Magnetic Storage Principles - Upgrading and Repairing PCs - page 446

organization for worldwide standardization in electrotechnology—approved as an IEC International
Standard the names and symbols for prefixes for binary multiples for use in the fields of data
processing and data transmission. Prior to this, a lot of confusion had existed as to whether a
megabyte stood for 1 million bytes (106) or 1,048,576 bytes (220). Even so, these new prefixes have
yet to be widely adopted, and confusion still reigns. The industry-standard abbreviations for the units
used to measure the capacity of magnetic (and other) drives are shown in Table 8.4 .
Table 8.4. Standard Abbreviations and Meanings
According to this prefix standard, 1 mebibyte (1 MiB = 2 20 B = 1,048,576 B) and 1 megabyte (1MB
= 10 6 B = 1,000,000 B) are not equal. Because these prefixes are not in widespread 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, although disk capacities can go either way.
This often leads to confusion in reporting 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 is defined as the product of the linear bits per inch (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 600Gb/sq. inch.