On Sat, Jul 07, 2007 at 04:31:21AM -0400, Matthew Szudzik wrote:
> > Disk: sd0 geometry: 3935/64/32 [8060926 Sectors]
> > Offset: 0 Signature: 0xAA55
> > Starting Ending LBA Info:
> > #: id C H S - C H S [ start: size ]
> > ------------------------------------------------------------------------
> > 0: 0B 0 1 32 - 3935 63 17 [ 63: 8060850 ] Win95 FAT-32
> > 1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> > 2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> > 3: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
>
> Ah, I now understand. Windows is assuming that each track has 63 sectors,
> and apparently requires that each partition fills an integer number of
> tracks. The ending CHS values are chosen so that the maximum number of
> 63-sector tracks are filled.
>
> So, to create the appropriate MBR partition for a drive, type the
> following (I assume that the drive is device sd0):
>
>
> # fdisk -i -e sd0
> fdisk: sysctl(machdep.bios.diskinfo): Device not configured
>
> -----------------------------------------------------
> ------ ATTENTION - UPDATING MASTER BOOT RECORD ------
> -----------------------------------------------------
>
> Do you wish to write new MBR and partition table? [n] y
> Enter 'help' for information
> fdisk: 1> print
> Disk: sd0 geometry: 3935/64/32 [8060926 Sectors]
> Offset: 0 Signature: 0xAA55
> Starting Ending LBA Info:
> #: id C H S - C H S [ start: size ]
> ------------------------------------------------------------------------
> 0: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> 1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> 2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> *3: A6 0 1 1 - 3934 63 32 [ 32: 8058848 ] OpenBSD
>
>
> Take careful note of the total number of sectors (8060926 in this case).
> Let NUM be the number of sectors divided by 63, rounded down to the
> nearest integer. Then, let SIZE be
> 63*(NUM - 1)
> In this example, NUM is 127951 and SIZE is 8060850. Continue as follows:
>
>
> fdisk: 1> edit 3
> Starting Ending LBA Info:
> #: id C H S - C H S [ start: size ]
> ------------------------------------------------------------------------
> *3: A6 0 1 1 - 3934 63 32 [ 32: 8058848 ] OpenBSD
> Partition id ('0' to disable) [0 - FF]: [A6] (? for help) 0
> Partition 3 is disabled.
> fdisk:*1> edit 0
> Starting Ending LBA Info:
> #: id C H S - C H S [ start: size ]
> ------------------------------------------------------------------------
> 0: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> Partition id ('0' to disable) [0 - FF]: [0] (? for help) 0B
> Do you wish to edit in CHS mode? [n] n
> offset: [0] 63
>
>
> At the next prompt, enter the number SIZE that we had previously
> calculated.
>
>
> size: [0] 8060850
> fdisk:*1> print
> Disk: sd0 geometry: 3935/64/32 [8060926 Sectors]
> Offset: 0 Signature: 0xAA55
> Starting Ending LBA Info:
> #: id C H S - C H S [ start: size ]
> ------------------------------------------------------------------------
> 0: 0B 0 1 32 - 3935 63 17 [ 63: 8060850 ] Win95 FAT-32
> 1: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> 2: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> 3: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
> fdisk:*1> quit
> Writing current MBR to disk.
>
>
> And notice that the 3935 that I could not explain is immediately accounted
> for. The problem is solved!
Nice work!
Notice that the 8060926 sectors the disk has in LBA mode is more than
3935 * 64 * 32 = 8058880, so there are 2046 more sectors in LBA
mode than in CHS mode.
Assuming 63 sectors per track instead and ignoring cylinder and head
count gives a spill of 13 sectors. I would have guessed Windows
used the classical fake geometry of 16383/16/63, and maybe it is:
if it also does not need to place partitions on cylinder
boundaries it all adds up.
Maybe you can use the 'e' command in disklabel to set drive parameters
to 63 sectors/track, 16 tracks/cylinder, 1008 sectors/cylinder,
7997 cylinders and 8060926 sectors. That would make the last disk
sector have CHS address 7996/15/13, 7996/15/14-63 being nonexistent,
and the last sector of the last whole track would be CHS 7996/14/63.
Partition in 7997/16/63 geometry:
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
------------------------------------------------------------------------
0: 0B 0 1 1 - 7996 14 63 [ 63: 8060850 ] Win95 FAT-32
That was a lot of guessing from me. Does it make sense?
--
/ Raimo Niskanen, Erlang/OTP, Ericsson AB
