METRIC UNITS

To avoid any confusion, it is worth stating explicitly that in this book, as in computer science in general, metric units are used instead of traditional English units (the furlong-stone-fortnight system). The principal metric prefixes are listed in Fig. 1-29. The prefixes are typically abbreviated by their first letters, with the units greater than 1 capitalized. Thus a 1-TB database occupies 10¹² bytes of storage and a 100 psec (or 100 ps) clock ticks every 10⁻¹⁰ seconds. Since milli and micro both begin with the letter “m” a choice had to be made. Normally, “m” is for milli and “µ” (the Greek letter mu) is for micro.

Exp.	Explicit	Prefix	Exp.	Explicit	Prefix
10−3	0.001	milli	103	1,000	Kilo
10−6	0.000001	micro	106	1,000,000	Mega
10−9	0.000000001	nano	109	1,000,000,000	Giga
10−12	0.000000000001	pico	1012	1,000,000,000,000	Tera
10−15	0.000000000000001	femto	1015	1,000,000,000,000,000	Pata
10−18	0.0000000000000000001	atto	1016	1,000,000,000.000,000,000	Exa
10−21	0.0000000000000000000001	zepto	1021	1,000,000,000,000,000,000,000	Zetta
10−24	0.0000000000000000000000001	yocto	1024	1,000,000,000,000,000,000,000,000	Yotta

Figure 1-29. The principal metric prefixes.

It is also worth pointing out that for measuring memory sizes, in common industry practice, the units have slightly different meanings. There Kilo means 2¹⁰ (1024) rather than 10³ (1000) because memories are always a power of two. Thus a 1-KB memory contains 1024 bytes, not 1000 bytes. Similarly, a 1-MB memory contains 2²⁰ (1,048,576) bytes and a 1-GB memory contains 2³⁰ (1,073,741,824) bytes. However, a 1-Kbps communication line transmits 1000 bits per second and a 10-Mbps LAN runs at 10,000,000 bits/sec because these speeds are not powers of two. Unfortunately, many people tend to mix up these two systems, especially for disk sizes. To avoid ambiguity, in this book, we will use the symbols KB, MB, and GB for 2¹⁰, 2²⁰, and 2³⁰ bytes respectively, and the symbols Kbps, Mbps, and Gbps for 10³, 10⁶ and 10⁹ bits/sec, respectively.