Joerg Wunsch wrote:
> Mike Smith <[EMAIL PROTECTED]> wrote:
> >  - The MBR partition table is not "obsolete", it's a part of the PC 
> >    architecture specification.
> Its design is antique.  Or rather: it's missing a design.  See other
> mail for the reasons.  For FreeBSD, it's obsolete since we don't need
> to rely on fdisk slices.  (Or rather: it's optional.  We can make good
> use of it when it's there, but we don't need to insist on it being
> there.)

No, it isn't.  We specifically have a copy of both the broken and fixed
fdisk tables in the kernel and do a bcmp() to see if the fdisk table that
is included in /boot/boot1 **uncoditionally** is in fact the dangerously
dedicated table.  If it is, then we specifically reject it or we end up
with a disk size of 25MB (50000 sectors).

> > You do realise that "DD" mode does include a (invalid) MBR partition
> > table (now valid, courtesy of a long-needed fix), right?
> Yes, of course, one that is basically ignored by everything.  It has
> always been there, back since 386BSD 0.1.  386BSD 0.0 didn't recognize
> fdisk tables at all, but could only live on a disk by its own (as any
> other BSD before used to).

No, it isn't ignored,  BIOS'es "know" that fdisk partitions end on cylinder
boundaries, and therefore can intuit what the expected geometry is for
the disk in question.  It does this in order to allow the CHS int 0x13
calls to work.  The problem is that the int13 code only allowed for 255 heads,
and the fake "end of disk" entry that is unconditionally in /boot/boot1
specified an ending head number 255 (ie: 256 heads).  When this gets put
into a byte register it is truncated to zero and we get divide by zero

> > I'd love to never hear those invalid, unuseful, misleading opinions
> > from you again.
> As long as you keep the feature of DD mode intact, i won't argue.  If
> people feel like creating disks that aren't portable to another
> controller, they should do.  I don't like this idea.

We can just as easily have bootable-DD mode with a real MBR and have
freebsd start on sector #2 instead of overlapping boot1 and mbr.   This
costs only one sector instead of 64 sectors (a whopping 32K, I'm sure that
is going to break the bank on today's disks).

I'd rather that we be specific about this.  If somebody wants ad2e or da2e
then they should not be using *any* fdisk tables at all.  Ie: block 0
should be empty.  The problem is that if you put /boot/boot1 in there, then
suddenly it looks like a fdisk disk and we have to have ugly magic to
detect it and prevent the fake table from being used.  I would prefer that
the fdisk table come out of /boot/boot1 so that we dont have to have it by
default, and we use fdisk to install the "DD magic table" if somebody wants
to make it bootable.

> But to be honest, see my other article: i never argued to make this
> the default or a recommended strategy in any form.  It should only
> remain intact at all.  Back to the subject, the current warning
> however, is pointless, and has the major drawback to potentially hide
> important console messages.

The console buffer is 32K these days.  You'd have to have around 300
disks to have any real effect on the kernel.

"All of this is for nothing if we don't go to the stars" - JMS/B5

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