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Re: stage 1

From: Jeroen Dekkers
Subject: Re: stage 1
Date: Mon, 1 Mar 2004 20:39:55 +0100
User-agent: Mutt/

On Sun, Feb 15, 2004 at 06:44:47PM +0100, Wolf Lammen wrote:
> C. Bugs
> If all the above went good, I would say 'wow', but, sadly, that is sometimes
> not the case. Especially with old hardware, there are some issues that I
> think need refining. The weak point is the 3. probing strategy, the floppy
> probing. IMHO, the current implementation renders this strategy useless (there
> might be
> some relation to Bug #482). So either remove it completely, or activate it
> in a correct fashion.
> What goes wrong here, is a misunderstanding of BIOS call INT 0x13, function
> 8. This call is used to retrieve the geometry of a drive. The drive and not
> the inserted media. So, if the capabilities of a drive and media differ (a
> 1.44 MB floppy in a 2.88 MB drive), GRUB currently uses a wrong geometry to
> access the floppy disk.
> Further on, GRUB invokes the floppy probing only, if the above BIOS call
> returns an error. I admit, I do not understand why it is programmed that way.
> The call might fail for two reasons: the drive is not accessible by BIOS, or
> the battery of the CMOS RAM is exhausted, so the parameters of the drive could
> not be read. But in either case, how could reading from a floppy succeed at
> all, and floppy probing could help out?
> Fortunately, modern computers are not that susceptible to this bug. First,
> the number of computers without floppy drive is growing. Second, modern floppy
> drives or modern BIOSes, whatever is responsible for the following effect,
> seems to abstract from the real format of a floppy. I was really surprised to
> see both my drives accessing a more than 10 years old 720 KB floppy using the
> geometry of a 1.44 MB disk and assuming a track size of 18 sectors! And they
> succeeded! I knew for sure it was formatted according to the then standard
> with 9 sectors/track, and I made sure, this value was written on the disk
> label as well. With such a drive the above bug does not show up, of course.
> Third, a normal GRUB installation of stage 1 and stage 1.5 fits completely
> into 18 sectors, so with nowadays diskettes and drives you do not reach into
> the critical area beyond the first track.
> However, it's still a bug and should be dealt with. I would like to hear
> some opinions on this subject.

I think this bug isn't causing any trouble because most people use
1.44 floppies in a 1.44 floppy drive today. But it is a bug and should
be fixed. I think the right fix is to move the jump to floppy_probe to
just after chs_mode. What do you think?

> D. Software Design
> Some of the complexity of stage 1 could be put down to the fact that it is
> designed as an allrounder. Whether you use it on a hard disk or on a floppy
> disk does not matter at all. This was convenient at times when there were only
> two media types bootable. You simply copy the boot track from a floppy
> disk to a hard disk, and all works fine.
> But this concept is broken for quite some time already. First, you see
> plenty of other bootable media such as a CD-ROM. Second, because of BIOS bugs,
> GRUB already patches stage 1 when it loads it onto a hard drive, so simple
> copying won't work any more already.
> In addition, the tight memory restrictions do not allow for more BIOS bug
> workarounds or enhancements of drives.
> So, why not step back from this concept and create dedicated stage 1s. The
> installer uses out of a pool the one suiting the destination media. Since the
> code could be stream-lined, more functionality and richer error diagnosis
> could be provided. For instance, a floppy stage 1 could deal with various
> special formats. A hard disk stage 1 does not die any more, if an 
> ECCcorrected load
> occurs. And so on.
> Using such a 'plugable' stage 1 concept requires a redesign of the current
> stage1/stage1.5 interface. IMHO, the current protocol between both stages
> looks a bit strange. Stage 1.5 knows too much about the interior of stage 1, 
> and,
> since stage 1 has a loader built in, a loading service should be provided
> to other stages.
> I'd like to hear some opinions to this too.

I see what you mean, but GRUB 0.94 is in maintaince mode now. But with
GRUB 2 it's a good time to redesign stage 1. I think splitting it up
in stage1s for floppy, hard disk and (eventually) CDs is a good idea.

Jeroen Dekkers

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