Disk layout for XO-1.5

Mitch Bradley wmb at laptop.org
Tue Jul 28 17:32:36 EDT 2009

lilo is nearly worst of breed in terms of putting magic stuff in the 
Master Boot Record.

david at lang.hm wrote:
> On Tue, 28 Jul 2009, Mitch Bradley wrote:
>> david at lang.hm wrote:
>>> On Tue, 28 Jul 2009, Mitch Bradley wrote:
>>>> Another important advantage to partitions is that the existence of 
>>>> a boot partition isolates the firmware from changes in the 
>>>> filesystem used for the root.
>>> can you explain this a bit more?
>> A conventional BIOS boots by reading a small chunk of 16-bit 
>> real-mode code from the Master Boot Record, which then does something 
>> magic, often involving chaining to some other chunk of code that is 
>> shoehorned into some location that is difficult to inspect (e.g. some 
>> reserved sectors near the beginning of a partition).
> I'll point out that lilo allows it to boot from stuff anywhere in the 
> filesystem, and does not have to understand the specific filesystem at 
> boot time.
> there are cases where it doesn't work well, but in the limited case of 
> the XO laptops I think it would work well (and avoid the complexity 
> you describe below)
> Davi dLang
>> OFW instead reads the kernel and initrd files directly from the 
>> filesystem - there are no hidden chunks of 16-bit code tucked into 
>> nooks and crannies.
>> Linux filesystems tend to evolve rapidly.  ext2/3 sprouted numerous 
>> extensions over the years, some of them backwards compatible and some 
>> not. Now we have ext4, which an ext2 reader can't necessarily 
>> handle.  At some point we might want to switch to btrfs, which would 
>> require yet another reader.  OFW will probably eventually acquire a 
>> btrfs reader just for diagnostic purposes, but from a stability and 
>> ease of migration standpoint, it's better to pin down the format of 
>> the boot partition, thus decoupling the evolution of the OS from the 
>> firmware.
>> UEFI has taken the same approach - a boot partition with a FAT 
>> filesystem.
>> The boot partition is useful not only for the kernel, but also for 
>> firmware extensions like extra diagnostics, alternate startup 
>> graphics, alternate fonts, localizations, ...

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