a few words on BIOS/FDISK geometry

[Jerry McAllister]

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
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