Representing text: ASCII and Unicode

Last time

Before we start: last time, in Representing images, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Every letter is a number

When you type a WhatsApp message, the phone cannot store the letters themselves. It can only store binary. So every letter, digit, space and symbol is given its own number, and that number is stored in binary.

Character set

The list of characters a computer can use, with the unique binary value for each one, is called a character set. When you type, each character is looked up in the character set, and its binary value is stored. The values are stored one after another, in order.

ASCII

The first character set Cambridge asks about is ASCII. Standard ASCII uses seven bits for each character, so it has one hundred and twenty-eight characters. Extended ASCII uses eight bits, giving two hundred and fifty-six.

Some ASCII codes

Here are some ASCII codes. Capital A is sixty-five, B is sixty-six, C is sixty-seven: the letters are in order. Small a is ninety-seven. And the digit zero is forty-eight, not zero.

Using the order

Because the codes are in order, you can work out a code you were not given. If A is sixty-five, then E, four letters later, is sixty-nine.

Capitals and small letters

A small letter's code is always thirty-two more than its capital. A is sixty-five and a is ninety-seven. In binary, only one bit changes: the thirty-two column.

How a word is stored

Here is how the word CAT is stored. C is 0 1 0 0 0 0 1 1, A is 0 1 0 0 0 0 0 1, and T is 01010100. The three codes are stored one after the other, in order.