ASCII to Binary

Task 1 - Characters are numbers

Here is the table of printable ASCII characters. For each character it gives the ASCII code in denary (Den) and in hex. Keep this table open for the rest of the worksheet.

Look at the table and answer the questions below. You do not need to convert anything to binary yet.

Task 2 - The conversion process

To turn a character into binary, follow these steps every time.

1. Look up the character in the ASCII table to find its code in denary.
2. Convert the denary code to 8-bit binary.
3. Check your answer by adding the column values back up.

For step 2, use the 8-bit column values and ask, from the left, "does this column fit into what is left?".

Because ASCII only goes up to 127, the 128 column will always be 0.

Worked example: the letter K

Step 1. Look up K in the table. Its code is 75.

Step 2. Convert 75 to binary.

Convert 75

128 no                          bit = 0
 64 fits      75 -  64 =  11    bit = 1
 32 no                          bit = 0
 16 no                          bit = 0
  8 fits      11 -   8 =   3    bit = 1
  4 no                          bit = 0
  2 fits       3 -   2 =   1    bit = 1
  1 fits       1 -   1 =   0    bit = 1

Answer    01001011

Step 3. Check: 64 + 8 + 2 + 1 = 75. Correct, so K is stored as 01001011.

Another route via hex. If you have done the Binary to Hexadecimal worksheet, you can also use the hex column. K is hex 4B. Turn each hex digit into a nibble: 4 = 0100 and B = 1011, giving 0100 1011. Same answer. Use whichever method you find easier, and use the other one to check.

Task 3 - Convert single characters

Work out each of these on paper using the three steps from Task 2. Write the ASCII code, then the 8-bit binary. Only press Check Answers once you have done them all.

Task 4 - Encode a word

A word or sentence is stored as one byte per character, one after another, in order. Here is how the word Cat is stored.

In memory these three bytes sit side by side: 01000011 01100001 01110100

Notice that upper case C and lower case a come from different parts of the table. Case matters. Cat and cat are stored differently.

Now encode the word Data, one character at a time.

Now encode your own initials, including a full stop after each one (for example J.S.). Write the bytes on paper. You can check them with the encoder in Task 7.

Task 5 - Decode a hidden message

Decoding is the same process backwards. For each byte, convert the binary to denary by adding up the column values, then look up the character in the ASCII table.

Worked example. 0100 1111

64 + 8 + 4 + 2 + 1 = 79. Code 79 in the table is O.

Decode this four-character message.

0100 0010    0111 1001    0111 0100    0110 0101

Task 6 - Spotting the patterns

ASCII was designed with some useful patterns. Once you know them you can work out many codes without the table.

Pattern 1. The alphabet is in order. A is 65, B is 66, and so on up to Z at 90. Lower case runs from a (97) to z (122). If you know A = 65 you can count on to any capital letter.

Pattern 2. Upper and lower case differ by 32. Compare these two rows.

The only difference is the bit in the 32 column. Switching that one bit switches between capital and small letters. This is how a computer can change the case of text very quickly.

Pattern 3. Digits are not stored as their own value. The character 7 is code 55, which is 0011 0111. The right-hand nibble is 0111, which is 7, and every digit character starts with 0011.

Task 7 - Check your work with the encoder

Now that you can do the conversion by hand, use the encoder below to check your answers. Type any text (up to 16 characters) and it shows each character, its ASCII code and its 8 bits, and then the full stream of bits as it would sit in memory.

1. Type the initials you encoded in Task 4 and compare with your paper answers.
2. Type Cat and then cat. Which bit changes?
3. Type a word with a space in it. What does the space become?
4. Type a £ sign or an accented letter such as é. What happens?

Task 8 - How much space does text take?

Because each ASCII character is stored in one byte, working out storage is simple.

Remember that spaces and punctuation are characters too. Hello, world! has 13 characters, so it needs 104 bits, or 13 bytes.

Task 9 - The limits of ASCII

ASCII was designed in the 1960s for English. With only 128 codes there is no room for characters such as £, é, ŵ, Chinese or Arabic script, or emoji. You saw this in Task 7, where the encoder could not convert them.

Extended ASCII used the spare eighth bit to add another 128 characters (codes 128 to 255), but different countries filled those codes with different characters, so text often came out garbled when it moved between computers.

The modern answer is Unicode, which gives a unique number to over 150,000 characters from almost every writing system. The most common way of storing Unicode, UTF-8, uses one byte for the first 128 characters and two to four bytes for everything else. Those first 128 characters are exactly the same as ASCII, so everything you have learned in this worksheet still applies to plain English text today.

Task 10 - Check your understanding