Understand how characters are represented using ASCII and Unicode character sets, and the effect of the character set on storage and the range of characters.

What this dot point is asking

AQA wants you to explain how characters are stored using ASCII and Unicode, and to describe the effect of the character set on the number of characters that can be represented and on the storage each character needs.

Character sets

The reason a standard is essential is that there is nothing about the bit pattern $01000001$ that "is" the letter A; it only means A because everyone agrees on the ASCII table. Before standard character sets, files moved between systems often became unreadable because each manufacturer used its own mapping.

ASCII

Two ordering facts are worth memorising because AQA tests them: the capital letters run from 65 ('A') to 90 ('Z'), and the lower-case letters from 97 ('a') to 122 ('z'). Because the codes are contiguous you can calculate any letter's code from a known one by adding the offset, and convert case by adding or subtracting 32.

Unicode

Unicode solved the problem that 128 characters cannot cover Chinese, Arabic, Greek, mathematical symbols and emoji all at once. By making the first 128 code points identical to ASCII, Unicode stays backward-compatible: an old ASCII file opens correctly in a Unicode-aware program.

The effect of the character set

Calculating storage for text

Because a character set fixes the number of bits per character, you can calculate how much storage a piece of text needs. With 7-bit ASCII (often stored in an 8-bit byte), the word "Computer" of eight characters needs eight bytes. With a wider Unicode encoding using, say, two bytes per character, the same word would need sixteen bytes, twice as much. This is the storage cost of Unicode's larger range: every character takes more bits, so a document covering many languages is larger than the same length of plain ASCII English. Questions may ask you to work out the size of a string in bits or bytes given the bits per character, so multiply the number of characters by the bits per character, then divide by 8 for bytes.

Why ordered codes are useful

The fact that character codes run in order is not just a curiosity; programs rely on it. Sorting words alphabetically works by comparing the codes of their characters. Checking whether a character is a digit, an upper-case letter or a lower-case letter is done by testing whether its code falls in the relevant range. Converting a letter between cases is done by adding or subtracting 32, because upper and lower case are exactly 32 apart in ASCII. These tricks all depend on the codes being contiguous and ordered, which is why AQA expects you to know that 'A' is 65 and 'a' is 97.

Try this

Q1. State how many characters can be represented using 7-bit ASCII. [1 mark]

• Cue. 128, because $2^7 = 128$.

Q2. State one advantage of Unicode over ASCII. [1 mark]

• Cue. It can represent many more characters, including other languages and emoji.