RAM Disks and ROM Disks

Warning

Migrated from: https://cwiki.apache.org/confluence/display/NUTTX/RAM+Disks+and+ROM+Disks

NSH mkrd Command

The typical way to create a RAM disk is by using the NuttShell (NSH) mkrd command. The syntax is:

mkrd [-m <minor>] [-s <sector-size>] <nsectors>

This command creates a RAM disk consisting of <nsectors> sectors, each of size <sector-size> (or 512 bytes if <sector-size> is not specified). The RAM disk is then registered as /dev/ram<minor>. If <minor> is not specified, mkrd attempts to register the RAM disk as /dev/ram0.

Internally, the NSH mkrd command is a simple wrapper around the OS boardctl() interface, using the BOARDIOC_MKRD command. “Under the hood,” this boardctl() command performs the following:

  1. Allocates kernel-space memory with kmm_malloc() of size <nsectors> times <sector-size>

  2. Zeros the allocated memory, and

  3. Calls the OS-internal function ramdisk_register() to create the RAM disk.

NSH ROMFS /etc Support

A ROM disk is a block device created from a read-only file system image stored in FLASH or other ROM. There is no NSH command available to create a ROM disk at runtime. However, it is possible to enable ROM disk support in NSH using the CONFIG_NSH_ROMFSETC option, as described in the section on NSH start-up scripts in the NSH User Guide.

Any application is able to create a ROM disk using the boardctl() interface with the BOARDIOC_ROMDISK command.

Creating RAM Disks in Board Bring-Up Logic

RAM disks may be created in board-specific initialization logic that runs in supervisor mode. That logic might look as follows:

int board_ramdisk(int minor, unsigned int sectsize, unsigned int nsectors)
{
  size_t allocsize = (size_t)sectsize * (size_t)nsectors;
  FAR uint8_t *buffer;

  /* Allocate the memory backing up the ramdisk */

  buffer = (FAR uint8_t *)kmm_zalloc(allocsize);
  if (buffer == NULL)
    {
      return -ENOMEM;
    }

  /* Then register the ramdisk */

  ret = ramdisk_register(minor, buffer, nsectors, sectsize,
                        RDFLAG_WRENABLED | RDFLAG_FUNLINK);
  if (ret < 0)
    {
      kmm_free(buffer);
    }

  return ret;
}

Alternatively, this could be replaced by a call to the OS internal function mkrd().

Creating ROM Disks in Board Bring-Up Logic

Note

Currently, the romdisk_register() function is only available within the OS. Certain logic in apps/ directly calls romdisk_register(), which violates the portable POSIX OS interface. The correct approach for an application is to create a ROM disk via boardctl(BOARDIOC_ROMDISK) as described above. Calling romdisk_register() directly is not only a violation of the NuttX portable interface, but also is not allowed in PROTECTED or KERNEL build modes.

ROM disks, i.e., read-only disks in FLASH, can be created by board bring-up logic in a way similar to RAM disks, with the following caveats:

  • The FLASH region is not allocated; the FLASH address, the sector size, and the number of sectors must already be known.

  • The romdisk_register() function is used instead of ramdisk_register().

A simple example could look like:

int board_romdisk(int minor, FAR uint8_t *buffer, unsigned int sectsize,
                  unsigned int nsectors)
{
  /* Register the romdisk */

  return romdisk_register(minor, buffer, nsectors, sectsize);
}

Calling romdisk_register() is equivalent to calling ramdisk_register() with the final parameter flags == 0.

Most ROM disks use the ROMFS file system, although CROMFS is another option. Creating ROMFS file system images involves several steps. Tools are available to simplify the process of building ROMFS images, but that topic is outside the scope of this Wiki page.