In order to read or write something from or to the disk, we have to specify a position on the disk, for example by giving a sector or block number. If the disk is a SCSI disk, then this sector number goes directly into the SCSI command and is understood by the disk. If the disk is an IDE disk using LBA, then precisely the same holds. But if the disk is old, RLL or MFM or IDE from before the LBA times, then the disk hardware expects a triple (cylinder,head,sector) to designate the desired spot on the disk.
The correspondence between the linear numbering and this 3D notation
is as follows: for a disk with C cylinders, H heads and S sectors/track
position (c,h,s) in 3D or CHS notation is the same as position
c*H*S + h*S + (s-1) in linear or LBA notation.
(The minus one is because traditionally sectors are counted from 1,
not 0, in this 3D notation.)
Consequently, in order to access a very old non-SCSI disk, we need to know its geometry, that is, the values of C, H and S.
Linux does not use the BIOS, but some other systems do.
The BIOS, which predates LBA times, offers with INT13
disk I/O routines that have (c,h,s) as input.
(More precisely: AH selects the function to perform,
CH is the low 8 bits of the cylinder number, CL
has in bits 7-6 the high two bits of the cylinder number
and in bits 5-0 the sector number, DH is the head number,
and DL is the drive number (80h or 81h).
This explains part of the layout of the partition table.)
Thus, we have CHS encoded in three bytes, with 10 bits for the cylinder number, 8 bits for the head number, and 6 bits for the track sector number (numbered 1-63). It follows that cylinder numbers can range from 0 to 1023 and that no more than 1024 cylinders are BIOS addressable.
DOS and Windows software did not change when IDE disks with LBA support were introduced, so DOS and Windows continued needing a disk geometry, even when this was no longer needed for the actual disk I/O, but only for talking to the BIOS. This again means that Linux needs the geometry in those places where communication with the BIOS or with other operating systems is required, even on a modern disk.
This state of affairs lasted for four years or so, and then disks appeared on the market that could not be addressed with the INT13 functions (because the 10+8+6=24 bits for (c,h,s) can address not more than 8.5 GB) and a new BIOS interface was designed: the so-called Extended INT13 functions, where DS:SI points at a 16-byte Disk Address Packet that contains an 8-byte starting absolute block number.
Very slowly the Microsoft world is moving towards using these Extended INT13 functions. Probably a few years from now no modern system on modern hardware will need the concept of `disk geometry' anymore.
At most 65536 cylinders (numbered 0-65535), 16 heads (numbered 0-15), 255 sectors/track (numbered 1-255), for a maximum total capacity of 267386880 sectors (of 512 bytes each), that is, 136902082560 bytes (137 GB). This is not yet a problem (in 1999), but will be a few years from now.
At most 1024 cylinders (numbered 0-1023), 256 heads (numbered 0-255), 63 sectors/track (numbered 1-63) for a maximum total capacity of 8455716864 bytes (8.5 GB). This is a serious limitation today. It means that DOS cannot use present day large disks.
If the same values for c,h,s are used for the BIOS Int 13 call and for the IDE disk I/O, then both limitations combine, and one can use at most 1024 cylinders, 16 heads, 63 sectors/track, for a maximum total capacity of 528482304 bytes (528MB), the infamous 504 MiB limit for DOS with an old BIOS. This started being a problem around 1993, and people resorted to all kinds of trickery, both in hardware (LBA), in firmware (translating BIOS), and in software (disk managers). The concept of `translation' was invented (1994): a BIOS could use one geometry while talking to the drive, and another, fake, geometry while talking to DOS, and translate between the two.
Some older BIOSes only allocate 12 bits for the field in CMOS RAM that
gives the number of cylinders. Consequently, this number can be at most
4095, and only 4095*16*63*512=2113413120 bytes are accessible.
The effect of having a larger disk would be a hang at boot time.
This made disks with geometry 4092/16/63 rather popular. And still today
many large disk drives come with a jumper to make them appear 4092/16/63.
See also
over2gb.htm.
There was a bug in the Phoenix 4.03 and 4.04 BIOS firmware that would cause the system to lock up in the CMOS setup for drives with a capacity over 3277 MB. See over3gb.htm.
Simple BIOS translation (ECHS=Extended CHS, sometimes called `Large
disk support' or just `Large')
works by repeatedly doubling the number of heads and halving the number
of cylinders shown to DOS, until the number of cylinders is at most 1024.
Now DOS and Windows 95 cannot handle 256 heads,
and in the common case that the disk reports 16 heads, this means that
this simple mechanism only works up to 8192*16*63*512=4227858432 bytes
(with a fake geometry with 1024 cylinders, 128 heads, 63 sectors/track).
Note that ECHS does not change the number of sectors per track, so if
that is not 63, the limit will be lower.
See
over4gb.htm.
Slightly smarter BIOSes avoid the previous problem by first adjusting the
number of heads to 15 (`revised ECHS'), so that a fake geometry with
240 heads can be obtained, good for 1024*240*63*512=7927234560 bytes.
Finally, if the BIOS does all it can to make this translation a success,
and uses 255 heads and 63 sectors/track (`assisted LBA' or just `LBA')
it may reach 1024*255*63*512=8422686720 bytes, slightly less than the
earlier 8.5 GB limit because the geometries with 256 heads must be avoided.
(This translation will use for the number of heads the first value H
in the sequence 16, 32, 64, 128, 255 for which the total disk capacity
fits in 1024*H*63*512, and then computes the number of cylinders C as
total capacity divided by (H*63*512).)
For another discussion of this topic, see Breaking the Barriers, and, with more details, IDE Hard Drive Capacity Barriers.
Hard drives over 8.4 GB are supposed to report their geometry as 16383/16/63. This in effect means that the `geometry' is obsolete, and the total disk size can no longer be computed from the geometry.