Computer measurements are expressed in both binary and decimal terms, often
using the same notation. Due to a mathematical coincidence, the fact that 2^10 (1024) is
almost the same number as 10^3 (1000), there are two similar but different ways to express
a megabyte or a gigabyte.
The problems with binary and decimal are probably more noticed in the area of hard disk
capacity than anywhere else. Hard disk manufacturers always use binary figures for their
products' capacity: a 72 GB hard disk has about 72,000,000,000 bytes of storage. However,
hard disk makers also use binary numbers where they are normally used--for example, buffer capacities are expressed in binary kilobytes or
megabytes--but the same notation ("kB" or "MB") is used as for decimal
figures. Hard disks are large, and larger numbers cause the discrepancy between decimal
and binary terms to be exaggerated. For example, a 72 GB hard disk, expressed in binary
terms, is "only" 67 GB. Since most software uses binary terms, this difference
in numbers is the source of frequent confusion regarding "where the rest of the
gigabytes went". In fact, they didn't go anywhere. It's just a different way of
expressing the same thing.
This is also the source of much confusion surrounding 2.1 GB hard disks (or 2.1 GB hard
disk volumes) and the 2 GB DOS limit on partition size.
Since DOS uses binary gigabytes, and 2.1 GB hard disks are expressed in decimal terms, a
2.1 GB hard disk can in fact be entirely placed within a single DOS partition. 2.1 decimal
gigabytes is actually 1.96 binary gigabytes. Another example is the BIOS limit on regular IDE/ATA hard disks, which is
either 504 MB or 528 MB, depending on which "MB" you are talking about.
Next: Geometry
Specifications and Translation
