MultiBoot 0.4

MultiBoot is a master boot record with ability to interactively select
which partition to boot. It doesn't try to do fancy things and extend
MBR format in fancy non-compatible ways. MBR which has multiboot
installed in it continues to be perfectly "normal" MBR for BIOS,
OSes and fdisk-like utilities.

mboot_mbr.S is a code which is stored in MBR's boot code section.
When you boot from it, you will see a menu:

MultiBoot 0.4

a - Partition 1
b - Partition 2
c - Partition 3
d - Partition 4

Choose: _

Letters and label texts are configurable. It's possible to have
less than four partitions selectable.

When user presses a letter or timeout expires, selected partition
is marked as active. MBR is updated if needed, in order to not confuse
OS being booted by booting it from seemingly "inactive" partition.
Then first sector of selected partititon is read and executed,
as usual.

It is possible to "boot" an extended partition this way. For this
to be useful, you need to write slightly modified multiboot code
in each extended partition table sector. The code is in mboot_ext.S.

If extended partition is selected, this code will be loaded
and will display another similar menu, where user can select which
of the extended partitions to boot. This method of booting OSes
is not standard and there is no guarantee it will work.
It was tested to work with Linux + (lilo installed to Linux's
partition superblock), though.

mbinstall is an utility which has mboot_mbr and mboot_ext binary
images embedded in it. You can install them into MBR and/or
extended partitions using it. It also can display or change
settings (letters, menu labels, timeout etc).

Build instructions: download mbinstall.c, mboot_mbr.S, mboot_ext.S
and mk, then run "sh mk". If you don't have dietlibc installed,
edit mk and remove "diet -Os" from the line which compiles mbinstall.

Usage:
mbinstall [-t timeout_ms] [-d default_key] [-e default_key_for_ext_partition]
                                [-1..9 c[:label]]
                                [-M] [-E] block_device
        -t timeout_ms   Timeout for boot menu
        -d default_key  Key which is assumed if timeout is reached
        -e default_key  Same for extended partition boot menu
        -N c[:label]    key 'c' selects N'th partition (1..4 primary, 5+ extd)
                        label: on-screen label for this partition
        -L NN:c:[:lbl]  Same as -N... for N>9
        -M, -E          Install boot code to MBR and/or extended partition
                        (normally I would just check for their presence)

        Special cases:
        -N -            makes partition N unselectable
        -d <bad_char>   disables timeout
        -t 0 -d c       no menu; boot partition 'c'
        -t 0 -d <bad>   blind boot: no menu, but you still have to press
                        a character corresponding to some partition

Without options, mbinstall shows current mbinstall configuration.
Examples:

# mbinstall /dev/sdb
signature 'MultiBoot 0.4' not found in MBR

No editing is done on MBR

# mbinstall /dev/sda
Partition 1: 1 - Partition 1
Partition 2: x - Extended
Default choice is 'x', timeout is 4950 msec
Partition 5: l - Linux
Partition 6: 6 - ExtPartition 6
Partition 7: 7 - ExtPartition 7
Partition 8: 8 - ExtPartition 8
Default choice for ext menu is 'l'