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					ASCII control characters

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes
originally intended not to carry printable information, but rather to control devices (such as
printers) that make use of ASCII, or to provide meta-information about data streams such
as those stored on magnetic tape. For example, character 10 represents the "line feed"
function (which causes a printer to advance its paper), and character 8 represents
"backspace".


 Binary Oct Dec Hex Abbr PR[1] CS[2] CEC[3]                    Description


0000 0000 000 0        00 NUL ␀ ^@             \0    Null character


0000 0001 001 1        01 SOH ␁ ^A                   Start of Header


0000 0010 002 2        02 STX ␂ ^B                   Start of Text


0000 0011 003 3        03 ETX ␃ ^C                   End of Text


0000 0100 004 4        04 EOT ␄ ^D                   End of Transmission


0000 0101 005 5        05 ENQ ␅ ^E                   Enquiry


0000 0110 006 6        06 ACK ␆           ^F         Acknowledgment


0000 0111 007 7        07 BEL ␇ ^G             \a    Bell


0000 1000 010 8        08    BS     ␈ ^H       \b    Backspace[4][8]


0000 1001 011 9        09 HT        ␉     ^I   \t    Horizontal Tab


0000 1010 012 10 0A LF              ␊ ^J       \n    Line feed


http://en.wikipedia.org/wiki/ASCII_code
0000 1011 013 11 0B VT              ␋ ^K       \v   Vertical Tab


0000 1100 014 12 0C FF              ␌ ^L       \f   Form feed


0000 1101 015 13 0D CR              ␍ ^M       \r   Carriage return[7]


0000 1110 016 14 0E SO              ␎ ^N            Shift Out


0000 1111 017 15 0F          SI     ␏ ^O            Shift In


0001 0000 020 16 10 DLE ␐                 ^P        Data Link Escape


0001 0001 021 17 11 DC1 ␑ ^Q                        Device Control 1 (oft. XON)


0001 0010 022 18 12 DC2 ␒ ^R                        Device Control 2


0001 0011 023 19 13 DC3 ␓                 ^S        Device Control 3 (oft. XOFF)


0001 0100 024 20 14 DC4 ␔ ^T                        Device Control 4


0001 0101 025 21 15 NAK ␕ ^U                        Negative Acknowledgement


0001 0110 026 22 16 SYN ␖ ^V                        Synchronous Idle


0001 0111 027 23 17 ETB ␗ ^W                        End of Trans. Block


0001 1000 030 24 18 CAN ␘ ^X                        Cancel


0001 1001 031 25 19 EM              ␙ ^Y            End of Medium




http://en.wikipedia.org/wiki/ASCII_code
0001 1010 032 26 1A SUB ␚ ^Z                        Substitute


0001 1011 033 27 1B ESC ␛                 ^[   \e   Escape[6]


0001 1100 034 28 1C FS              ␜     ^\        File Separator


0001 1101 035 29 1D GS              ␝     ^]        Group Separator


0001 1110 036 30 1E RS              ␞ ^^            Record Separator


0001 1111 037 31 1F US              ␟ ^_            Unit Separator




0111 1111 177 127 7F DEL ␡                ^?        Delete[5][8]


    1. Printable Representation, the Unicode characters reserved for representing
       control characters when it is necessary to print or display them rather than
       have them perform their intended function. Some browsers may not display
       these properly.
    2. Control key Sequence, the traditional key sequences for inputting control
       characters. The caret (^) represents the "Control" or "Ctrl" key that must be
       held down while pressing the second key in the sequence. The caret-key
       representation is also used by some software to represent control characters.
    3. Character Escape Codes in C programming language and many other
       languages influenced by it, such as Java and Perl.
    4. The Backspace character can also be entered by pressing the "Backspace",
       "Bksp", or ← key on some systems.
    5. The Delete character can also be entered by pressing the "Delete" or "Del" key.
       It can also be entered by pressing the "Backspace", "Bksp", or ← key on some
       systems.
    6. The Escape character can also be entered by pressing the "Escape" or "Esc"
       key on some systems.
    7. The Carriage Return character can also be entered by pressing the "Return",

http://en.wikipedia.org/wiki/ASCII_code
       "Ret", "Enter", or ↵ key on most systems.
    8. The ambiguity surrounding Backspace comes from mismatches between the
       intent of the human or software transmitting the Backspace and the
       interpretation by the software receiving it. If the transmitter expects Backspace
       to erase the previous character and the receiver expects Delete to be used to
       erase the previous character, many receivers will echo the Backspace as "^H",
       just as they would echo any other uninterpreted control character. (A similar
       mismatch in the other direction may yield Delete displayed as "^?".) "^H"
       persists in messages today as a deliberate humorous device — for example,
        "there's a sucker^H^H^H^H^H^Hpotential customer born every minute". A
        less common variant of this involves the use of "^W", which in some user
        interfaces means "delete previous word". The example sentence would
        therefore also work as "there's a sucker^W potential customer born every
        minute".



