GCSE · Computer Science · AQA · Spec 8525

Buses

You can't just shout 'book!' in a library. You say which shelf, say what to do, and wait for it to arrive. Your computer does the same with three buses.

Computer Science · Buses

Follow one memory read

Step through it. Only the bus that is working lights up.

The processor wants the value stored in memory location 12. Before you press Next: which bus do you think goes first?

Phase: Start: all three buses are quiet

Address bus

—

Control bus

—

Data bus

—

Output

Step 1: The processor needs the value stored in location 12. It can't just shout 'give me that!' at memory. It has to say where, say what to do, then wait for the thing to come back.

1 / 4

Step through the trace — every value is the lesson’s, not run by the page.

Step 1 of 4: The processor needs the value stored in location 12. It can't just shout 'give me that!' at memory. It has to say where, say what to do, then wait for the thing to come back..

Computer Science · Structure

The system bus: three roads, one set of places

Tap a bus to see what it carries and which way it runs. Arrowheads show direction.

Tap any part of the diagram to see what it does.

Computer Science · Buses

Which bus does it travel on?

Pick an item, then pick the bus that carries it. Each item belongs to exactly one bus.

Still to sort

Address bus (0)

Names a location

Where the line is: If it says where, it's the address bus. It never carries the thing stored there.

Data bus (0)

Carries the data or instruction itself

Where the line is: If it's the content being moved, it's the data bus, and that includes instructions.

Control bus (0)

Signals that coordinate the components

Where the line is: If it tells a component what to do or when, it's the control bus.

7 of 7 still to sort.

Computer Science · Buses

Which of these sounds like you?

A processor reads a value from memory, and later writes a value back to it.

Which is closest to what you think right now?

The address bus carries the data, because it's the bus that points at the memory.All three buses run both ways, so the address bus can bring addresses back to the processor.The address bus carries a location outwards from the processor. The data bus carries data both ways.A wider data bus means the computer can reach more memory locations.

How sure are you?

I'm sureI think soI'm guessing

The address bus carries the data, because it's the bus that points at the memory.

This is a common misconception

Easy swap to make, but the address bus only carries the location: where to look. The thing stored there comes back on the data bus. Think of the address as the shelf number and the data as the book.

All three buses run both ways, so the address bus can bring addresses back to the processor.

This is a common misconception

Not quite. The data bus and the control bus are two-directional, but the address bus is not. An address goes outwards from the processor, and what comes back from memory is data, on the data bus.

The address bus carries a location outwards from the processor. The data bus carries data both ways.

This matches the evidence

Yes. One-way for the location, two-way for the data, because data is read into the processor and written out of it.

A wider data bus means the computer can reach more memory locations.

This is a common misconception

Width does matter, but this is the wrong bus. A wider data bus moves more bits in each transfer. It's the address bus that decides how many locations can be named: n address lines identify 2 to the power n addresses.

The address bus names the place, one way out. The data bus carries the thing, both ways. Width means different things on each.

Computer Science · Buses

Each wire doubles it

An address bus with 8 wires can identify 256 different locations. How many different locations can a 16-wire address bus identify? Drag to your guess, then lock it in.

Your estimate

50000 locations

An address bus with 8 wires can identify 256 different locations. How many different locations can a 16-wire address bus identify? Drag to your guess, then lock it in. Estimate between 0 locations and 100000 locations.

0 locations100000 locations

Computer Science · Buses

Now you do a write

The processor wants to store the value 9 in memory location 40. Describe how the three buses work together to do this. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The address 40 is placed on the address bus, travelling from the processor to memory. (1 mark)
A memory write signal is sent on the control bus. (1 mark)
The value 9 travels on the data bus, from the processor towards memory. (1 mark)
The address and the write signal go first; the data follows. (1 mark)

Model answer

First the processor puts the address 40 on the address bus, so memory knows which location to use. It sends a memory write signal on the control bus, so memory knows to write rather than read. Then the value 9 travels along the data bus from the processor to memory and is stored in location 40.

