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CHAPTER 6
INSIDE THE SYSTEM UNIT
How computers represent data.
We’re all used to counting with decimal numbers, which consist of 10 digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9).
Computers
count with
binary numbers
, which consist of only two digits (0 and 1). A binary number is called a
binary digit, or bit for short. A
bit
is the smallest unit of information that a computer can work with (Figure 6.1).
Think of a bit as being similar to a light switch: it has only two possible states, and is always in one or the other. If
there is one light switch, the switch is on, or the switch is off. If there are two light switches, there are four
possibilities: both switches are on; both switches are off; the first switch is on, and the second switch is off; or the
first switch is off, and the second switch is on. Three switches allow for eight possibilities, and so on—up to eight
switches, which results in 256 possible combinations.
Representing Characters: Character Codes
Character codes translate numerical data into characters
readable by humans;
American Standard Code for Information Interchange
(ASCII)—Eight bits equals one
character; used by minicomputers and personal computers;
Extended Binary Coded Decimal Interchange Code
(EBCDIC)—Eight bits equals one character; used by mainframe computers;
Unicode
—Sixteen bits equals one
character; over 65,000 combinations; used for foreign language symbols.
The measurements used to describe data transfer rates and data storage capacity.
The basic unit of information in a computer is the bit, a single-digit binary number (either 1 or 0). An eight-bit
sequence of numbers, called a
byte
, is sufficient to represent the basic letters, numbers, and punctuation marks in
most European languages. For this reason, the term byte is synonymous with the term character (a letter or
number). Larger units of data are described by the terms
kilobyte
(
K
or
KB
, approximately one thousand bytes),
megabyte
(
M
or
MB
, approximately one million bytes),
gigabyte
(
G
or
GB
, approximately one billion bytes), and
terabyte
(
T
or
TB
, approximately one trillion bytes). Data transfer rates are measured in
bits per second
(
bps
).
Common terms to describe data transfer rates include
Kbps
(approximately
one thousand bits per second
),
Mbps
(approximately
one million bits per second
), and
Gbps
(approximately
one billion bits per second
).
The components found inside the system unit, and their uses.
Inside the system unit, there is the motherboard, power supply, cooling fan, speaker, internal drive bays, external
drive bays, and various expansion cards (such as the sound card and network interchange card).
8
bits = 1 Byte
1024
Bytes = 1 Kilobyte (KB)
1,048,576
Bytes = 1 Megabyte (MB)
1,043,741,824
Bytes = 1 Gigabyte (GB)