Re: maximum hard drive size in Latitude cpi

J. Clarke <jclarke.usenet@xxxxxxxxxxxxxxxx> wrote:
> Something I think that all the self-proclaimed experts around here ought to
> do is take the sophomore or junior level operating systems lab at a
> university with a decent computer science department. Once they've

Thank you, but some of us are university professors of computer science,
hardware and software engineering, with twenty years of experience in
writing boot loaders and operating systems, and indeed, hardware, of
various sorts. Not to mention the books on it, not to mention the hundred
odd peer reviewed articles.

> actually written a boot loader they won't be so adamant about how "Windows
> can this" and "Windows can that".

Windows can do whatever it wants to do. That is a given of computing!

> The difficulty that the overlook is that Windows doesn't get burned into
> firmware and it doesn't load itself by teleportation--it has to be read off
> of the disk, and while it is being read off of the disk the machine is
> under the control of something other than Windows, specifically it is under
> the control of the boot loader built into the firmware on the motherboard,
> and that firmware accesses the disk via the BIOS routines and has the
> limitatoins of that BIOS. Once the firmware has transferred control to
> Windows, _then_ Windows can query the disk for its capacity and respond
> accordingly, but until that happens the Windows has no part in the matter.

It's hard to answer this without saying "so what"! The bios only needs
access to the bit of the disk it needs to have access to in order to
boot the o/s kernel. After that the kernel takes over. Windows kernel
needs (or needed, up to winnt) to be placed in the first 2GB of the disk
for that to happen, because it actually used signed 32-bit byte offsets
(as far as I recall) in its internal early setup routines. The bios can
always reach that far in any remotely modern system.

> Linux addresses this issue by having a small boot partition with a copy of
> the OS kernel on it,

No it doesn't. I am a linux kernel driver author - I should at least
KNOW. YOU (not linux) CAN address "this [bios] issue" by placing the
o/s kernel in a zone where the bios can boot it, which may well be in a
small boot partition near the front of the disk, whch will confine it
nicely, thanks. Or on a floppy. Anywhere the bios can boot it without
problems, in fact!

That's neither here nor there. The O/P's confusion apparently comes
because he thinks there is some kind of "magic overlay" intervening in
the disk i/o (or he doesn't know what disk i/o means). No. Not. Nix. Nyet.
I hope that is absolutely clear!

He doesn't seem to grok that figuring out how big the disk is an
administrative thing the o/s does while starting up, and has nothing to
do with disk i/o. The o/s is free to ask the disk how big it is, or ask
the bios how big it thinks the disk is, or simply try reading further
and further ahead until it gets an error! Anything will work.

Windows apparently asks the bios and then handicaps itself to the relayed
size. There is no good reason for doing that. It means that one must get
the bios to lie about it if the bios doesn't know. The so-called
"overlay" merely fixes the bios before the o/s boots in order that it
will give the right answer when asked. As soon as the o/s has got a
millisecond or two int bootup, the bios is history. Gone. Inaccessible.
Nada.

> with the necessary drivers built into the kernel.

Absolutely.

> Novell accomplishes this by using MS-DOS as a second stage boot loader--you

No, LINUX (or anybody) accomplishes it that way. See freedos. See
loadlin.exe. This is all irrelevant (heh - and your theory doesn't take
into account that according to the OP msdos can't "see" the disk yet).

> boot DOS, then boot Novell from a folder that has the OS kernel and disk
> drivers in it. I'm not sure exactly _what_ Microsoft has done, but they do
> seem to have managed to get NT and its descendants to under some
> circumstances boot from partitions larger than the BIOS can access--it's my

That's because it's completely irrelevant. The only thing that matters is
the (bios) commands placed in the MBR or in secondary boot sequences
elsewhere. THOSE commands merely have to use an addressing scheme that
can correctly address the next stage in the boot procedure. For linux,
the boot process is generally either two stage (lilo plus kernel) or
three stage (grub plus grub secondary loader plus kernel), but any
sequence can be followed. You can boot it from winnt if you want!

The only important point is that the bios commands placed in the early
stage boot loaders must use an addressing scheme supported by the bios to
load the next stage. That means, if you are using LBA addressing
commands for the BIOS, the next stage can be located anywhere within
2^28 512B sectors of the start of the disk. I.e. 128GB (I think the
sector offset field in those bios commands is 28 bits wide, by memory).


> understanding though that there are limitations on where the kernel and
> drivers can be located on that partition--sometimes a defrag can move one

Thanks, but we understand just fine.


> Overlays work, but they shouldn't be used until all other options have been

No they don't - they don't exist when the o/s starts running! They can
only work by enhancing (or replacing) the bios, which is only helpful if
you place your kernel is some outlandish position, or by hooking
interrupts in msdos. The bios is HISTORY after the o/s has started
booting, as soon as the cpu leaves real mode.

> explored, and they should be used with an understanding of how they operate
> and what their limitations are and what problems can occur as a result of

Exactly.

> their use. For example "fdisk /mbr", a common step in the removal of a
> boot-sector virus, usually wipes the overlay and all of a sudden the system
> is unusable until the overlay is restored.

No - it's usable perfectly fine thanks. The mswindows fdisk /mbr merely
restores a standard MBR.

> Ontrack's latest overlay for Windows does, I am surprised to find, address
> the 48-bit issue and support Windows 2K and XP with NTFS (with some
> limitations), so they're still working at it--Ontrack's is the one that is

No "support" is required. You can boot windows from linux if you like!

> usually bundled with disk drives, but they seldom include the latest
> version and it's never full featured.

> You might want to google '"Dynamic Disk Overlay" +NTFS' (include the double

It turns up nothing for me! It points to

http://www.sysopt.com/forum/showthread.php?threadid=148235

which is just some silly commercial company promoting its bogusware via a
bogus agony aunt page.

> quotes, not the single) for more information about this. Also, Ontrack's
> page for their latest version is at <http://www.ontrack.com/diskmanager/>.
> Note that toward the bottom of the page there is a link to a "tech paper"
> that gives the rundown on what the current version can and cannot do.
>>

I'd have to ask if you are employed by ontrack. There is no real
substance in what you are trying to say. To my eyes it looks like
pompous, self-important rubbish.

> --John
> to email, dial "usenet" and validate
> (was jclarke at eye bee em dot net)

Peter
.

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