On Thu, Nov 3, 2011 at 11:55 AM, Marko Vojinovic <vvma...@gmail.com> wrote:
This is how bootloading works... First, there is bios, which is programmed > to > look for and execute the boot code in the MBR, and it does so at some > point. > The "look for and execute" means that bios needs to access the MBR of the > disk, read it into RAM memory, analyze whether the data contained there is > executable, and if it is to point the processor to execute it. > > The "program" that gets executed like that is called the stage 1 > bootloader, > and it is very very small (like 512 bytes or so), since the MBR doesn't > have > more space available. What this program does is to tell the bios to access > some physical part of the hard disk, load it into memory and execute it. > The > bios knows nothing about partitions and filesystems, so it cannot be just > pointed to "beginning of the partition /dev/sda6", but rather it must be > given > "physical" address of the place where it should look for data. This is > explained in more detail in > > http://en.wikipedia.org/wiki/Logical_block_addressing > > The stage 1 bootloader has the LBA address of the beginning of the > /dev/sda6 > partition, the LBA address of the end of the data to be read in, and the > code > to instruct the bios to go there, read that much data into memory, and > execute > it. If the bios is not designed to access the LBA address that far on the > disk, it will fail and the machine won't boot. If it can, it goes there, > reads > the data into memory, and executes it. > > This data is called the stage 2 bootloader. It is a larger and more > complicated program. It understands filesystems, it has a configuration > file (in > Fedora it is the /boot/grub/grub.conf file, feel free to take a look), it > can > interact with a user and offer various choices for the OS to boot. Once the > bios loads it into memory and executes it, the stage 2 bootloader reads up > its > configuration file, on the /dev/sda6 partition. If the file isn't there, > the boot > fails (so the Ubuntu bootloader won't work if you deleted the Ubuntu system > partition). Once the configuration is processed, the user is presented with > options to load various OS's. Once the user makes a choice, the bootloader > will do whatever is specified for that OS in the config file. Typically, > it will > load the kernel file (/boot/vmlinuz-something) into memory, and execute > it. It > doesn't need the bios for this anymore, since it knows how to access the > disk > itself. > > Another typical situation is to not load a kernel, but instead read some > other > stage 2 bootloader that resides on, say, /dev/sda1, and let that take over > and > repeat the whole thing for another OS. This is called chainloading, and > that > is how Windows gets booted from the Linux bootloader. > > At any rate, eventually some kernel gets loaded into memory, and the > bootloader instructs the processor to execute that. And that's where the > *real* fun begins... ;-) But that's another story,,, :-) > > > > Of course, once the OS gets booted, > > > > from which location? > > In Linux, the kernel is typically a file called vmlinuz-<something>, and it > resides in the /boot directory of the filesystem tree. This directory can > be > basically anywhere --- on its own partition, on the / partition, on some > other > disk, etc... The stage 2 bootloader just needs to know where to look for > it. > > In Windows, the kernel is (IIRC) the file C:\Windows\ntoskrnl.exe, or > something > like that. The stage 2 bootloader of Windows knows where and how to find > it. > The Windows' stage 2 bootloader gets loaded into memory and gets executed > by > the Linux stage 2 bootloader, if you choose to boot Windows when asked (the > chainloading process). > > In other OS's the kernel file may be called whatever and be situated > whereever, > depending on the OS. ;-) > > > SUSE would automatically delete the MBR (which right now points to > Ubuntu) > > and would set the other defaults.....in it? > > Yes, SuSE would wipe and rework the MBR so that it contains the stage 1 > bootloader that points to SuSE's new stage 2 bootloader, instead of the old > Ubuntu's stage 2 bootloader (which was deleted during SuSE installation). > Otherwise the transition from stage 1 to stage 2 in the boot sequence above > would be broken. > > Yes. The above explanation should give you a clear idea what would happen > if > you leave Ubuntu stage 1 loader in MBR, and delete the stage 2 loader > (along > with the rest of the Ubuntu installation). > > Nothing is permanent, of course, it would just be a hassle to fix. Neither > Windows nor Linux would boot, and you would need to boot from the > installation > DVD or something called the "Rescue CD", and use the rescue environment to > reinstall both stages of the Linux bootloader (or reinstall Linux > completely). > You would need to know how to use the rescue environment and how to > reconfigure > the GRUB (the Linux bootloader) so that it loads everything correctly. This > requires reading the documentation, which is on the Internet and you can > have > a hard time accessing it, since your computer doesn't boot... > > It happened to me once, back in the day --- I had to go to a nearby > Internet > caffe to read the docs, pay them to print me the details, go back home > only to > find out that I was reading the docs for the wrong version of the > software, so > I had to visit the Internet caffe once more, pay again for the printing of > new > docs, and then go home and fix it. I wasted a whole afternoon, realizing > in the > end that it is a Good Idea to learn about these things *before* I make a > mess... ;-) > Ah, thanks, it is pretty nice explanation and I would definitely read the docs - not only for rescue but also other details you pointed, the matter is only of time... I would do that. -- THX
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