Rolling Your Own Bootloader: Difference between revisions

The easiest way to boot another OS is a mechanism called ''chainloading''. Windows stores something akin to a second-stage bootloader in the boot sector of the ''partition'' it was installed in. When installing Linux, writing e.g. LILO or GRUB to the ''partition'' boot sector instead of the MBR is also an option. Now, the thing your MBR bootsector can do is to ''relocate'' itself (copying from 0x07c0:0x0000 to, traditionally, 0x0060:0x0000), parse the partition table, display some kind of menu and let the user chose which partition to boot from. Then, your (relocated) MBR bootsector would load that ''partition'' boot sector to 0x07c0:0x0000, and jump there. The partition boot sector would be none the wiser that there already was a bootsector loaded ''before'', and could actually load yet ''another'' bootsector - which is why it's called ''chainloading''. It doesn't really matter where you decide to relocate the boot sector as long as you don't overwrite the ''[[IVT'']] (if IF in EFLAGS is set), the ''[[Memory Map (x86)#BIOS Data Area (BDA'')|BDA]] or the ''[[Memory Map (x86)#Extended BIOS Data Area (EBDA)|EBDA'']].