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All About Hard drive Geometry plus the 1024 Cyndrical tube Limit pertaining to Disks.

 All About Drive Geometry plus the 1024 Cylinder Limit to get Disks. Article

All About Disk Geometry and The 1024 Cylinder Limit For Hard disks.

Large Disk mini-HOWTO

Andries Brouwer, [email protected] nl

v1. 0, 960626

1 . The condition

Suppose you have a drive with more than 1024 cylinders. Imagine moreover you have an operating system that uses the BIOS. Then you certainly have a problem, since the usual INT13 BIOS software to drive I/O uses a 10-bit discipline for the cylinder on what the I/O is done, to ensure that cylinders 1024 and past are hard to get at.

Fortunately, Cpanel does not make use of the BIOS, thus there is no trouble.

Well, apart from two things:

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

(2) It is necessary for all operating systems that use a single disk to agree on in which the partitions are. In other words, when you use both Linux and, state, DOS using one disk, then both must interpret the partition desk in the same way. It has some implications for the Linux nucleus and for fdisk.

Below an extremely detailed information of all relevant details. Remember that I used kernel edition 2 . zero. 8 supply as a reference point. Other variations may differ a bit.

2 . Booting

When the strategy is booted, the BIOS says sector zero (known as the MBR - the Master Start Record) from the first drive (or via floppy), and jumps to the code located there - usually several bootstrap termes conseilles. These tiny bootstrap applications found there typically have not any own hard disk drive drivers and use BIOS services. Which means that a Linux kernel can only be booted when it is completely located within the first 1024 cylinders.

This matter is very easily solved: make sure that the kernel (and perhaps other data used during bootup, including LILO map files) can be found on a rupture that is entirely contained in the 1st 1024 cylinders of a disk that the BIOS can gain access to - most likely this means the first or second drive.

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

3. Hard disk drive geometry and partitions

For those who have several operating systems on your hard disk drives, then every uses a number of disk partitioning. A difference on in which these partitioning are may have huge consequences.

The MBR includes a partition table describing where the (primary) partitions will be. There are some table entries, for four primary partitions, and each seems like

struct zone 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 means Cylinder/Head/Sector).

Hence, this information can be redundant: the positioning of a canton is given both by the 24-bit begin and end areas, and by the 32-bit commence and span fields.

Cpanel only uses the start and length fields, and can consequently handle partitions of only 2^32 groups, that is, partitions of at most of the 2 TB. That is two hundred times larger than the hard disk drives available today, therefore maybe will probably be enough for ten years or so.

Unfortunately, the BIOS INT13 call uses CHS coded in 3 bytes, with 10 bits for the cylinder quantity, 8 pieces for your head number, and 6 bits for the track sector number. Conceivable cylinder quantities are 0-1023, possible head numbers will be 0- 255, and likely track sector numbers will be 1-63 (yes, sectors on a track happen to be counted via 1, not really 0). With these twenty-four bits one can address 8455716864 bytes (7. 875 GB), two hundred times larger than the disks obtainable in 1983.

A lot more unfortunately, the conventional IDE software allows 256 sectors/track, 65536 cylinders and 16 heads. This by itself allows usage of 2^37 = 137438953472 octet (128 GB), but combined with BIOS limit to 63 sectors and 1024 cyl only 528482304 bytes (504 MB) continue to be addressable.

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

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