a few words on BIOS/FDISK geometry

[Yuri GV]

Yea, it must must be implemented as a doc.

> Wow,
>
> Have you put this up as a FAQ somewhere?   It seems like a good idea
> to do so.
>
> ////jerry
>
>> I find most of the BIOS/MBR/FDISK disk geometry gospel that has recently
>> appeared in freebsd-questions to be confusing if not actually incorrect.
>> In the interest of world peace of mind, I feel compelled to offer my own
>> model of reality.  It really isn't that complicated.
>>
>> There are two common ways in which disk sector addresses are expressed.
>> These are the LBA (Logical Block Address) number and the C/H/S 
>> (Cylinder/
>> Head/Sector) numbers.  As pretty much everyone knows, the LBA and C/H/S
>> values are related by an expression similar to:
>>
>> 	LBA = (C*NH + H)*NS + S
>>
>> where NH is the number of heads and NS is the number of sectors/track.
>> C/H/S used to be the most common address representation but LBA has 
>> since
>> gained popularity because it is conceptually simpler (is only one 
>> number)
>> and because C/H/S numbers are typically limited to inconveniently small
>> values.  The physical significance of the NH and NS values has been
>> largely eroded by the advancement of technology.  We now only use these
>> values when converting between sector address representations.
>>
>> The system BIOS provides a basic disk access facility sometimes called
>> "int13".  There are different int13 "functions" for things like reading,
>> writing and obtaining disk parameters such as geometry.  The original
>> "basic" int13 functions, implemented by essentially all versions of PC
>> BIOS, expect sector addresses to be in C/H/S format.  There is also a 
>> set
>> of "extended" int13 functions, implemented by newer BIOS, that expect
>> sector addresses to be in 64 bit LBA format.
>>
>> The disk geometry assumed by the basic int13 functions is what we mean
>> by the term "BIOS geometry".  The BIOS may describe different geometries
>> for a single disk drive in different contexts.  We only care about the
>> geometry the BIOS uses to interpret the disk addresses used with the
>> basic int13 functions.  Note that the BIOS geometry may not be related
>> to any physical or logical geometry used by the disk itself.
>>
>> The common FreeBSD master bootstrap program may be installed and
>> configured with the "boot0cfg" command.  It uses the basic int13
>> functions by default but may be configured to use the extended functions
>> (the "packet" option).  When a FreeBSD partition is booted, the boot0
>> program boots the boot1 program in the second sector of the partition.
>> The boot1 program in turn boots the boot2 program.  I don't know if
>> these programs use basic or extended int13 functions or at what point
>> in the bootstrap sequence the bootstrap programs stop using the BIOS.
>>
>> The MBR (Master Bootstrap Record) partition table (aka FreeBSD slice
>> table) which is stored in the first sector of most PC disk drives
>> contains the starting address of each partition in both C/H/S and LBA
>> format.  There are 10 bits in the cylinder field, 8 bits in the head
>> field, 6 bits in the sector field and 32 bits for the LBA field.
>> By (MS?) convention cylinder and head numbers begin at 0 but the first
>> sector number is 1.  There is allegedly some important program (unknown
>> to me) which limits the number of heads to 255.  Programs that use the
>> basic BIOS int13 functions to access partitions defined in an MBR can
>> address at most 1024 cylinders, 255 heads and 63 sectors (somewhat less
>> than 8 GB).
>>
>> (An explanation of the many disk sizes to which PC systems are sometimes
>> limited is tempting but way beyond the scope of this posting.)
>>
>> The FreeBSD fdisk program needs to know the disk geometry only when
>> filling in the C/H/S fields in the MBR partition table.  If it gets the
>> geometry wrong, bootstrap programs that use the basic int13 functions
>> may fail.  (Programs that use the extended int13 functions will not
>> be affected!)
>>
>> The FreeBSD fdisk program sometimes gets the BIOS geometry wrong and we
>> have to correct it.  How can we determine the correct BIOS geometry of a
>> disk drive in this case?  BIOS configuration user interfaces can be
>> confusing and the disk drive geometries they report may not always be
>> those used by the basic int13 functions.  The only (usually) reliable 
>> way
>> to get a BIOS disk geometry may be to ask the BIOS via one of the int13
>> functions or to read it out of one of the data structures left behind by
>> the BIOS POST (power on self test).
>>
>> Sometimes if we boot a FreeBSD kernel with the "-v" option it will tell
>> us the BIOS geometries during the autoconfiguration monologue.  I am not
>> sure that I trust it.  Sometimes software will report disk controller
>> interface geometry instead.  (Hint: if a geometry specifies more than
>> 255 heads or 63 sectors/track, you know it is not the BIOS geometry.)
>> I sometimes boot grub (see /usr/ports/sysutils/grub) off a floppy and 
>> ask
>> it about a disk drive with the "geometry" command.  As far as I know,
>> this will reliably report the BIOS geometry.
>>
>> Modern BIOS geometry most frequently uses 255 heads and 63 sectors/track
>> because that maximizes the addressable part of the disk drive using the
>> basic int13 functions.  Cylinder numbers greater than 1023 don't really
>> matter because whatever you put in the MBR will be wrong.  I typically
>> manually set them to 1023.  Remember that you can only boot partitions
>> beyond cylinder 1023 if all bootstrap programs use the extended int13
>> functions.
>>
>> Dan Strick
>> strick at covad.net