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ASCII

The ASCII character set is a standard defining, for character data, which bit-patterns represent which letters, numerals, and punctuation marks.

We have seen how letters have numerical values, like A is 65.

In memory, A is represented as 01000001.

This relationship is based on the ASCII character set.

ASCII characters use 1 byte of memory, and are numbered 0 to 127.

Practice Exercise

ASCII character codes use only 7 of a byte's 8 bits. The other bit was used in older data transmission protocols.

ASCII codes represent:

  • The letters of the American alphabet (both capital and small letters)

  • The ASCII character codes for the uppercase letters A-Z start at 65.

  • The lowercase letters a-z are also in ascending order starting at 97.

  • Digit characters

  • The digit characters for 0-9 are in order starting at 48.

  • Punctuation

  • These codes are scattered throughout the ASCII table.

  • About 30 non-printable control codes

Drill

Go to http://www.asciitable.com/ and take a close look at how the letters and digits are organized in the table.

Practice Exercise

Control Characters

The first 32 ASCII characters are non-printable codes originally used to control teletypewriter machines. While some have no relevance in modern computing, many have been repurposed for programs that run in a terminal. For example, ASCII 10 is the "line feed" code that told a teletype machine to rotate its platen up one line - this is now our familiar newline character, \n. This would normally have been followed by a "carriage return" code causing the machine's typing carriage to move all the way back to the left. On Windows systems, both characters are used to separate lines. On non-Windows systems only the \n newline is used.

In a terminal the first 26 control characters (after 0) can be typed using the Ctrl key and the corresponding letter: the 10th letter is J, so Ctrl-J types a newline. Ctrl-H types a backspace, Ctrl-G sounds a bell or beep if your terminal has sound enabled, Ctrl-I types a tab. ASCII 27 is Ctrl-[, the Esc code. ASCII 3 ("end of text") has been repurposed as Ctrl-C, "interrupt" (stop the current program in the terminal.)

Drill

ASCIIData/com.example.characters.drills.ASCIICharacters * In a for loop, iterate through the numbers 0 to 127, printing each number and the character representation of that number (by casting it to a char) on a separate line.

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