[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: GRUB problem (long, description of BOOT)

Alvin Oga wrote:
hi ya mike

:-) i'm even more sleepy now :-)

HEY! I SAID it was long!


but, some comments

On Tue, 27 Sep 2005, Mike McCarty wrote:

I think you need a little more information about how
boot is accomplished on IBM PC style computers.

if a user can't get the machine to boot .. this much
detail is probably more than they need ??

a simple post of their config is enough that shows
	root=   and boot=
along with the output fdisk -l

but ..

Discs, to be used, must be formatted. The formatting
takes place in levels. The lowest level (sometimes
called low-level format) places tracks and sectors on
each surface usable on the disc. Modern hard discs reserve
one surface to hold tracking information alone, no

the "tracking info" is dependant on the filesystem

It is not. It is used by the uController on the disc itself
to servo. A FS format should not disturb the low level
format. Anyone who redoes the low level format is taking
his figurative life into his own hands.

some intermingle track info ( aka servo data ) with data

Not without changing the ROM contained on the disc.

Low-level formatting should not be done on modern
hard discs except at the factory.

and then there's the joe-blow-me-too-factories that wipe
out the low level formatting with "dd" or equivalent
includhing windoze ghost

I'm afraid that dd cannot see that information, nor
can it destroy it.


(Floppy discs have all three levels done at once
by a single program, usually.

I should have qualified this with "under DOS/Windows

fdformat or superformat ( low level formatting ) is done separately from the filesystem formatting

Yes, fdformat does the first two levels, but not the third.

	mkdos, format a:, mkfs.vfat, ...

Which does the third level. (Though not the fourth.)


Each Partition has a type (OS, more or less) and a status.

and an MBR

There is only one MBR per physical disc. Each partition has
a BR, not an MBR.

The status could be either Bootable (also called Active) or
non-bootable (Inactive). Only up to one (1) partition may be in
an Active state, and if so, it must be a Primary Partition.

not 100% sure but we all know any of these are bootable, each having
enough info to make it bootable ( maybe a difference in terminology )
	/dev/hda1	windoze
	/dev/hda2	debian
	/dev/hda3	redhat

	each partition has its own MBR allowing it to be bootable

I'm afraid you have some misconceptions yourself.

pretty pics

255 heads, 63 sectors/track, 4865 cylinders

you forgot to describe the cyliner/head/sector mapping  :-)

Well, I didn't want to get into the details of LBA. But, since
you mention it....

The original BIOS interface (INT 13) had reserved bit fields in
various registers to request reads and writes. As discs got bigger,
it turned out that the bit fields were not large enough to contain
the maximum number of cylinders (equiv. tracks). So some more unused
bits were co-opted. But eventually even those extra bits ran out.
So, some BIOS allowed one to use a logical/physical interface, called
LBA (Linear Block Addressing) which "fiddled" the actual number of
heads (usually less than 10) into something huge, while reducing the
actual number of cylinders (equiv. tracks) by the same factor.
In this manner, one could use larger discs while maintaining a
sort of compatible interface.

it's sorta hard to have 255 heads on that itty bitty thingie
that flies over the disk platter  :-)

/dev/hda1 * 1 2433 19543041 7 HPFS/NTFS

Here is your primary partition which is active.

important to note that its active, but is NOT required to boot

Depends on what boot program one is using. Since the current
context is Windows, that is the context I used.


This is a primary partition, not bootable.

it is not flagged .. but it can still be bootable
if lilo has boot=/dev/hdb1 or boot=/dev/hdb

It is not bootable by the standard MBR Windows installs, which
is the current context.


the boot sequence is configurable in the BIOS setting

It may or may not be configurable in the ROM boot program


outside of any partition). It then looks for a Boot Marker
(0x55, 0xAA) as the last two bytes of the first sector.

the important thing to remember ... as part of the MBR
and the 4 disk partitions

	512 bytes
	  2 bytes for boot flag
	 64	4 partitons of 16 bytes each
446 is the actual mbr

As I said, some consider the PT to be separate from the MBR,
some consider it to be part of the MBR.


How many times did you quote that? :-)

This message made from 100% recycled bits.
You have found the bank of Larn.
I can explain it for you, but I can't understand it for you.
I speak only for myself, and I am unanimous in that!

Reply to: