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ASCII Table
Introduction to Computer Systems
American System for Coded Information Interchange
Introduction to Computer Systems
• Bytes
Bits are almost always bundled together into 8-bit
collections, and these collections are called bytes.
– Why are there 8 bits in a byte? A similar question is, "Why
are there 12 eggs in a dozen?" The 8-bit byte is something
that people settled on through trial and error over the past 50
years.
• With 8 bits in a byte, you can represent 256 values
ranging from 0 to 255:
–
–
–
–
–
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0 = 00000000
1 = 00000001
2 = 00000010 ...
254 = 11111110
255 = 11111111
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Introduction to Computer Systems
• Computers store text documents, both on disk and in
memory, using ASCII codes.
– In the ASCII character set, each binary value
between 0 and 127 is given a specific character.
Most computers extend the ASCII character set to
use the full range of 256 characters available in a
byte. The upper 128 characters handle special
things like accented characters from common
foreign languages.
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Introduction to Computer Systems
• Bytes are frequently used to hold individual
characters in a text document.
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– For example, if you use Notepad in Windows 95/98 to create
a text file containing the words, "Four score and seven years
ago," Notepad would use 1 byte of memory per character
(including 1 byte for each space character between the
words -- ASCII character 32). When Notepad stores the
sentence in a file on disk, the file will also contain 1 byte per
character and per space.
– Try this as an experiment:. Use the explorer and look at the
size of the file. You will find that the file has a size of 30
bytes on disk: 1 byte for each character.
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Introduction to Computer Systems
Introduction to Computer Systems
• If you were to look at the file as a computer looks at
it, you would find that each byte contains not a letter
but a number -- the number is the ASCII code
corresponding to the character (see below). So on
disk, the numbers for the file look like this:
F
o
u
r
70
111
117
114
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a n d
32
97
110
100
32
s
e
v
e
n
115
101
118
101
110
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• Lots of Bytes
– To talking about lots of bytes, we use prefixes like
kilo, mega and giga.
• as in kilobyte, megabyte and gigabyte (also shortened to
K, M and G, as in Kbytes, Mbytes and Gbytes or KB, MB
and GB).
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Introduction to Computer Systems
Introduction to Computer Systems
• By looking in the ASCII table, you can see a
one-to-one correspondence between each
character and the ASCII code used. Note the
use of 32 for a space -- 32 is the ASCII code
for a space. We could expand these decimal
numbers out to binary numbers (so 32 =
00100000) if we wanted to be technically
correct -- that is how the computer really
deals with things.
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Name
Abbr.
Size
Kilo
K
2^10 = 1,024
Mega
M
2^20 = 1,048,576
Giga
G
2^30 = 1,073,741,824
Tera
T
2^40 = 1,099,511,627,776
Peta
P
2^50 =
1,125,899,906,842,624
Exa
E
2^60 =
1,152,921,504,606,846,97
6
Zetta
Z
2^70 =
1,180,591,620,717,411,30
3,424
Yotta
Y
2^80 =
1,208,925,819,614,629,17
4,706,176
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Introduction to Computer Systems
• A kilo is about a thousand, mega is about a
million, giga is about a billion, and so on.
– So when someone says, "This computer has a 2
gig hard drive," what he or she means is that the
hard drive stores 2 gigabytes, or approximately 2
billion bytes, or exactly 2,147,483,648 bytes.
– How could you possibly need 2 gigabytes of
space?
• When you consider that one CD holds 650 megabytes,
you can see that just three CDs worth of data will fill the
whole thing! Terabyte databases are fairly common these
days, and there are probably a few petabyte databases
floating around the Pentagon by now.
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