A computer understands the process of its transformation into a form that allows organizing a more convenient transfer, storage or automatic processing of this data. For this purpose, various tables are used. The ASCII encoding is the first system developed in the United States for working with English-language text, which subsequently became widespread throughout the world. The article below is devoted to its description, features, properties and further use.
Display and storage of information in a computer
Symbols on a computer monitor or one or another mobile digital gadget are formed on the basis of sets of vector forms of all kinds of signs and a code that allows you to find among them the symbol that needs to be inserted in the right place. It is a series of bits. Thus, each character must uniquely correspond to a set of zeros and ones, which stand in a specific, unique order.
How it all began
Historically, the first computers were in English. To encode symbolic information in them, it was enough to use only 7 bits of memory, while for this purpose 1 byte, consisting of 8 bits, was allocated. The number of characters understood by the computer in this case was equal to 128. The number of such characters included the English alphabet with its punctuation marks, numbers and some special characters. The English-language seven-bit encoding with the corresponding table (code page), developed in 1963, was named the American Standard Code for Information Interchange. Usually the abbreviation "ASCII encoding" was used to denote it and is still used to this day.
Transition to multilingualism
Over time, computers have become widely used in non-English-speaking countries as well. In this regard, there was a need for encodings that would allow the use of national languages. It was decided not to reinvent the wheel, and to take ASCII as a basis. The encoding table in the new edition has expanded significantly. The use of the 8th bit made it possible to translate 256 characters into the computer language.
Description
ASCII encoding has a table that is divided into 2 parts. Only the first half is considered to be the generally accepted international standard. It includes:
- Characters with ordinal numbers from 0 to 31, encoded by sequences from 00000000 to 00011111. They are reserved for control characters that control the process of displaying text on the screen or printer, giving a sound signal, etc.
- Characters with NN in the table from 32 to 127, coded by sequences from 00100000 to 01111111, constitute the standard part of the table. These include a space (N 32), letters of the Latin alphabet (lowercase and uppercase), ten-digit numbers from 0 to 9, punctuation marks, brackets of different styles and other symbols.
- Characters with ordinal numbers from 128 to 255, encoded by sequences from 10000000 to 11111111. These include letters of national alphabets other than Latin. It is this alternative part of the table that ASCII encoding is used to convert Russian characters into computer form.
Some properties
The peculiarities of the ASCII encoding include the difference between the letters "A" - "Z" of lower and upper cases by only one bit. This circumstance greatly simplifies the conversion of the register, as well as its check for belonging to the specified range of values. In addition, all letters in the ASCII encoding system are represented by their own ordinal numbers in the alphabet, which are written in 5 digits in the binary system, preceded by 011 2 for lower case letters and 010 2 for upper case.
Among the features of the ASCII encoding can be considered the representation of 10 digits - "0" - "9". In the second number system, they start with 00112 and end with 2 numbers. For example, 0101 2 is equivalent to decimal five, so the character "5" is written as 0011 01012. Based on this, you can easily convert BCDs to an ASCII string by adding 00112 to each nibble on the left.
"Unicode"
As you know, thousands of characters are required to display texts in the languages of the Southeast Asian group. Such a number of them is in no way described in one byte of information, so even extended ASCII versions could no longer satisfy the increased needs of users from different countries.
Thus, the need arose to create a universal text encoding, which was developed by the Unicode consortium in cooperation with many leaders of the global IT industry. Its specialists created the UTF 32 system. In it, 32 bits were allocated for encoding 1 character, making up 4 bytes of information. The main drawback was a sharp increase in the amount of required memory by as much as 4 times, which entailed many problems.
At the same time, for most countries with official languages belonging to the Indo-European group, the number of characters equal to 2 32 is more than redundant.
As a result of further work of specialists from the Unicode consortium, the UTF-16 encoding appeared. It became the option for transforming symbolic information that suited everyone both in terms of the amount of required memory and the number of encoded characters. That is why UTF-16 was accepted by default and it requires 2 bytes to be reserved for one character.
Even this rather advanced and successful version of "Unicode" had some drawbacks, and after the transition from the extended version of ASCII to UTF-16, it doubled the weight of the document.
In this regard, it was decided to use the variable length encoding UTF-8. In this case, each character of the source text is encoded with a sequence of 1 to 6 bytes long.
Relationship with American standard code for information interchange
All characters of the Latin alphabet in UTF-8 of variable length are encoded in 1 byte, as in the ASCII encoding system.
The peculiarity of UTP-8 is that in the case of a text in Latin without using other characters, even programs that do not understand Unicode will still allow you to read it. In other words, the basic part of the ASCII text encoding is simply merged into the new variable-length UTF. Cyrillic characters in UTP-8 occupy 2 bytes, and, for example, Georgian ones - 3 bytes. The creation of UTF-16 and 8 solved the main problem of creating a single code space in fonts. Since then, font manufacturers can only fill the table with vector forms of text characters based on their needs.