The original ASCII standard used only short descriptive phrases for each control character.
The ambiguity this left was sometimes intentional (where a character would be used
slightly differently on a terminal link than on a data stream) and sometimes more
accidental (such as what "delete" means).

Probably the most influential single device on the interpretation of these characters was the
Teletype corporation model 33 series, which was a printing terminal with an available
paper tape reader/punch option. Paper tape was a very popular medium for long-term
program storage up through the 1980s, lower cost and in some ways less fragile than
magnetic tape. In particular, the Teletype 33 machine assignments for codes 17 (Control-Q,
DC1, also known as XON), 19 (Control-S, DC3, also known as XOFF), and 127 (DELete)
became de-facto standards. Its noncompliant use of code 15 (Control-O, Shift In) as "left
arrow", usually interpreted as "delete previous character" was also adopted by many early
timesharing systems but eventually faded out.

The use of Control-S (XOFF, an abbreviation for "transmit off") as a handshaking signal
warning a sender to stop transmission because of impending overflow, and Control-Q
(XON, "transmit on") to resume sending, persists to this day in many systems as a manual
output control technique. On some systems Control-S retains its meaning but Control-Q is
replaced by a second Control-S to resume output.

Code 127 is officially named "delete" but the Teletype label was "rubout". Since the
original standard gave no detailed interpretation for most control codes, interpretations of


http://en.wikipedia.org/wiki/ASCII_code
this code varied. The original Teletype meaning was to make it an ignored character, the
same as NUL (all zeroes). This was specifically useful for paper tape, because punching
the all-ones bit pattern on top of an existing mark would obliterate it. Tapes designed to be
"hand edited" could even be produced with spaces of extra NULs (blank tape) so that a
block of characters could be "rubbed out" and then replacements put into the empty space.

As video terminals began to replace printing ones, the value of the "rubout" character was
lost. Unix systems, for example, interpreted "Delete" to mean "remove the character before
the cursor". Most other systems used "Backspace" for that meaning and used "Delete" to
mean "remove the character after the cursor". That latter interpretation is the most common
today.

Many more of the control codes have taken on meanings quite different from their original
ones. The "escape" character (code 27), for example, was originally intended to allow
sending other control characters as literals instead of invoking their meaning. This is the
same meaning of "escape" encountered in URL encodings, C language strings, and other
systems where certain characters have a reserved meaning. Over time this meaning has
been coopted and has eventually drifted. In modern use, an ESC sent to the terminal
usually indicates the start of a command sequence, usually in the form of an ANSI escape
code. An ESC sent from the terminal is most often used as an "out of band" character used
to terminate an operation, as in the TECO and vi text editors.

The inherent ambiguity of many control characters, combined with their historical usage,
has also created problems when transferring "plain text" files between systems. The
clearest example of this is the newline problem on various operating systems. On printing
terminals there is no question that you terminate a line of text with both "Carriage Return"
and "Linefeed". The first returns the printing carriage to the beginning of the line and the
second advances to the next line without moving the carriage. However, requiring two
characters to mark the end of a line introduced unnecessary complexity and questions as to
how to interpret each character when encountered alone. To simplify matters, plain text
files on Unix systems use line feeds alone to separate lines. Similarly, older Macintosh
systems, among others, use only carriage returns in plain text files. Various DEC operating
systems used both characters to mark the end of a line, perhaps for compatibility with
teletypes, and this de facto standard was copied in the CP/M operating system and then in
MS-DOS and eventually Microsoft Windows. The DEC operating systems, along with
CP/M and early versions of MS-DOS, tracked file length only in units of disk blocks and
used Control-Z (SUB) to mark the end of the actual text in the file. Control-C (ETX, End
of TeXt) might have made more sense, but was already in wide use as a program abort



http://en.wikipedia.org/wiki/ASCII_code
signal. UNIX's use of Control-D (EOT, End of Transmission) appears on its face similar,
but is used only from the terminal and never stored in a file.

