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How does GRUB work and what is Multiboot?
GRUB Is a multi stage bootloader, it loads from whichever storage medium you install it too.
Then depending if you have the correct file system support (stage 1.5) file, it will then load the specified kernel and boot.
Multiboot is the specification that GRUB is built to. It specifies the data-structure passed to the kernel and the way
the bootloader should leave the processors registers/environment.
Also see:
http://wiki.osdev.org/GRUBQuote:
Segmentation. Where is it used? How is it used? Do I need to know about it?
Segmentation was used before protected (32-Bit) mode, it's not used today and unless you are planning on writing a bootloader you don't need to worry.
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Where does stuff go in memory?
Hmmm, a little ambiguous...
There is a basic memory layout for the x86 architecture (See:
http://wiki.osdev.org/Memory_Map_(x86)), however there is a lot more to memory maps then this.
The BIOS implements ways of obtaining a more accurate memory map (ref. E820 See:
http://wiki.osdev.org/Detecting_Memory_(x86)#E802h).
NOTE: Grub passes this E820 memory map to your kernel as specified in the MultiBoot Specification
This will list most memory areas, if they are free, used, unknown or reclaimable.
Hope that answers your questions.