Different encodings are preferred on different operating systems. To be able to read and edit texts typed in a different encoding, Russian text conversion programs are used. Some text editors contain built-in transcoders and allow you to read text regardless of encoding.
Now you know how many characters are in ASCII and how and why it was developed. Of course, today the most widespread standard in the world is "Unicode". However, we must not forget that it was created on the basis of ASCII, therefore, the contribution of its developers to the field of IT should be appreciated.
According to the International Telecommunication Union, in 2016, three and a half billion people used the Internet with varying regularity. Most of them do not even think about the fact that any messages sent by them via PCs or mobile gadgets, as well as texts that are displayed on all kinds of monitors, are actually combinations of 0 and 1. This presentation of information is called encoding. It provides and greatly facilitates its storage, processing and transmission. In 1963, the American ASCII encoding was developed, which this article is devoted to.
Presentation of information in a computer
From the point of view of any electronic computer, text is a collection of individual characters. These include not only letters, including capital letters, but also punctuation marks and numbers. In addition, special characters "=", "&", "(" and spaces are used.
The set of symbols that make up the text is called the alphabet, and their number is called the cardinality (denoted as N). To define it, the expression N = 2 ^ b is used, where b is the number of bits or the informational weight of a particular character.
It has been proven that an alphabet with a capacity of 256 characters can represent all the necessary characters.
Since 256 is the 8th power of two, the weight of each character is 8 bits.
The unit of measurement of 8 bits is called 1 byte, so it is customary to say that any character in a text stored on a computer takes up one byte of memory.
How is coding done
Any texts are entered into the memory of a personal computer by means of keyboard keys on which numbers, letters, punctuation marks and other symbols are written. They are transferred to the RAM in a binary code, that is, each character is associated with a decimal code familiar to humans, from 0 to 255, which corresponds to a binary code - from 00000000 to 11111111.
Byte character encoding allows the text processor to access each character separately. At the same time, 256 characters are enough to represent any character information.
ASCII character encoding
This abbreviation in English stands for code for information interchange.
Even at the dawn of computerization, it became obvious that you can come up with a wide variety of ways to encode information. However, to transfer information from one computer to another, it was required to develop a single standard. So, in 1963, an ASCII encoding table appeared in the United States. In it, any symbol of the computer alphabet is associated with its ordinal number in binary representation. Initially, ASCII was used only in the United States and later became the international standard for PCs.
ASCII codes are divided into 2 parts. Only the first half of this table is considered an International Standard. It includes characters with ordinal numbers from 0 (encoded as 00000000) to 127 (code 01111111).
Serial number | ASCII text encoding | Symbol |
0000 0000 - 0001 1111 | Characters with N from 0 to 31 are called control characters. Their function is to "guide" the process of displaying text on a monitor or printing device, giving a sound signal, etc. |
|
0010 0000 - 0111 1111 | Characters with N from 32 to 127 (standard part of the table) - upper and lower case letters of the Latin alphabet, 10 digits, punctuation marks, as well as various brackets, commercial and other symbols. The character 32 denotes a space. |
|
1000 0000 - 1111 1111 | Characters with N from 128 to 255 (alternative part of the table or code page) can have different variants, each of which has its own number. The code page is used to specify national alphabets that are different from Latin. In particular, it is with its help that ASCII encoding for Russian characters is carried out. |
In the encoding table, uppercase and follow one after another in alphabetical order, and numbers - in ascending order of values. This principle also applies to the Russian alphabet.
Control characters
The ASCII encoding table was originally created to receive and transmit information on such a device that has not been used for a long time, such as a teletype. In this regard, non-printable characters have been included in the character set, used as commands to control this device. Similar commands were used in such pre-computer messaging methods as Morse code, etc.
The most common "teletype" character is NUL (00, "zero"). It is still used in most programming languages to this day, denoting a line terminator.
Where is ASCII encoding used?
The US Standard Code is needed for more than just entering text information from the keyboard. It is also used in graphics. Specifically, in ASCII Art Maker, images of different extensions represent a spectrum of ASCII characters.
Such products are of two types: they perform the function of graphic editors by converting images into text, and converting "pictures" into ASCII graphics. For example, the famous emoticon is a prime example of an encoding character.
ASCII can also be used when creating an HTML document. In this case, you can enter a certain set of characters, and when viewing the page, a character will appear on the screen that corresponds to this code.
ASCII is also necessary for the creation of multilingual sites, since characters that are not included in a specific national table are replaced by ASCII codes.
Some features
To encode text information in ASCII encoding, 7 bits were originally used (one was left empty), but today it works as 8-bit.