While the codes mentioned above have retained some semblance of their original meanings,
many of the codes originally intended for stream delimiters or for link control on a
terminal have lost all meaning except their relation to a letter. Control-A is almost never
used to mean "start of header" except on an ANSI magnetic tape. When connecting a
terminal to a system, or asking the system to recognize that a logged-out terminal wants to
log in, modern systems are much more likely to want a carriage return or an ESCape than
Control-E (ENQuire, meaning "is there anybody out there?").

[edit]


ASCII printable characters

Code 32, the "space" character, denotes the space between words, as produced by the large
space-bar of a keyboard. Codes 33 to 126, known as the printable characters, represent
letters, digits, punctuation marks, and a few miscellaneous symbols.

Seven-bit ASCII provided seven "national" characters and, if the combined hardware and
software permit, can use overstrikes to simulate some additional international characters: in
such a scenario a backspace can precede a grave accent (which the American and British
standards, but only those standards, also call "opening single quotation mark"), a tilde, or a
breath mark (inverted vel).


  Binary Dec Hex Glyph             Binary Dec Hex Glyph         Binary Dec Hex Glyph


                       (blank) 0100 0000 64 40         @      0110 0000 96 60          `
0010 0000 32 20
                         (␠ )

                                 0100 0001 65 41        A     0110 0001 97 61          a
0010 0001 33 21           !

                                 0100 0010 66 42        B     0110 0010 98 62          b
0010 0010 34 22           "

                                 0100 0011 67 43        C     0110 0011 99 63          c
0010 0011 35 23           #



http://en.wikipedia.org/wiki/ASCII_code
0010 0100 36 24           $      0100 0100 68 44   D   0110 0100 100 64   d


0010 0101 37 25          %       0100 0101 69 45   E   0110 0101 101 65   e


0010 0110 38 26           &      0100 0110 70 46   F   0110 0110 102 66   f


0010 0111 39 27           '      0100 0111 71 47   G   0110 0111 103 67   g


0010 1000 40 28           (      0100 1000 72 48   H   0110 1000 104 68   h


0010 1001 41 29           )      0100 1001 73 49   I   0110 1001 105 69   i


0010 1010 42 2A           *      0100 1010 74 4A   J   0110 1010 106 6A   j


0010 1011 43 2B           +      0100 1011 75 4B   K   0110 1011 107 6B   k


0010 1100 44 2C           ,      0100 1100 76 4C   L   0110 1100 108 6C   l


0010 1101 45 2D           -      0100 1101 77 4D   M   0110 1101 109 6D   m


0010 1110 46 2E           .      0100 1110 78 4E   N   0110 1110 110 6E   n


0010 1111 47 2F           /      0100 1111 79 4F   O   0110 1111 111 6F   o


0011 0000 48 30           0      0101 0000 80 50   P   0111 0000 112 70   p


0011 0001 49 31           1      0101 0001 81 51   Q   0111 0001 113 71   q


0011 0010 50 32           2      0101 0010 82 52   R   0111 0010 114 72   r




http://en.wikipedia.org/wiki/ASCII_code
0011 0011 51 33           3      0101 0011 83 53     S     0111 0011 115 73       s


0011 0100 52 34           4      0101 0100 84 54     T     0111 0100 116 74       t


0011 0101 53 35           5      0101 0101 85 55     U     0111 0101 117 75       u


0011 0110 54 36           6      0101 0110 86 56     V     0111 0110 118 76       v


0011 0111 55 37           7      0101 0111 87 57     W     0111 0111 119 77       w


0011 1000 56 38           8      0101 1000 88 58     X     0111 1000 120 78       x


0011 1001 57 39           9      0101 1001 89 59     Y     0111 1001 121 79       y


0011 1010 58 3A           :      0101 1010 90 5A     Z     0111 1010 122 7A       z


0011 1011 59 3B           ;      0101 1011 91 5B     [     0111 1011 123 7B       {


0011 1100 60 3C           <      0101 1100 92 5C      \    0111 1100 124 7C       |


0011 1101 61 3D           =      0101 1101 93 5D     ]     0111 1101 125 7D       }


0011 1110 62 3E           >      0101 1110 94 5E     ^     0111 1110 126 7E       ~


0011 1111 63 3F           ?      0101 1111 95 5F     _


[edit]


Structural features

        The digits 0-9 are represented with their values in binary prefixed with 0011
         (this means that bcd-ASCII is simply a matter of taking each bcd nibble

http://en.wikipedia.org/wiki/ASCII_code
        separately and prefixing 0011 to it.
       Lowercase and uppercase letters only differ in bit pattern by a single bit
        simplifying case conversion to a range test (to avoid converting characters that
        are not letters) and a single bitwise operation




http://en.wikipedia.org/wiki/ASCII_code

				
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