4.26. x86

4.26.1. Features

barebox can act as a bootloader for PC based systems. In this case a special binary layout will be created to be able to store it on some media the PC BIOS can boot from. It can boot Linux kernels stored also on the same boot media and be configured at runtime, with the possibility to store the changed configuration on the boot media.

4.26.2. Restrictions

Due to some BIOS and barebox restrictions the boot media must be prepared in some special way:

  • barebox must be stored in the MBR (Master Boot Record) of the boot media. Currently its not possible to store and boot it in one of the partition sectors to use it as a second stage loader). This is no eternal restriction. It only needs further effort to add this feature.
  • barebox currently cannot run a chained boot loader. Also, this is no external restriction, only further effort needed.
  • barebox comes with limited filesystem support. There is currently no support for the most common and popular filesystems used in the *NIX world. This restricts locations where to store a kernel and other runtime information
  • barebox must be stored to the first n sectors of the boot media. To ensure this does not collide with partitions on the boot media, the first partition must start at a sector behind the ones barebox occupies.
  • barebox handles its runtime configuration in a special way: It stores it in a binary way into some reserved sectors (“persistant storage”).

4.26.3. Boot Preparations

To store the barebox image to a boot media, it comes with the tool setupmbr in the directory scripts/setupmbr/ . To be able to use it on the boot media of your choice, some preparations are required.

4.26.4. Keep Sectors free

Build the barebox image and check its size. At least this amount of sectors must be kept free after the MBR prior the first partition. Do this simple calulation:

sectors = (size of barebox image + 511) / 512

To be able to store the runtime configuration, further free sectors are required. Its up to you and your requirements, how large this persistant storage must be. If you need 16 kiB for this purpose, you need to keep additional 32 sectors free.

For this example we are reserving 300 sectors for the barebox image and additionaly 32 sectors for the persistant storage. So, the first partition on the boot media must start at sector 333 or later.

Run the fdisk tool to setup such a partition table:

[jb@host]~> fdisk /dev/sda
Command (m for help): p

Disk /dev/sda: 20.7 MB, 212680704 bytes
16 heads, 63 sectors/track, 406 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

Device Boot      Start         End      Blocks   Id  System

Change the used units to sectors for easier handling.

Command (m for help): u
Changing display/entry units to sectors

Command (m for help): p

Disk /dev/sda: 20.7 MB, 212680704 bytes
16 heads, 63 sectors/track, 406 cylinders, total 409248 sectors
Units = sectors of 1 * 512 = 512 bytes

Device Boot      Start         End      Blocks   Id  System

Now its possible to create the first partition with the required offset:

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First sector (63-409247, default 63): 333
Last sector or +size or +sizeM or +sizeK (333-409247, default 409247): +18M
Command (m for help): p

Disk /dev/sda: 20.7 MB, 212680704 bytes
16 heads, 63 sectors/track, 406 cylinders, total 409248 sectors
Units = sectors of 1 * 512 = 512 bytes

        Device Boot      Start         End      Blocks   Id  System
/dev/sda                   333       35489       17578+  83  Linux

That’s all. Do whatever is required now with the new partition (formatting and populating the root filesystem for example) to make it useful.

In the next step, barebox gets installed to this boot media:

[jb@host]~> scripts/setupmbr/setupmbr -s 32 -m ./barebox -d /dev/sda
This command writes the barebox image file ‘./barebox’ onto the device
/dev/sda.

The -s option will keep the persistant storage sectors free and untouched and set flags in the MBR to forward its existance, size and location to barebox at runtime. setupmbr also does not change the partition table.

The barebox image gets stored on the boot media like this:

sector 0   1             33                              333
       |---|-------------|--------------- ~~~ ------------|--------------
      MBR    persistant              barebox                 first
              storage               main image              partition

If the -s option is omitted, the “persistant storage” part simply does not exist:

sector 0   1                              333
       |---|--------------- ~~~ ------------|--------------
      MBR               barebox                 first
                       main image              partition

NOTE: the setupmbr tool is also working on real image file than on device nodes only. So, there is no restriction what kind of file will be modified.