Large Disk mini-HOWTO: Disk geometry and partitions

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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.

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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, and people resort to all kinds of trickery, both in hardware and in software.

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