Character sets, and why ASCII was not enough

A character set is an agreement

A computer stores only numbers, so text needs a table mapping each character to a code. ASCII uses $7$ bits, giving $128$ codes — enough for English letters, digits and punctuation. Extended ASCII uses $8$ bits for $256$.

Two facts about ASCII worth knowing

The codes are contiguous and ordered: 'A' is $65$, so 'C' is $67$; 'a' is $97$, exactly $32$ more than 'A'. That gap of $32$ is one bit, which is why changing case is a single bit operation. Digit '0' is $48$, so the character '7' minus $48$ gives the number $7$ — the standard way to convert a digit character to its value.

Why Unicode

$256$ codes cannot hold Vietnamese, Chinese, Arabic, Greek and emoji at the same time. Unicode gives every character in every writing system a unique code point. UTF-$8$ stores them in one to four bytes, and is designed so that the first $128$ code points are byte-identical to ASCII — old files still work.

The trade-off

Unicode text takes more storage than ASCII for the same message. That is the price of covering every language, and it is the comparison Cambridge asks for.