All About Disk Geometry and the 1024 Cylinder Limit for Disks.

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  • Topic: Booting, Disk partitioning, Master boot record
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  • Published : October 8, 1999
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All About Disk Geometry and The 1024 Cylinder Limit For Disks.

Large Disk mini-HOWTO
Andries Brouwer, aeb@cwi.nl
v1.0, 960626

1. The problem

Suppose you have a disk with more than 1024 cylinders. Suppose moreover that you have an operating system that uses the BIOS. Then you have a problem, because the usual INT13 BIOS interface to disk I/O uses a 10-bit field for the cylinder on which the I/O is done, so that cylinders 1024 and past are inaccessible.

Fortunately, Linux does not use the BIOS, so there is no problem.

Well, except for two things:

(1) When you boot your system, Linux isn't running yet and cannot save you from BIOS problems.This has some consequences for LILO and similar boot loaders.

(2) It is necessary for all operating systems that use one disk to agree on where the partitions are.In other words, if you use both Linux and, say, DOS on one disk, then both must interpret the partition table in the same way.This has some consequences for the Linux kernel and for fdisk.

Below a rather detailed description of all relevant details.Note that I used kernel version 2.0.8 source as a reference.Other versions may differ a bit.

2.Booting

When the system is booted, the BIOS reads sector 0 (known as the MBR - the Master Boot Record) from the first disk (or from floppy), and jumps to the code found there - usually some bootstrap loader.These small bootstrap programs found there typically have no own disk drivers and use BIOS services.This means that a Linux kernel can only be booted when it is entirely located within the first 1024 cylinders.

This problem is very easily solved: make sure that the kernel (and perhaps other files used during bootup, such as LILO map files) are located on a partition that is entirely contained in the first 1024 cylinders of a disk that the BIOS can access - probably this means the first or second disk.

Another point is that the boot loader and the BIOS must agree as to the disk geometry.It may help to give LILO the `linear' option. More details below.

3.Disk geometry and partitions

If you have several operating systems on your disks, then each uses one or more disk partitions.A disagreement on where these partitions are may have catastrophic consequences.

The MBR contains a partition table describing where the (primary) partitions are.There are 4 table entries, for 4 primary partitions, and each looks like

struct partition { char active;/* 0x80: bootable, 0: not bootable */ char begin[3];/* CHS for first sector */ char type; char end[3];/* CHS for last sector */ int start;/* 32 bit sector number (counting from 0) */ int length; /* 32 bit number of sectors */ };

(where CHS stands for Cylinder/Head/Sector).

Thus, this information is redundant: the location of a partition is given both by the 24-bit begin and end fields, and by the 32-bit start and length fields.

Linux only uses the start and length fields, and can therefore handle partitions of not more than 2^32 sectors, that is, partitions of at most 2 TB.That is two hundred times larger than the disks available today, so maybe it will be enough for the next ten years or so.

Unfortunately, the BIOS INT13 call uses CHS coded in three bytes, with 10 bits for the cylinder number, 8 bits for the head number, and 6 bits for the track sector number.Possible cylinder numbers are 0-1023, possible head numbers are 0- 255, and possible track sector numbers are 1-63 (yes, sectors on a track are counted from 1, not 0). With these 24 bits one can address 8455716864 bytes (7.875 GB), two hundred times larger than the disks available in 1983.

Even more unfortunately, the standard IDE interface allows 256 sectors/track, 65536 cylinders and 16 heads.This in itself allows access to 2^37 = 137438953472 bytes (128 GB), but combined with the BIOS restriction to 63 sectors and 1024 cylinders only 528482304 bytes (504 MB) remain addressable.

This is not enough for present-day disks,...
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