3. System Initialization — RTEMS Filesystem Design Guide 5.4-rc2 (21st August 2026) documentation

# 3. 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

## 3.1. 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.

    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.

### 3.1.1. 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.

    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.

    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.