Re: dual grub install?

[Tom Davies]

Hi :)
Aaargh, sorry for the double (triple?) posting! 

Boot-loaders have a few different sections.  Before the boot-loader is the Bios part of the booting process.  The first part of the boot-loader (as far as we need to worry about) is the MBR = Master Boot Record. 

Each machine has only 1 Bios (hopefully unless we start going all Latin).  The Bios is told which MBR to look for 1st.  If it doesn't find a useful MBR on it's first device it looks at the 2nd in the list and keeps going until it has run out of list.  You set the boot-order list although by default most machines are set to look first for an MBR on the main internal hard-drive.  Bios often calls it the "primary" "master" hard-drive.

There could be useful MBRs on any or all hard-drive (internal or external), usb-sticks, Cds/Dvds, floppies, even memory cards can have an MBR.  When you set the "Boot order" in the Bios it makes the Bios looks first for the MBR of whatever you had at the top of the boot-order list. 

The MBR holds the 1st stage of the boot-loader but it's main function is to point the boot-process at the exact location of the main bulk of the boot-loader's 2nd stage.  Many decades ago i guess boot-loaders were tiny enough to fit entirely inside the MBR but nowadays everything is so much larger (some would say bloated) and has so much more functionality that a 2nd stage is needed. 

Each and every partition on the boot-drive can have a boot-loader BUT the MBR can only point to one of them.  I think Grub2 (and Grub1 and probably LILO) can cheat by pointing to another boot-loader, such as the Windows one, and pretend that it (Grub or LILO) was really only the MBR.  I think that is what "chain loading" is. 

Windows is completely happy then.  As far as it is concerned it is the only OS on the machine and it was started by the MBR.  Blissful ignorance of the
Bios - MBR - Grub - Ntldr
Ntldr is the wonderful name of the Windows boot-loader.  It's an "NT loader".  Grub stands for "GRand Unified Bootloader" of course.  Apparently quite a few people that have to maintain pcs install grub purely to avoid having to rely on the flaky Windows one even on machines that really do only have Windows. 

Regards from
Tom :)