Hardware Reference
In-Depth Information
Accordingtothisprefixstandard,1mebibyte(1MiB=2
20
B=1,048,576B)and1mega-
byte (1MB = 10
6
B = 1,000,000 B) are not equal. Because these prefixes are not in wide-
spread use (and they might never be), M in most cases can indicate both decimal
millions
of bytes
and binary
megabytes
. Similarly, G is often used to refer to decimal
billions of
bytes
and binary
gigabytes
. In general, memory values are expressed by using the binary
values,althoughdiskcapacities cangoeitherway.Thisoftenleadstoconfusioninreport-
ing disk capacities because many manufacturers tend to use whichever value makes their
products look better. For example, drive capacities are often rated in decimal billions (G
- Giga), whereas most BIOS chips and operating system utilities, such as the Windows
FDISK, rate the same drive in binary gigabytes (Gi - Gibi). Note also that when bits and
bytes are used as part of some other measurement, the difference between bits and bytes
is often distinguished by the use of a lower- or uppercase
B
. For example, megabits are
typically abbreviated with a lowercase
b
, resulting in the abbreviation
Mbps
for
megabits
per second
, whereas
MBps
indicates
megabytes per second
.
Areal Density
Areal density is often used as a technology growth-rate indicator for the hard disk drive
industry.
Areal density
isdefinedastheproductofthelinearbitsperinch(BPI),measured
along the length of the tracks around the disk, multiplied by the number of tracks per inch
(TPI), measured radially on the disk (see
Figure 8.11
). The results are expressed in units
of megabits or gigabits per square inch (Mb/sq. inch or Gb/sq. inch) and are used as a
measure of efficiency in drive recording technology. Current high-capacity drives record
at areal densities exceeding 400Gb/sq. inch.
Figure 8.11
Areal density, combining tracks per inch and bits per inch.
