As far as I understand it booting for a IBM compatible PC is as follows:
- The computer runs the Power-On Self Test.
- The computer BIOS locates the boot device and loads and runs the MBR (Master Boot Record).
Modern systems might have EUFI rather than BIOS. UEFI can load both MBR and GPT drives. I will ignore those for now.
- The MBR contains the partition table for the disk. One of the partitions will be marked as 'active'. The BIOS loads the boot sector from this the active partition into memory and then executes it.
- If this was a older windows boot sector then the program in the boot sector will start and initialize the NTLDR file, which is the operating system loader.
- NTLDR uses boot.ini to present options
You can point the NT loader (ntldr) to other installation. E.g. if your old boot.ini looks like this:
[boot loader]
timeout=10
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows NT Workstation Version 4.00"
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows NT Workstation Version 4.00 [VGA mode]" /basevideo /sos
C:\="MS-DOS"
then you can add a line with C:\bootsect.lnx="Boot Linux
and put the linux bootsector in a file c:\bootsect.lnx. Then you can use the same menu to select between windows boot options and linux. Selecting the linux boot option will trigger whatever commands are in that file, which might a GNU/Linux bootloader. That would present you with a second (and possibly unwanted) menu to select from.
This means that nltdr does not really boot anything but windows, but you can let it hand over to boot process to another boot loader.
I think the Windows booter which reads boot.ini is always installed at MBR, while GRUB can be installed at MBR or the boot sector in any primary partition. (Well, I'm not sure...) – Xiè Jìléi – 2012-07-19T16:03:50.137
@XièJìléi You can indeed install GRUB to a partition - but you still have a bootloader on the MBR to load GRUB with. So there's not much of a point. – new123456 – 2012-07-19T16:54:39.427