The 7-Bit History of ASCII

In the 1960s, ASCII used only 7 bits ($2^7 = 128$ characters). The final bit was reserved for a Parity Bit to check for hardware errors during transmission. Because of this design, standard ASCII only covers the range 0x00 to 0x7F.

In modern systems, parity bits are no longer used for character encoding, so the first bit of a standard ASCII character is always set to 0.

UTF-8 Memory Representation

UTF-8 is a variable-width encoding (1 to 4 bytes). It is backwards compatible because it uses the Most Significant Bit (MSB) of every byte to tell the CPU how to read the data.

Number of BytesByte 1 (Leader)Byte 2Byte 3Byte 4
1 Byte0xxxxxxx
2 Bytes110xxxxx10xxxxxx
3 Bytes1110xxxx10xxxxxx10xxxxxx
4 Bytes11110xxx10xxxxxx10xxxxxx10xxxxxx

  • Single-Byte (ASCII): Starts with 0.

    • Pattern: 0xxxxxxx
    • Range: U+0000 to U+007F (0-127).
    • Example: A is 01000001.

  • Multi-Byte (Non-ASCII): Starts with 1.

    • The first byte tells you the length (e.g., 110xxxxx means 2 bytes total).
    • All following bytes (continuation bytes) start with 10.
    • Example: My name Hùng takes 5 bytes in memory because ù requires a 2-byte sequence (11000011 10111001).