The letters in the top and bottom columns differ from each other by only one single bit. This greatly reduces the complexity of the check.
Using ASCII in Microsoft Office
If necessary, this type of text encoding can be used in Microsoft text editors such as Notepad and Office Word. However, when typing in this case, it will not be possible to use some functions. For example, you will not be able to bold, because ASCII only preserves the meaning of the information, ignoring its general appearance and shape.
Standardization
The ISO organization has adopted the ISO 8859 standards. This group defines eight-bit encodings for different language groups. Specifically, ISO 8859-1 is Extended ASCII, which is a table for the United States and Western Europe. And ISO 8859-5 is a table used for the Cyrillic alphabet, including the Russian language.
For a number of historical reasons, the ISO 8859-5 standard has been in use for a very short time.
For the Russian language, at the moment, encodings are actually used:
- CP866 (Code Page 866) or DOS, which is often referred to as the alternative GOST encoding. It was actively used until the mid-90s of the last century. At the moment, it is practically not used.
- KOI-8. The encoding was developed in the 1970-80s, and at the moment it is a generally accepted standard for mail messages on the Runet. It is widely used in the OS of the Unix family, including Linux. The "Russian" version of KOI-8 is called KOI-8R. In addition, there are versions for other Cyrillic languages, such as Ukrainian.
- Code Page 1251 (CP 1251, Windows - 1251). Developed by Microsoft to provide support for the Russian language in the Windows environment.
The main advantage of the first CP866 standard was the preservation of pseudographic characters in the same positions as in Extended ASCII. This made it possible to run without changes foreign-made text programs, such as the well-known Norton Commander. At the moment, CP866 is used for programs developed under Windows that work in full-screen text mode or in text windows, including FAR Manager.
Computer texts written in the CP866 encoding are quite rare lately, but it is precisely this encoding that is used for Russian file names in Windows.
"Unicode"
At the moment, it is this encoding that has received the most widespread use. Unicode codes are divided into areas. The first (U + 0000 to U + 007F) includes ASCII characters with codes. This is followed by the areas of signs of various national scripts, as well as punctuation marks and technical symbols. In addition, some of the "Unicode" codes are reserved in case there is a need to include new characters in the future.
Now you know that in ASCII, each character is represented as a combination of 8 zeros and ones. To non-specialists, this information may seem unnecessary and uninteresting, but don't you want to know what is happening “in the brains” of your PC ?!
The set of characters with which text is written is called alphabet.
The number of characters in the alphabet is his power.
Formula for determining the amount of information: N = 2 b,
where N is the cardinality of the alphabet (number of characters),
b - number of bits (informational weight of the character).
The alphabet with a capacity of 256 characters can accommodate almost all the necessary characters. This alphabet is called sufficient.
Because 256 = 2 8, then the weight of 1 character is 8 bits.
The 8-bit unit was named 1 byte:
1 byte = 8 bits.
The binary code of each character in computer text takes up 1 byte of memory.
How is text information represented in the computer memory?
The convenience of byte encoding of characters is obvious, since a byte is the smallest addressable part of memory and, therefore, the processor can access each character separately, performing text processing. On the other hand, 256 characters is quite a sufficient number to represent a wide variety of character information.
Now the question arises, what kind of eight-bit binary code to associate with each character.
It is clear that this is a conditional matter, you can come up with many encoding methods.
All characters of the computer alphabet are numbered from 0 to 255. Each number corresponds to an eight-digit binary code from 00000000 to 11111111. This code is simply the ordinal number of the character in the binary system.
The table in which all the characters of the computer alphabet are assigned serial numbers is called the encoding table.
Different coding tables are used for different types of computers.
The international standard for the PC has become the table ASCII(read asci) (American Standard Code for Information Interchange).
The ASCII table is divided into two parts.
The international standard is only the first half of the table, i.e. symbols with numbers from 0 (00000000), up to 127 (01111111).
ASCII encoding table structure
Serial number |
The code |
Symbol |
0 - 31 |
00000000 - 00011111 |
Symbols with numbers from 0 to 31 are usually called control characters. |
32 - 127 |
00100000 - 01111111 |
Standard part of the table (English). This includes lowercase and uppercase letters of the Latin alphabet, decimal digits, punctuation marks, all kinds of brackets, commercial and other symbols. |
128 - 255 |
10000000 - 11111111 |
Alternative part of the table (Russian). |
The first half of the ASCII table
 |
I draw your attention to the fact that in the encoding table, letters (uppercase and lowercase) are arranged in alphabetical order, and numbers are ordered in ascending order of values. This observance of the lexicographic order in the arrangement of characters is called the principle of sequential coding of the alphabet.
For the letters of the Russian alphabet, the principle of sequential coding is also observed.
