Dual booting (multiple booting).
The boot sequence is the following:
(1) The bios determines the boot device.
(2) The bios reads the master boot record (MBR) from
the boot device. This is one sector in a fixed
location (first physical sector on the disk).
Of course, the controller can play with the bios's head
if it wants to. The MBR contains a few dozen machine
instructions and the Partition Table. The partition table
consists of 4 entries, which is why there is a max of
4 primary partitions on a physical disk.
(3) The bios executes the code in the MBR which looks
for the first enabled partition in the partition table,
loads the Partition Boot Sector (PBS), the first disk sector
in the partition, and jumps to it.
Note that up to this point the boot process is OS independent.
The MBR is the same on every hard disk of every (Ix86)
os implementation.
(4) Code in the PBS typically loads the OS loader and jumps to
it, or in the case of a boot manager, the PBS loads the boot
manager and jumps to it. This IS os dependent.
From this point it depends on the OS/boot manager that's
running what happens next. NT, for example, has a true but
limited multiboot capability that is governed by the boot.ini
file. Win 95 does not have a true multiboot capability, but it
does have a pseudo-multiboot capability to boot itself or,
at a point in the boot process, stop booting itself and
restore the boot files for the previously installed version
of MS dos (this differs from true multiboot in that the
previous MS dos doesn't coexist intact with Win 95 but
must be restored in the boot process).
In any case, it is perfectly possible to have a full,
bootable system on any hard drive. Only the primary
active partition of the drive the bios selects as a boot
device will get booted; everything else will be ignored.
I'm not guessing on this one; I have done it many times.
Every (I86-formatted) hard drive has an MBR
and is capable of being booted. I've never seen a bios
that allowed selection of more than the first IDE or the
first SCSI drive, but apparently that's not so of newer
bioses.
In any event, except for DISMOUNTABLE media (eg, floppy),
the partition that the MBR boots gets the designation
as the C drive. This is what we really mean when we
say the "first active partition on the first physical disk."
However, this doesn't apply to booting from a floppy (which
isn't labeled C). I don't know what happens from a CDRom,
but I'd be surprised if that were labeled C either.
A boot manager acts just like a booting OS in this sequence,
except that rather than boot a full blow system they present
the user with a choice, then typically load
and execute the PBS of the users choice OS. This may
or may not be the from the physical partition boot sector.
Typically not, but rather from a collection of PBS's stored
by the boot manager. Oh, the OS booted by a boot manager
will almost certainly label ITS primary boot partition as
the C drive (that is, the partition where its PBS is kept.)
Note that the C drive is where the PBS and the os BOOT LOADER
load from; it doesn't have to be the partition where the main
OS resides (the os system directory -- Win95, WinNT, etc).
NT keeps only one PBS, the PBS of whatever version of DOS or
Windows that was on the machine when NT is installed. That
PBS image is kept in the root directory of the NT boot
partition, the C drive, that is (the place where
the MBR of the boot drive points). Thus NT can boot any
number of copies of NT (because it's "behind" the PBS of
NT, which has already executed by the time NTLoader starts),
but only one choice of "anything else", which is whatever
it has saved the PBS image for. That could be another os
that has multiboot capabilities. In my case, it's Win 95
from which I can then boot the previous version of DOS
and Win 3.11 if I choose.
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