[syslinux] After USB boot problems on Gigabyte GA-M55Plus-S3G

[H. Peter Anvin]

On 01/21/2014 12:22 AM, Thomas Schmitt wrote:
> 
>> If we take the ENDing CHS values as valid:
>>  982 / 254 / 63 
>> and we calculate the corresponding LBA:
>>   ( ( 982 + 1 )
>>  x ( 254 + 1 )
>>  x ( 63 ) )
>>  - 1
>>    ___________
>>    15'791'894 
> 
> Is this calculation generally valid ? It seems to assume that
> the end is aligned to a full cylinder. (The start is not.)

It isn't.  To calculate the LBA from an arbitrary CHS address when the
geometry is C':H':S':

LBA = (C * H' * S') + (H * S') + (S - 1)

or equivalently

LBA = ((C * H') + H) * S' + S - 1

> It is clear that S/H must be 63 and H/C must be 255, because we
> see the maximum possible values here. (H/C could be 256. But
> that is outruled by Wikipedia about MBR.)
> 
> Nevertheless, this does not match the end LBA calculated
> from start LBA and number of blocks.
> 
> Do i miss an essential point here ?

I think the essential point is that the CHS geometry offered on the
preformatted stick seems to be "impossible".  Most likely the
manufacturer simply did not care.

Consider this:

8 GB Corsair:
Partition type  0x0b  (Wikipedia: FAT32, CHS or LBA)
Start LBA       63 = CHS    0   1   1
End   LBA 15794175 = CHS  982 254  63

If LBA 63 = CHS 0/1/1 then we know S'=62, but that is completely
inconsistent with the ending sector being 63.  In other words, the CHS
information in this partition table is complete garbage.

>> If there is nothing to loose (some special feature in the USB drive, 
>> data, etc.), probably it would be best to create a new partition 
>> table, a new partition and then format them again.
> 
> Seems advisable, given the numerical madness.
> 
> This is were i see a gap in SYSLINUX wiki. The world could need
> prescriptions for a non-weird partitioning and a LBA driven
> filesystem.

Fortunately geometry is becoming a fading problem.  We're dealing with
an old board and BIOS here.

> Biggest riddle for me, though, is how to achieve this with Linux
> tools and MS-Windows tools. (I know /sbin/fdisk -lu. Everything
> else i do by byte manipulations in images. Plumber style.)

	-hpa