Disk - Capacity (Raw Capacity|Size)


Raw capacity is a characteristic of a computer storage and is the total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of bits or bytes (e.g. 10.4 megabytes).

This cd-rom can hold more information than all of the paper that's here below me

Example of Maximum disk capacity

In 1998, with a 24 bit address mode and because all hard disks are low-level formatted with a standard 512-byte sector, the maximum disk capacity described by the partition table is calculated as follows:

  Maximum capacity = sector size x cylinders (10 bits) x 
                     heads (8 bits) x sectors per track (6 bits)

Using the maximum possible values yields:

512 x 1024 x 256 x 63 (or 512 x 2^24) = 8,455,716,864 bytes or 7.8 GB
Year Cylinders Head Sectors per track Total per byte Sector size Maximum Capacity
Address Byte 1998 10 8 6 24 512 7.8 GB
Maximum capacity
per byte (2^n)
1998 1024 256 63 15,75 MB 512 7.8 GB

Some drives are sold by formatted size, others by unformatted size. The formatted size of a drive is approximately 85% of its unformatted size.

The calculation results in a maximum capacity of slightly less than 8 gigabytes (GB). Before BIOS INT 13h extensions drive geometry translation (also known as logical block addressing (LBA) were introduced in 1998, the active, primary partition could not exceed 7.8 GB, regardless of the file system used.

Since 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 This in effect means that the “geometry” is obsolete, and the total disk size can no longer be computed from the geometry, but is found in the LBA capacity field returned by the IDENTIFY command.
  • 137.4 GB are supposed to report an LBA capacity of 0xfffffff = 268435455 sectors (137438952960 bytes).
  • Now the actual disk size is found in the new 48-capacity field.

Maximum capacity by Byte

Byte Capacity
1 2
2 4
3 8
4 16
5 32
6 64
7 128
8 256
9 512
10 1024

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