Help in setting up a RAM Disk

Chris Johns chrisj at
Sat Mar 5 04:26:45 UTC 2011

On 5/03/11 7:52 AM, Robert S. Grimes wrote:
> I have many megabytes of RAM, and want to set aside a large portion for
> use as a RAM disk. I've looked about a bit on the message list, and
> couldn't find the answers I need, and some info I did found seemed
> contradictory - that, and I suspect things have changed a bit in the
> last 4 years anyway!
>  From what I've read, it seems I don't really want to use just the IMFS,
> as it uses the C heap; I'd like to keep the disk ram usage separate from
> my app's heap, out of concern for fragmentation. Looking at
>, I think I have figured
> out the basics, but then come my questions:
> 1. It seems I want to use RFS, is that correct? How do I configure it?
> (I could not find the configuration documentation).

The RFS is just one file system. There is also DOSFS. The RFS does not 
suffer from the issues DOSFS does.

> 2. RFS then needs a block device, specifically the RAM Disk, right? How
> do I configure that?

The RFS test code here:

is a complete working example with a RAM and flash disk. The disk set up 
is in main.c.

The RAM can be configured with:

#include <rtems.h>
#include <rtems/bdbuf.h>
#include <rtems/blkdev.h>
#include <rtems/diskdevs.h>
#include <rtems/error.h>
#include <rtems/fsmount.h>
#include <rtems/ramdisk.h>
#include <rtems/shell.h>


  * The RAM Disk configuration.
rtems_ramdisk_config rtems_ramdisk_configuration[] =
     block_size: 512,
     block_num:  1984 * 2,
     location:   NULL

  * The number of RAM Disk configurations.
size_t rtems_ramdisk_configuration_size = 1;

  * Create the RAM Disk Driver entry.
rtems_driver_address_table rtems_ramdisk_io_ops = {
   initialization_entry: ramdisk_initialize,
   open_entry:           rtems_blkdev_generic_open,
   close_entry:          rtems_blkdev_generic_close,
   read_entry:           rtems_blkdev_generic_read,
   write_entry:          rtems_blkdev_generic_write,
   control_entry:        rtems_blkdev_generic_ioctl

setup_ramdisk (void)
   rtems_device_major_number major;
   rtems_status_code         sc;

    * Register the RAM Disk driver.
   printf ("Register RAM Disk Driver: ");
   sc = rtems_io_register_driver (RTEMS_DRIVER_AUTO_MAJOR,
   if (sc != RTEMS_SUCCESSFUL)
     printf ("error: ramdisk driver not initialised: %s\n",
             rtems_status_text (sc));
     return 1;

   printf ("successful\n");

   return 0;

In the test application in the link above I handle the formatting and 
mounting using an RTEMS shell script. Look in shell-init. There is a 
mount call which is simple to use. Check the main_mount.c file in the 
shell for an example.

> 3. Is the Block Device Cache required in this setup, and if so, how is
> that configured?

Yes the block device cache is needed. You do not need to create a big 
cache if you have a RAM disk operating.

> 4. Is there a way to pre-populate the filesystem with a set of files? In
> other words, is there a reasonable way to bind a "disk image" into the
> final output of my build process, such that when my app is loaded and
> begins execution, it can mount this filesystem and use the files it
> finds there?

I am not sure if this relates to the RAM or if this is a separate disk. 
I think there is a TARFS but I have not used it. There is also the flash 
disk driver that is a block driver. You can use this with the same flash 
device as you boot with. The RFS test application has a configuration 
for the mcf5235 board.

> What I'm looking for is a nice "how-to". Given that our project team
> will have several RTEMS-based systems in it, and I'm the first one out
> the gate, I'll become a local resource for the others. Therefore, I'll
> be putting together one for our use here, and will post it to the RTEMS
> wiki for others to use too.



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