philhofer / mlb2

Minimal Linux Bootloader 2

Minimal Linux Bootloader, adapted/adopted from Sebastian Plotz by Wiktor Kerr, and then later adapted for use in virtualized booting environments by Phil Hofer for use in the distill project.

This is a single stage x86 bootloader that can boot a single Linux kernel. There is no support for initrd and only one kernel can be configured to boot at any time.

MLB was originally written by Sebastian Plotz. Wiktor Kerr changed the code very slightly and wrote the original Makefile and mlbinstall. Phil Hofer optimized the assembly a bit more and wrote a new Makefile and mlb2install.

http://sebastian-plotz.blogspot.de/

Dependencies

a C compiler
ld(1)
make(1)
nasm(1)

Building

Run make, optionally with CC, LD, CFLAGS, LDFLAGS, etc. set via the command line, i.e. make CC=gcc 'CFLAGS=-02'

The mlb2install binary has the MBR boot code embedded in it, so it can simply be copied to its final destination, or you can run make install. (The Makefile will respect the conventional PREFIX and DESTDIR arguments for the install prefix and staging prefix, respectively.)

Installing

To install MLB, simply invoke

mlb2install <target> <kernel-lba> <command line>

where:

<target> is where you want your bootloader installed (e.g. a file, a block device, whatever)
<kernel-lba> is the LBA (in 512-byte sectors) of the kernel within the target device
<command line> is the command line to pass to the kernel - don't forget to pass root= as mlbinstall will not calculate the root device. Currently the command line cannot be longer than 99 bytes (more just won't fit in the MBR).

Typically, the kernel will live in reserved (unformatted) space at the beginning of the drive before the first real MBR partition, or it will begin at the first partition. (You can simply dd your kernel to the right place once you've arranged the partitions so that it won't be clobbered by other disk manipulation.)

mlb2install is quite noisy if it detects any problems. Restore your MBR from backup, fix the problems, and try again. If it succeeds, it won't say anything.

Error codes

If MLB fails to boot your kernel, it will print a one-letter error code. Currently, there are two error codes:

Error What it means
----- ----------------------------------------
  R   Failed reading data from disk
  M   Failed moving data to its final location