directory structure:

- Creation of the /dev directory

- Registration of devices under /dev directory

Base Filesystem
===============

RTEMS initially mounts a RAM based file system known as the base file system.
The root directory of this file system tree serves as the logical root of the
directory hierarchy (Figure 3). Under the root directory a '/dev' directory is
created under which all I/O device directories and files are registered as part
of the file system hierarchy.

.. code-block:: shell

    Figure of the tree structure goes here.

A RAM based file system draws its management resources from memory. File and
directory nodes are simply allocated blocks of memory. Data associated with
regular files is stored in collections of memory blocks. When the system is
turned off or restarted all memory-based components of the file system are
lost.

The base file system serves as a starting point for the mounting of file
systems that are resident on semi-permanent storage media. Examples of such
media include non- volatile memory, flash memory and IDE hard disk drives
(Figure 3). File systems of other types will be mounted onto mount points
within the base file system or other file systems that are subordinate to the
base file system. The framework set up under the base file system will allow
for these new file system types and the unique data and functionality that is
required to manage the future file systems.

Base Filesystem Mounting
------------------------

At present, the first file system to be mounted is the 'In Memory File
System'. It is mounted using a standard MOUNT() command in which the mount
point is NULL.  This flags the mount as the first file system to be registered
under the operating system and appropriate initialization of file system
management information is performed (See figures 4 and 5). If a different file
system type is desired as the base file system, alterations must be made to
base_fs.c. This routine handles the mount of the base file system.

.. code-block:: shell

    Figure of the mount table chain goes here.

Once the root of the base file system has been established and it has been
recorded as the mount point of the base file system, devices are integrated
into the base file system. For every device that is configured into the system
(See ioman.c) a device registration process is performed. Device registration
produces a unique dev_t handle that consists of a major and minor device
number. In addition, the configuration information for each device contains a
text string that represents the fully qualified pathname to that device's place
in the base file system's hierarchy. A file system node is created for the
device along the specified registration path.

.. code-block:: shell

    Figure  of the Mount Table Processing goes here.

Note: Other file systems can be mounted but they are mounted onto points
(directory mount points) in the base file system.