Same three buses, same order as the read. The one thing that flips is the direction the data travels on the data bus.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

A bus is a set of parallel wires carrying signals between the processor, main memory and the input/output controllers.
Three buses make up the system bus: the address bus, the data bus and the control bus. Each has its own job.
Address bus: carries the location to read or write, from the processor outwards. One-directional.
Have a goYour mate Jo is very confident that the address bus brings the data back from memory. What has Jo mixed up?
The address bus carries a location, not data, and only outwards from the processor.
An address names the place and travels one way, out from the processor, so it can't bring anything back.
Data bus: carries data and instructions, in both directions. Data is read into the processor and written out of it.
Have a goThe processor stores a value into memory. On the data bus, does that value travel into the processor or out of it?
Out of the processor, towards memory.
Writing sends data out and reading brings data in, which is why the data bus is two-directional.
The control bus carries control signals that coordinate the components, such as memory read, memory write, the clock and interrupts. It is two-directional.
In a memory access, the address goes on the address bus, a read or write signal goes on the control bus, then the data travels on the data bus.
Have a goPriya says 'the data arrives first, then you tell memory the address.' In a memory read, what actually goes first, and on which bus?
The address, on the address bus.
Memory has to be told where to look before it can hand anything back; the read signal follows on the control bus, and only then does the data travel.
Width means the number of wires, which is how many bits the bus carries at the same time.
A wider data bus moves more bits per transfer.
A wider address bus can address more locations: n address lines identify 2 to the power n addresses.
Have a goA tiny address bus has just 3 wires. How many different addresses can it identify?
8
n lines identify 2 to the power n addresses, so 2 to the power 3 is 8, not 3 or 6.

The big picture

The system bus is three buses with three different jobs. The address bus carries a location outwards from the processor, the control bus carries signals such as memory read and write, and the data bus carries the data itself in both directions. In a memory access they act in that order, and their width decides how many bits move at once and how many locations can be addressed.

Key points

1Three buses, three jobs: address (where), control (what to do), data (the thing itself).

2Only the address bus is one-way, outwards from the processor. The data bus and control bus run both ways.

3A memory access goes in order: address, then read or write signal, then data.

4Wires equal width. A wider data bus moves more bits per transfer, and n address wires can name 2 to the power n locations.

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Worked example

Problem

A computer has a 12-wire address bus and an 8-wire data bus. (a) How many different memory locations can it address? (b) How many bits can move along the data bus in one transfer?

⚠ Watch out

Saying the address bus carries data. It doesn't: it only carries the location, outwards from the processor. The stored value comes back on the data bus.

🧠

Memory hook

A, C, D: Address, Control, Data. It's alphabetical, and it's also the order a memory access happens in.

✓

Check yourself

Which bus would you widen to let a computer address more memory, and which would you widen to move more bits at a time? Say what each does.

Flashcards

(13)

What is a bus?

A set of parallel wires that carries signals between the processor, main memory and the input/output controllers.

What are the three buses that make up the system bus?

The address bus, the data bus and the control bus.

What does the address bus carry?

A memory address, naming the location to read from or write to.

Which direction does the address bus run?

One way only: from the processor outwards.

What does the data bus carry?

The data and instructions being transferred between the processor, main memory and input/output devices.

Why is the data bus two-directional?

Data is both read into the processor and written out of it.

What does the control bus carry?

Control signals that coordinate the components, such as memory read and write signals, the clock signal and interrupt signals.

Is the control bus one-way or two-way?

Two-directional.

In a memory read, which bus acts second, and what goes on it?

The control bus: the read signal goes out after the address and before the data comes back.

What is the width of a bus?

The number of wires it has, which is the number of bits it can carry at the same time.

What does a wider data bus change?

More bits move in each transfer.

What does a wider address bus change?

More memory locations can be addressed.

How many different addresses can n address lines identify?

2 to the power n.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow one memory read

The system bus: three roads, one set of places

Which bus does it travel on?

Which of these sounds like you?

Each wire doubles it

Now you do a write

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