Re: Disk mapping confusion with the device command

[Molle Bestefich]

Molle Bestefich wrote:
> [side note]
> For some reason, I've never seen a BIOS adjust the sectors/track
> number.  I haven't done the math, but one could suspect that there's
> no gain in adjusting the s/t number, at least when already adjusting
> the head numbers?  Could probably be easily determined - find out how
> many divisions are involved in the calculation, for each division
> there's a rounding error which can be minimized if you fiddle with the
> numbers.  (One would have to trust that all BIOS manufacturers
> actually chose the equation with the least number of divisions.....)
> [/side note]
>
> The discussion is a bit moot when it comes to actually booting new
> systems, however.  They all support LBA extensions to int13 anyway.
> The only thing that matters with new systems is to make sure that the
> CHS numbers in MBR cause the partitions to _land on cylinder
> boundaries_ - otherwise neither DOS nor Win9x/ME will boot.  I suspect
> that Win2000/XP/2003 will not either, though it will take some testing
> before I'm 100% sure.

(Would probably take some code disassembly to get to know the exact
DOS boot loader / filesystem code / FDISK behaviour as well....)

> [side note] Not a particular hard feature to implement, but for some
> reason neither FDISK nor Partition Magic can do this.  Guess neither
> tool expects that people move disks from one BIOS to another or fiddle
> with BIOS settings or 'dd' from a smaller drive to a larger one.
> [/side note]

I realize I've gone, uhm, a bit off-topic from the original
discussion, but since I'm venting anyway, I have a last bit I'd like
to have added to the puzzle :-).

As I said above, aligning the start of a partition to a cylinder
boundary thus allowing various braindead operating systems to work is
an easy feat.  (I'm assuming that no known filesystem uses absolute
disk sector numbers, only start-of-partition-relative numbers.  If
anyone thinks I'm wrong, speak up.)

PartitionMagic can't do this feat - it will bark an error message at
you and quit.  But it does have another interesting feature.  If the
start of the partition is aligned correctly, but the end isn't (I
think!), it adjusts the Head number in the end CHS coordinates.  It
leaves Cylinder and Sector maxed out, but fiddles with the Head
number.  I'm imagining that perhaps the PowerQuest folks did thorough
analyses and found that some other tool or operating system code
messes up your disk if you just max out both numbers, but I can't be
entirely sure.  If someone can shed some light on this, I'd be very
interested in knowing what the motivation is :-).  Also it might very
well be something that's worth fixing in Linux FDISK.