Code
Combinations of 01 for each
character.
Acronym: Co = Combs-ozo-feCh (pronounced Cohmz-ohzoh-fetch).
- A code relates specific combinations of bits (0s and 1s) to specific bytes
(characters).
- To communicate, computers must agree on which combinations represent which
characters.
Note: The term code is used in a different sense when it refers
to programming code. But we'll get to that when we review the fifth term of the
Digital Dozen: Program.
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ASCII Code A number of codes are used in computing, but the code common to nearly all
computers is called ASCII (ASK-ee). ASCII is an acronym for American Standard Code for Information
Interchange,
which is quite a mouthful. But it makes some sense when you break it down into
its parts:
-
American Standard--ASCII
was created in America by a
committee in the 1960s, but has
become a worldwide standard.
-
Code--ASCII specifies a unique 01
combination for each character.
-
Information Interchange--ASCII
enables a machine, which works
with only 0s and 1s, to
interchange information with
humans, who work with many
characters.
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But Max, which
combinations make which characters? |
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Good question, C.W. 8 bits can be arranged into 256 unique combinations from
all off's (00000000) to all on's (11111111) and every combination in between.
I've listed some of these combinations and the characters they make in the
Partial ASCII Chart below. But here are a few things to consider first.
Decimal vs. Binary
Arithmetically,
each 01 combination has a decimal equivalent. When speaking of a
particular character, it's easier to refer to the decimal rather than the
binary value. For example, it's easier to refer to the
Space character as ASCII 32 instead of ASCII 0010 0000.
Computers Count Funny
Another oddity is that computers begin counting with ZERO. For example,
looking at your hand, you would count your fingers as "1, 2, 3, 4, 5."
But a computer would count them as "0, 1, 2, 3, 4." Likewise, because
the first ASCII character is numbered ASCII 0, the last is numbered ASCII 255
(instead of 256).
Standard ASCII
As originally conceived, ASCII used only seven bits, yielding 128 possible
01 combinations. All ASCII-based computers use the same characters for ASCII
0-127. Hence, they're "standard."
Extended ASCII
To meet the demand for more characters, an eighth bit was
added, doubling the number of possible 01 combinations to 256. However, the
specific characters assigned to ASCII 128-255 vary by country.
The originators of ASCII had the challenge of deciding which 01 combination should
be assigned to which character. But it didn't really matter what they decided, as
long as they all agreed. Here's some of what they came up with.
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| ASCII Categories |
Partial ASCII Chart
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| Decimal |
Binary |
Character |
| ASCII
characters fall into three categories.
I. 0-31: Control Characters.
Control characters control certain computer operations. They are typically
invisible although some can be displayed and printed as symbols like, believe it or not,
hearts or happy faces!
For example, when sending a message, ASCII 1 is Start of Heading (SOH)
and ASCII 2 is Start of Text (STX).
Probably more familiar to you are keyboard keys like ASCII 13 [Enter] and ASCII 27 [Esc].
II. 32-127 = Alphanumerics
ASCII 32 is the Space character, invisible to us, but
"seen" by the computer. Common punctuation symbols follow. Surprisingly, the
numbers 0-9 don't begin until ASCII 48.
Observe that the uppercase alphabet begins at ASCII 65 (A); the lowercase
alphabet begins at ASCII 97 (a).
ASCII 127 is the [Delete] key.
III. 128-255 = Miscellaneous.
These extended ASCII characters vary by country but typically
include foreign characters, line-draw symbols, and mathematical symbols. ASCII
255 (the 256th character) has no symbol. I've seen it used to put an invisible
character into a password.
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0
1
2
3
4
8
13
27
32
33
34
48
49
50
51
65
66
67
68
97
98
99
100
127
128
129
130
131
170
171
241
246
255
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00000000
00000001
00000010
00000011
00000100
00001000
00001101
00011011
00100000
00100001
00100010
00110000
00110001
00110010
00110011
01000001
01000010
01000011
01000100
01100001
01100010
01100011
01100100
01111111
10000000
10000001
10000010
10000011
10101010
10101011
11110001
11110110
11111111
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Null
SOH
STX
ETX
EOT
Backspace
Enter
Esc
Space
!
"
0
1
2
3
A
B
C
D
a
b
c
d
Del
Ç
ü
é
â
¬
½
±
÷
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ASCII Screen Display |
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Let's take a very simplified look at how a computer displays a character on
screen:
In the figure on the right, (1) Pressing a key sends an electronic signal, which (2) sets
a bank of 8
switches off or on, which sends (3) a unique combination of electrical pulses to
an ASCII Character Generator which projects (4) dots on the screen in the shape
of the character assigned to that off/on pulse combination.
Although only one bank of switches is shown, we can compose messages of almost unlimited
length because today's computers contain millions of banks of switches. The interesting thing is, the computer doesn't really
know that it's displaying a character at all, just a group of onscreen dots that
look to us like a character!
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Plain Text (DOS Text)--ASCII may be referred to as Plain Text or DOS
Text. Messages sent in ASCII are limited to its 256 characters, which include no
provision for formatting. For example, ASCII text can't be bolded or italicized.
Numeric Numbers--ASCII handles
numeric numbers (for use in calculations) differently
from text numbers (for display only). Every text number requires 8 bits. Numeric numbers
use the decimal equivalent of 01 combinations. For example, a numeric 3 requires
only two (11) instead of eight bits; a numeric 13 only four (1101) instead of
sixteen bits.
Other Codes--Since ASCII is
limited to text, numbers, and symbols, many
other bit codes have been created (ANSI, JPEG, MPEG, etc.) to display colors,
pictures, and sound.
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So what good is ASCII to me?
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Well, K.N., here are just a few of the ways a knowledge of ASCII may benefit
you.
Email--Email messages are sent in ASCII since it's understood by all
modern computers regardless of make, model, or size. Which means you don't have to
worry whether your recipient's computer can read your message. Note: Modern email
programs use HTML to format ASCII text. HTML stands for HyperText Markup
Language and is used primarily to create Internet web pages.
Special Characters--ASCII contains special
characters, such as the é in résumé. Learn how to access these characters,
and you can impress people with your advanced computer skills!
Sorting--Computers sort lists in ASCII order. By knowing the order, you can create lists that
take advantage of this. For example, to make a file or folder list first, begin
its name with a number instead of a letter.
Software AcroMap --3
of 5 Terms
The Software AcroMap has grown a new link in its spine with a new rib
and definition. Memorize this new portion. By
learning the AcroMap, you are building a framework of computer literacy in your
brain that you can call up in the future as needed.
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