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3. Units and Sizes

A kilobyte (kB) is 1000 bytes. A megabyte (MB) is 1000 kB. A gigabyte (GB) is 1000 MB. A terabyte (TB) is 1000 GB. This is the SI norm. However, there are people that use 1 MB=1024000 bytes and talk about 1.44 MB floppies, and people who think that 1 MB=1048576 bytes. Here I follow the recent standard and write Ki, Mi, Gi, Ti for the binary units, so that these floppies are 1440 KiB (1.47 MB, 1.41 MiB), 1 MiB is 1048576 bytes (1.05 MB), 1 GiB is 1073741824 bytes (1.07 GB) and 1 TiB is 1099511627776 bytes (1.1 TB).

Quite correctly, the disk drive manufacturers follow the SI norm and use the decimal units. However, Linux kernel boot messages (for not-so-recent kernels) and some fdisk-type programs use the symbols MB and GB for binary, or mixed binary-decimal units. So, before you think your disk is smaller than was promised when you bought it, compute first the actual size in decimal units (or just in bytes).

Concerning terminology and abbreviation for binary units, Knuth has an alternative proposal, namely to use KKB, MMB, GGB, TTB, PPB, EEB, ZZB, YYB and to call these large kilobyte, large megabyte, ... large yottabyte. He writes: `Notice that doubling the letter connotes both binary-ness and large-ness.' This is a good proposal - `large gigabyte' sounds better than `gibibyte'. For our purposes however the only important thing is to stress that a megabyte has precisely 1000000 bytes, and that some other term and abbreviation is required if you mean something else.

3.1 Sectorsize

In the present text a sector has 512 bytes. This is almost always true, but for example certain MO disks use a sectorsize of 2048 bytes, and all capacities given below must be multiplied by four. (When using fdisk on such disks, make sure you have version 2.9i or later, and give the `-b 2048' option.)

3.2 Disksize

A disk with C cylinders, H heads and S sectors per track has C*H*S sectors in all, and can store C*H*S*512 bytes. For example, if the disk label says C/H/S=4092/16/63 then the disk has 4092*16*63=4124736 sectors, and can hold 4124736*512=2111864832 bytes (2.11 GB). There is an industry convention to give C/H/S=16383/16/63 for disks larger than 8.4 GB, and the disk size can no longer be read off from the C/H/S values reported by the disk.

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