American Standard Code for Information Interchange (ASCII)

In the early 1960s, computers could not communicate with one another: each machine represented the keys of its keyboard in its own way, so text produced on one computer meant nothing to another. The need for a common standard was recognised, and the encoding scheme ASCII — American Standard Code for Information Interchange — was developed to standardise how characters are represented. ASCII remains the most commonly used coding scheme even today.

Why 7-bit ASCII can encode exactly 128 characters

Initially, ASCII used 7 bits to represent each character. Since a binary digit can take only 2 values (0 or 1), a string of 7 bits can form:

2^7 = 128 distinct patterns

So 7-bit ASCII can encode 128 different characters — enough for every key on an English keyboard: uppercase and lowercase letters, digits, punctuation marks and control characters.

Limitation: ASCII can encode the character set of the English language only. Other scripts (such as Indian languages) have no place in the 128 codes — this is precisely the gap that ISCII and UNICODE (next sections) were created to fill.

A sample of printable ASCII codes

Some printable characters and their decimal code values (from the ASCII table):

Notice the pattern: uppercase letters start at 65 (A) and run consecutively; lowercase letters start at 97 (a). Uppercase and lowercase versions of the same letter have different codes — the computer treats 'A' (65) and 'a' (97) as different characters.

Worked example — encoding a whole word (Example 2.2)

Task: encode the word DATA and convert the encoded values into binary, the form a computer understands.

Look up each letter's ASCII value, then write each value as a 7-bit binary code:

• D → ASCII 68 → 7-bit binary 1000100
• A → ASCII 65 → 7-bit binary 1000001
• T → ASCII 84 → 7-bit binary 1010100
• A → ASCII 65 → 7-bit binary 1000001

Replacing every letter with its code gives the word's ASCII form and its binary form:

So the text "DATA" travels through the computer as the bit stream 1000100 1000001 1010100 1000001.

You can verify these codes yourself in Python:

for ch in "DATA": …