The ASCII scheme, short for the American Standard Code for Information Interchange, is a character encoding standard that represents text in computers and other devices that work with text. The ASCII scheme is designed to include 128 characters in total, which consist of both control characters and printable characters.
Overview of ASCII
ASCII is a 7-bit character encoding, meaning that each character is represented by a unique 7-bit binary number. Since there are 2^7 (128) possible combinations of 7 bits, the ASCII scheme is capable of encoding 128 distinct characters.
The 128 characters in the ASCII scheme can be divided into two main groups:
- Control characters (0-31): The first 32 characters in the ASCII table are non-printable control characters. These characters are used to control devices such as printers and display screens or to organize data in a structured manner. Examples of control characters include carriage return, line feed, and tab.
- Printable characters (32-127): The remaining 96 characters are printable characters, which include alphabetic characters (both uppercase and lowercase), numeric digits, punctuation marks, and a few special characters, such as the space character.
ASCII Character Ranges
The 128 characters in the ASCII scheme can be further broken down into the following ranges:
- 0-31 (Control characters): These are non-printable characters used for controlling devices and organizing data.
- 32 (Space character): The space character is used to separate words and other text elements.
- 33-47 (Punctuation marks and symbols): This range includes various punctuation marks and symbols, such as the exclamation mark, quotation marks, and the percent symbol.
- 48-57 (Numeric digits): The characters in this range represent the numeric digits 0 through 9.
- 58-64 (Punctuation marks and symbols): This range includes additional punctuation marks and symbols, such as the colon, semicolon, and the at symbol.
- 65-90 (Uppercase alphabetic characters): The characters in this range represent the uppercase letters of the English alphabet, from A to Z.
- 91-96 (Punctuation marks and symbols): This range includes more punctuation marks and symbols, such as the square brackets and the caret symbol.
- 97-122 (Lowercase alphabetic characters): The characters in this range represent the lowercase letters of the English alphabet, from a to z.
- 123-127 (Punctuation marks and symbols): This range includes the final punctuation marks and symbols in the ASCII table, such as the curly braces and the tilde symbol.
ASCII Table
The ASCII table is a visual representation of the ASCII scheme, displaying each character along with its corresponding decimal, hexadecimal, and binary values.
This table shows a selection of characters from the ASCII scheme, along with their corresponding decimal, hexadecimal, and binary values.
ASCII and Modern Character Encoding
While ASCII was sufficient for encoding the English alphabet and a limited set of special characters and control codes, it was not suitable for representing the wide range of characters used in other languages and writing systems. Consequently, modern character encoding schemes have been developed to accommodate a broader range of characters and symbols.
One such modern character encoding scheme is Unicode, which is designed to encode characters from virtually every writing system in the world. Unicode can represent over a million distinct characters, far surpassing the 128 characters of the ASCII scheme. UTF-8 is a popular encoding for Unicode that is backward compatible with ASCII, meaning that the first 128 characters of UTF-8 are the same as the ASCII characters.
Examples of ASCII Usage
Encoding and Decoding Text
One common use of the ASCII scheme is to encode and decode text data. For example, when you save a text file on your computer, the text is typically encoded using the ASCII scheme or a compatible character encoding, such as UTF-8. When you open the file, the encoded data is decoded back into the original text.
text = "Hello, ASCII!"
encoded = [ord(c) for c in text]
print("Encoded:", encoded)
decoded = "".join([chr(c) for c in encoded])
print("Decoded:", decoded)
In this Python example, the text variable contains a string of ASCII characters. The ord function is used to convert each character in the string into its corresponding ASCII code, creating a list of encoded data. The chr function is then used to convert the encoded data back into the original text.
Data Transmission
The ASCII scheme is also used in data transmission, particularly in protocols that involve the exchange of textual data. For example, the Simple Mail Transfer Protocol (SMTP) for email relies on the ASCII scheme to represent the content of email messages.