Though it was down voted ... possibly because someone thought it was not answering the question ... I think @Rony's answer is a good start at explaining what the
boot flag is about. (I was actually planning to begin my answer with an example similar to the one he provided.)
I was all set to ramble off an answer about how the
boot flag is, at this point in time, an often ignored (as @Rony's example shows) historical remnant from a period when hard drives were smaller and bootloaders were much less sophisticated.
But then I discovered this had already been said in this answer to this question: What is the "Bootable flag" option when installing a distro?
What's more there was also a link to a short article about the Boot flag which says
- "Its primary function is to indicate to a MS-DOS/MS Windows-type boot loader which partition to boot. In some cases it is used by Windows XP/2000 to assign the active partition the letter "C:"."
Well, this is embarrassing ...
When I claimed that the
boot flag was a "historical remnant" I was assuming this was the case because clearly GRUB had no need to use it. Surely Microsoft would also have "moved on".
The well known quote usually attributed to Oscar Wilde turned out to be too true in this instance.
It appears that the MBR and PBR (Partition Boot Record) loaders used by the Windows operating systems DO expect the
boot flag to be set correctly.
To test this I cleared the boot flag from all the partitions of a Windows 8 VM. (See below. If you're curious, here's a link to the pastebin of the complete BootInfo Script result)
Disk /dev/sda: 26.8 GB, 26843545600 bytes
255 heads, 63 sectors/track, 3263 cylinders, total 52428800 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
Partition Boot Start Sector End Sector # of Sectors Id System
/dev/sda1 2,048 718,847 716,800 7 NTFS / exFAT / HPFS
/dev/sda2 718,848 52,426,751 51,707,904 7 NTFS / exFAT / HPFS
When I cleared the flag from both partitions, I got the error message
FATAL: INT18: BOOT FAILURE when I attempted to boot. (I am not sure if that is from the Windows MBR bootloader or the VM's equivalent of a BIOS.)
Just to see what would happen, I also set the
boot flag on the "wrong" partition,
/dev/sda2 instead of
/dev/sda1. Doing that resulted in the window shown in the image below.
This experience makes me wonder if Microsoft is still using the same MBR boot sector loader which they used for MS-DOS and Windows 3.0/3.1?