1.2 Memory and Storage

1.2.3 Units - click to expand

This section covers:

  • The units of data storage:

    • Bit

    • Nibble (4 bits)

    • Byte (8 bits)

    • Kilobyte (1,000 bytes or 1 KB)

    • Megabyte (1,000 KB)

    • Gigabyte (1,000 MB)

    • Terabyte (1,000 GB)

    • Petabyte (1,000 TB)

  • How data needs to be converted into a binary format to be processed by a computer

  • Data capacity and calculation of data capacity requirements

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Other sections in this topic:

🔗1.2.1 Primary storage (Memory)

🔗1.2.2 Secondary storage

🔗1.2.4 Data storage

🔗1.2.5 Compression

This page looks at how we deal with the actual electronic signals that the computer system uses. We will see how humans deal with the large numbers of signals that a system uses, and how we carry out some basic conversion and addition of binary and hexadecimal numbers.

How data needs to be converted into a binary format to be processed by a computer

Computers use electrical devices called transistors. The transistors are a little bit like a switch, they can be either turned on or off. One individual on or off signal isn't very useful and can only represent one small thing such as whether a pixel on a screen is turned on or off. We need a way of grouping the positions of these switches.

Because each switch can be in only two different states (on or off), we can use a binary counting system, which only uses two numbers, 0 and 1.

In the exam, you may get asked to explain why we use a binary format to represent information on a computer. The answer is that computers use on and off signals, binary has two states 0 and 1, the binary 0 and 1 state can therefore represent the on and off electrical signals.

The Units of Data Storage:

We have already looked at how computers store data and programs using primary and secondary storage devices. Everything on a computer is stored using signals which are either on or off. Modern RAM stores these signals in devices called transistors and a stick of RAM will contain billions of transistors. It becomes obvious that we need some words to describe these signals.

An individual bit cannot store very much (literally one single on or off), so we needed to group them into larger amounts. At first, there were several proposals as to how many bits should be grouped together (known as the word length to super geeky people), but eventually a standard group of eight bits became the next unit that we have to remember.

Sometimes, we need to look at a smaller number of bits, and computer scientists, in their wonderfully humorous way, decided to give this smaller amount a funny name.

You may be asked to define any of these three basic units in the exam. A bit is one single on or off, a nibble is four bits and a byte is eight bits of two nibbles.

Larger Units

We now need to consider even larger amounts of storage. Because we use binary to represent the data in a computer system (this may be one reason why we use eight bits, which is two to the power three, or two cubed), we decided to group the bytes into 1024 bytes, which is two to the power 10, which we called a kilobyte. This seemed sensible at the time, but didn't really fit in with the SI system where kilo meant only one thing, - one thousand. Some die-hard computer geeks may still insist on saying that there are 1024 bytes in a kilobyte, but for your exam, you can use either definition, and the fact is that the maths is so much easier if you use 1000.

We also need terms much larger than this:

When we talk about petabytes of data, we are talking about absolutely massive amounts of data, but there are even larger units that represent simply astronomical amounts of data. You only need to know the ones in the table above, but you must be able to convert between one and the other.

Remember the order:

byte, kilobyte, megabyte, gigabyte, terabyte, petabyte

And then remember that each one is 1000 times larger or smaller than the next. If you are asked to convert a unit from one to the next, divide or multiply by 1000.

If you are asked to convert between units that are more than one row apart, then multiply or divide by 1000 for each row you have had to move from. The maths isn't hard, but you should practice it a few times.

Calculations of Data Requirements

You may be asked in the exam to calculate how much data a certain image, sound or document will use. Look at the next few ;pages to see how we calculate file size for each type of file.

You must be able to calculate how much data certain files take up. To do this properly, you need to understand how text, images and sound are stored in binary format, which is covered in the next section of this web site. The Craig and Dave video below on Data Capacity and calculations is really good.

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Other Resources for this topic

Past Paper Questions

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🔗 1.2.3 Units Past Papers 1

🔗 1.2.3 Units Past Papers 2

Craig and Dave Videos (Requires login)

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🔗1.2.4 Data Storage