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ASCII Controls

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Michael P. Frank ASCII-controls-v2.2.doc 9/15/06 Page 1 of 9 Discussion of the Intended Meanings of the Nonprintable ASCII Characters Michael P. Frank FAMU-FSU College of Engineering Friday, September 15, 2006 This document discusses how the invisible ASCII characters (that is, the page C0 control characters, codes 0016-1F16, along with the SP (2016) and DEL (7F16) characters) were apparently intended to be used originally, and also something about how they are used in practice presently. This discussion is based loosely on ANSI X3.4-1986, ISO 646-1983, ISO/IEC 6429, and other official standards specifications. However, some of the below material is also a matter of interpretation, or educated guesswork based on experience. The symbol glyphs that I use to graphically represent the control characters in the examples are my own notation, and are not standard. The choice of these characters is explained in a companion document, “A Proposed Set of Mnemonic Symbolic Glyphs for the Visual Representation of C0 Controls and Other Nonprintable ASCII Characters.” The section on transmission control characters needs to be updated to incorporate information from ANSI X3.28-1976, American National Standard for the Use of the Communication Control Characters of American National Standard Code for Information Interchange in Specified Data Communication Links. Likewise, the sections covering the use of the shift in, shift out, and escape characters need to be updated to integrate information from ANSI X3.41-1974, Code Extension Techniques for Use with the 7-Bit Coded Character Set of American National Standard Code for Information Interchange (or equivalently, ISO 2022-1973). 1. Characters Addressed in this Document Table 1 gives the complete list of characters whose meanings are discussed in this document. Table 1. Table of characters discussed in this document. The first column gives the code point in ANSI X3.4’s notation. The next four columns give the hexadecimal, decimal, octal, and binary equivalents. Next is the control sequence that can be used for typing the control character on some systems. Next come a couple of choices of glyphs for visual display of the control character, the first of these being the one intended for this use in the Arial Unicode MS font, the other being my suggestion for a more visually suggestive glyph that is still available in that same font. Next is the official 2-3 character abbreviation for the character name, followed by the official character type. Next is the character type, also represented in the background color of the table row, according to OC=other control (magenta), TC=transmission control (red), FE=format effector (green), CE=code extension (yellow), DC=device control (cyan), IS=information separator (blue). Finally is the section number that addresses that character. x/y Hex Dec Oct Bin Ctl Syms Abbr Name Type Sec 0/0 00 0 000 000 0000 ^@ ␀ ∅ NUL null OC 2.1 0/1 01 1 001 000 0001 ^A ␁ ⌈ SOH start of heading TC 2.2Michael P. Frank ASCII-controls-v2.2.doc 9/15/06 Page 2 of 9 x/y Hex Dec Oct Bin Ctl Syms Abbr Name Type Sec 0/2 02 2 002 000 0010 ^B ␂ ⊥ STX start of text TC 2.2 0/3 03 3 003 000 0011 ^C ␃ ⌋ ETX end of text TC 2.2 0/4 04 4 004 000 0100 ^D ␄ ↯ EOT end of transmission TC 2.2 0/5 05 5 005 000 0101 ^E ␅ ⍰ ENQ enquiry TC 2.2 0/6 06 6 006 000 0110 ^F ␆ ☑ ACK acknowledge TC 2.2 0/7 07 7 007 000 0111 ^G ␇ ☎ BEL bell OC 2.3 0/8 08 8 010 000 1000 ^H ␈ ⌫ BS backspace FE 2.4 0/9 09 9 011 000 1001 ^I ␉ ↦ HT horizontal tabulation FE 2.4 0/10 0A 10 012 000 1010 ^J ␊ ↓ LF line feed FE 2.4 0/11 0B 11 013 000 1011 ^K ␋ ↧ VT vertical tabulation FE 2.4 0/12 0C 12 014 000 1100 ^L ␌ ⇟ FF form feed FE 2.4 0/13 0D 13 015 000 1101 ^M ␍ ↲ CR carriage return FE 2.4 0/14 0E 14 016 000 1110 ^N ␎ ⊙ SO shift-out CE 2.5 0/15 0F 15 017 000 1111 ^O ␏ ⊗ SI shift-in CE 2.5 1/0 10 16 020 001 0000 ^P ␐ ⌥ DLE data link escape TC 2.2 1/1 11 17 021 001 0001 ^Q ␑ ⊚ DC1 device control one DC 2.6 1/2 12 18 022 001 0010 ^R ␒ ⊛ DC2 device control two DC 2.6 1/3 13 19 023 001 0011 ^S ␓ ⊜ DC3 device control three DC 2.6 1/4 14 20 024 001 0100 ^T ␔ ⊝ DC4 device control four DC 2.6 1/5 15 21 025 001 0101 ^U ␕ ☒ NAK negative acknowledge TC 2.2 1/6 16 22 026 001 0110 ^V ␖ ⌚ SYN synchronous idle TC 2.2 1/7 17 23 027 001 0111 ^W ␗ ▣ ETB end of transmission block TC 2.2 1/8 18 24 030 001 1000 ^X ␘ ✖ CAN cancel OC 2.7 1/9 19 25 031 001 1001 ^Y ␙ ■ EM end of medium OC 2.7 1/10 1A 26 032 001 1010 ^Z ␚ � SUB substitute character OC 2.7 1/11 1B 27 033 001 1011 ^[ ␛ ⇑ ESC escape CE 2.7 1/12 1C 28 034 001 1100 ^\ ␜ ∰ FS file separator IS 2.8 1/13 1D 29 035 001 1101 ^] ␝ ∯ GS group separator IS 2.8 1/14 1E 30 036 001 1110 ^^ ␞ ∮ RS record separator IS 2.8 1/15 1F 31 037 001 1111 ^_ ␟ ∫ US unit separator IS 2.8 2/0 20 32 040 010 0000 ^` ␠ ␣ SP space FE 2.4 7/15 7F 127 177 111 1111 ^? ␡ ⌦ DEL delete OC/FE 2.4Michael P. Frank ASCII-controls-v2.2.doc 9/15/06 Page 3 of 9 2. Character Descriptions In this section we give our detailed descriptions of the use of the above-listed characters. We break this section up into subsections corresponding to different classes of characters. In some cases (e.g. DEL), our classifications below may differ slightly from ANSIs. 2.1. Use of the Null character. NUL (∅∅∅∅) The null character NUL (∅; ^@; 0016; 000,00002) is intended to be used as filler for the empty space and time that exists in between meaningful data. For example, the characters ∅∅∅∅∅… may repeat indefinitely throughout the empty regions of a storage medium, or during the time between messages in a transmission medium. See also TC9/SYN (⌚) below, which is used to fill gaps when it is important not to lose synchronization with character boundaries, which may be difficult to discern in the ∅ symbol, due to its all-0s binary pattern. NUL rarely if ever appears as a character within ASCII text files on a storage medium; when it does, its meaning is application-dependent. In string-manipulation libraries for some


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