The second half of the ASCII table
Unfortunately, there are currently five different Cyrillic encodings (KOI8-R, Windows. MS-DOS, Macintosh and ISO). Because of this, problems often arise with the transfer of Russian text from one computer to another, from one software system to another.
Chronologically, one of the first standards for encoding Russian letters on computers was KOI8 ("Information exchange code, 8-bit"). This encoding was used back in the 70s on computers of the ES computer series, and from the mid 80s it began to be used in the first Russified versions of the UNIX operating system.
From the beginning of the 90s, the time of the dominance of the MS DOS operating system, the CP866 encoding remains ("CP" stands for "Code Page").
Apple computers running Mac OS use their own Mac encoding.
In addition, the International Organization for Standardization (International Standards Organization, ISO) approved another encoding called ISO 8859-5 as a standard for the Russian language.
Currently, the most common encoding is Microsoft Windows, abbreviated as CP1251.
Since the late 90s, the problem of character coding standardization has been solved by the introduction of a new international standard called Unicode... This is a 16-bit encoding i.e. it allocates 2 bytes of memory for each character. Of course, this doubles the amount of memory used. But on the other hand, such a code table allows the inclusion of up to 65536 characters. The complete specification of the Unicode standard includes all the existing, extinct and artificially created alphabets of the world, as well as many mathematical, musical, chemical and other symbols.
Let's try to use an ASCII table to imagine how words will look in computer memory.
Internal representation of words in computer memory
Sometimes it happens that a text consisting of letters of the Russian alphabet, received from another computer, cannot be read - some kind of "gibberish" is visible on the monitor screen. This is due to the fact that computers use different encoding of the characters of the Russian language.
Overlay symbols
The BS (backspace) character allows the printer to overwrite one character. In ASCII, it was provided for the addition of diacritics to letters in this way, for example:
- a BS "→ á
- a BS `→ à
- a BS ^ → â
- o BS / → ø
- c BS, → ç
- n BS ~ → ñ
Note: in old fonts, the apostrophe "was drawn with a slant to the left, and the tilde ~ was shifted up, so that they just fit the role of acute and tilde on top.
If the same symbol is superimposed on a character, then the effect of a bold font is obtained, and if an underscore is superimposed on the character, then underlined text is obtained.
- a BS a → a
- a BS _ → a
Note: this is used, for example, in the man help system.
National ASCII variants
The ISO 646 (ECMA-6) standard provides for the possibility of placing national characters in place @ [ \ ] ^ ` { | } ~ ... In addition to this, in place # can be accommodated £ , and in place $ - ¤ ... This system is well suited for European languages where only a few extra characters are needed. The ASCII version without national characters is called US-ASCII, or "International Reference Version".
Subsequently, it turned out to be more convenient to use 8-bit encodings (code pages), where the lower half of the code table (0-127) is occupied by US-ASCII characters, and the upper half (128-255) is occupied by additional characters, including a set of national characters. Thus, the upper half of the ASCII table, before the widespread adoption of Unicode, was actively used to represent localized characters, letters of the local language. The lack of a unified standard for placing Cyrillic characters in the ASCII table caused many problems with encodings (KOI-8, Windows-1251, and others). Other languages with a non-Latin script also suffered from the presence of several different encodings.
| .0 | .1 | .2 | .3 | .4 | .5 | .6 | .7 | .8 | .9 | .A | .B | .C | .D | .E | .F | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0. | NUL | SOM | EOA | EOM | EQT | WRU | RU | BELL | BKSP | Ht | LF | VT | FF | CR | SO | SI |
| 1. | DC 0 | DC 1 | DC 2 | DC 3 | DC 4 | ERR | SYNC | LEM | S 0 | S 1 | S 2 | S 3 | S 4 | S 5 | S 6 | S 7 |
| 2. | ||||||||||||||||
| 3. | ||||||||||||||||
| 4. | BLANK | ! | " | # | $ | % | & | " | ( | ) | * | + | , | - | . | / |
| 5. | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | : | ; | < | = | > | ? |
| 6. | ||||||||||||||||
| 7. | ||||||||||||||||
| 8. | ||||||||||||||||
| 9. | ||||||||||||||||
| A. | @ | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O |
| B. | P | Q | R | S | T | U | V | W | X | Y | Z | [ | \ | ] | ← | |
| C. | ||||||||||||||||
| D. | ||||||||||||||||
| E. | a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | |
| F. | p | q | r | s | t | u | v | w | x | y | z | ESC | DEL |
On those computers where the minimum addressable unit of memory was a 36-bit word, 6-bit characters were initially used (1 word = 6 characters). After the transition to ASCII on such computers, they began to place either 5 seven-bit characters in one word (1 bit remained superfluous), or 4 nine-bit characters.
ASCII codes are also used to identify the pressed key during programming. For a standard QWERTY keyboard, the code table looks like this:
