GCSE · Computer Science · AQA · Spec 8525

Registers

An instruction sits at address 20 in memory. The CPU can't just grab it: it must send the address out and wait for the contents to come back. Let's watch.

Computer Science · Inside the CPU

Follow one instruction round the registers

Main memory: address 20 holds the instruction LOAD 50, which means “put the data from address 50 into the accumulator”. Address 50 holds the number 7. The PC starts on 20.

Predict first: the 7 passes through one register on its way to another. Which register does it end up stored in?

Phase: Fetch — the starting point

MAR

—

MDR

—

CIR

—

Acc

—

Address bus

—

Data bus

—

Control bus

—

Output

PC = 20 (the address of the next instruction)

Step 1: The PC holds 20. That isn't a count of anything — it's the address where the next instruction is waiting.

1 / 12

Press Next to move one step at a time. A dash means that store isn't holding anything for this instruction yet. The log underneath keeps a record of every move.

Step 1 of 12: The PC holds 20. That isn't a count of anything — it's the address where the next instruction is waiting..

Watch out: MAR and MDR sound alike. The MAR holds an Address — where to look. The MDR holds whatever comes back from that address: an instruction or a data item.

Computer Science · Structure

Where the registers live

Zoom out from what you just watched. Tap any part to see its job.

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

The fork at the MDR

Instruction or data? Follow the route

A value has just arrived in the MDR. Pick what it is, and follow where it goes.

What arrived in the MDR?

2 possible routes.

On A value arrives in the MDR on the data bus. 2 branches to choose from.

Quick honesty check

What do the PC and MAR really hold?

Which of these sounds most right to you right now?

The program counter counts how many instructions the program has. That's why it goes up by one each time.The MAR goes to memory and holds the data it finds there.The PC holds the next instruction, ready to be carried out.The PC holds the address of the next instruction. That address is copied to the MAR, which sends it to memory on the address bus.

How sure are you?

I'm sureI think soI'm guessing

The program counter counts how many instructions the program has. That's why it goes up by one each time.

This is a common misconception

Tempting — it's called a counter, and it does go up by one. But the number in it isn't a tally. It's an address: where the next instruction is waiting. It goes up by one so that it points to the next instruction in the sequence. Think bookmark, not scoreboard.

The MAR goes to memory and holds the data it finds there.

This is a common misconception

This is the classic mix-up. The MAR only holds an address — the “where”. It sends that address out on the address bus. What's stored there comes back on the data bus and lands in a different register: the MDR.

The PC holds the next instruction, ready to be carried out.

This is a common misconception

Close, but the PC holds where the instruction is, not the instruction itself. The instruction arrives later, in the MDR, and is then copied into the CIR.

The PC holds the address of the next instruction. That address is copied to the MAR, which sends it to memory on the address bus.

This matches the evidence

Exactly right. The PC and the MAR both hold addresses. The contents of that address come back separately, into the MDR.

Addresses go out; contents come back. The PC and MAR deal in addresses — the MDR catches whatever returns.

Your turn

Write the fetch in your own words

Describe how the registers are used to fetch an instruction from main memory. [5 marks]

Your answer

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

The mark scheme

The address in the PC is copied into the MAR. (1 mark)
The MAR sends the address along the address bus to main memory. (1 mark)
The instruction stored at that address comes back on the data bus and is held in the MDR. (1 mark)
The instruction is copied from the MDR into the CIR, where the control unit takes it to decode it. (1 mark)
The PC is increased by one, so it points to the next instruction. (1 mark)

Model answer

The program counter holds the address of the next instruction, and this address is copied into the MAR. The MAR sends the address along the address bus to main memory. The instruction stored at that address is sent back to the CPU on the data bus and held in the MDR. Because it is an instruction, it is copied from the MDR into the CIR, where the control unit can take it and decode it. During the fetch, the control unit also tells the PC to increase by one, so it points to the next instruction.

Check your answer for the two places learners slip: the address goes to the MAR (not the MDR), and the PC goes up by one so it points to the next instruction — it isn't counting them.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Five tiny stores inside the CPU — and one question that tells them apart: is it holding an address, or what was stored there?

What you need to know

Registers are very small, very fast memory locations inside the CPU. The CPU's other key components are the control unit, the ALU, the buses and the clock.
PC (program counter): holds the memory address of the next instruction to fetch. During each fetch it is increased by one.
MAR (memory address register): holds the address being looked up in main memory, and sends it along the address bus.
MDR (memory data register): holds whatever comes back from memory on the data bus — an instruction or a data item.
CIR (current instruction register): holds the binary representation of the instruction to be executed; the control unit takes it from here to decode it.
Acc (accumulator): holds data being used in calculations and the results of calculations.

The big picture

Registers are very small, very fast memory locations inside the CPU. Five of them each have their own job. The program counter (PC) holds the address of the next instruction and goes up by one during each fetch. The memory address register (MAR) holds an address and sends it to main memory on the address bus. The memory data register (MDR) catches whatever comes back on the data bus. If that is an instruction, it is copied to the current instruction register (CIR) for the control unit to decode; if it is data, it is stored in the accumulator (Acc).

Key points

1Address versus contents is the big split: the PC and the MAR hold addresses; the MDR, CIR and accumulator hold what was stored at an address.

2The PC is a pointer, not a tally: it is increased by one so that it points to the next instruction in the sequence.

3After the MDR the route forks: an instruction is copied to the CIR; a data item is stored in the accumulator.

4The control unit uses the control bus to tell the PC to increase by one, and to put an address taken from a decoded instruction into the MAR.

5Fetch, decode, execute — then the cycle repeats, starting from the address now in the PC.

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Worked example

Problem

You catch the CPU mid-cycle and see: PC = 63, MAR = 62, MDR = LOAD 90, CIR = LOAD 90. (LOAD 90 means “put the data stored at address 90 into the accumulator”.) (a) Which address was this instruction fetched from? (b) What will the MAR hold next, and what puts it there? (c) Where will the value from address 90 end up?

⚠ Watch out

Saying the MAR holds the data it finds. The MAR only ever holds an address. The value found at that address arrives in a different register — the MDR.

🧠

Memory hook

Where versus what. The PC and MAR hold WHERE (an address). The MDR, CIR and Acc hold WHAT (the instruction or data found there). And the PC is a bookmark, not a scoreboard.

✓

Check yourself

Without scrolling up: the PC holds 8. Which register gets the 8 first, which bus takes it to memory, where does the reply land — and where does it go next if it's an instruction?

Flashcards

(14)

What is a register?

A very small, very fast memory location inside the CPU.

Name the five registers in this lesson.

Memory address register (MAR), memory data register (MDR), current instruction register (CIR), program counter (PC) and accumulator (Acc).

What does the program counter (PC) hold?

The memory address of the next instruction the CPU needs to fetch.

What happens to the PC during the fetch stage, and why?

The control unit tells it, on the control bus, to increase by one — so it points to the next instruction in the sequence.

What does the memory address register (MAR) hold?

The memory address being searched for in RAM. It is copied from the PC (or put there by the control unit) and sent along the address bus.

What does the memory data register (MDR) hold?

The instruction or data item that main memory has just sent back to the CPU on the data bus.

What does the current instruction register (CIR) hold?

The binary representation of the instruction to be executed. The control unit takes it from the CIR to decode it.

What does the accumulator (Acc) store?

Data being used in calculations and the results of calculations.

Which bus carries an address from the MAR to main memory?

The address bus.

Which bus brings an instruction or data item back from main memory to the MDR?

The data bus.

The MDR is a fork in the road. What are the two turnings?

Instructions turn off to the CIR, to be decoded. Data items turn off to the accumulator.

Which two registers hold memory addresses?

The PC and the MAR.

Apart from copying the PC, how else can an address get into the MAR?

The control unit can put an address taken from a decoded instruction into the MAR, using the control bus.

Besides the registers, what are the other key components inside the CPU?

The control unit, the arithmetic logic unit (ALU), the buses and the clock.

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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Registers

Follow one instruction round the registers

Where the registers live

Instruction or data? Follow the route

What do the PC and MAR really hold?

Rebuild the register grid

Write the fetch in your own words

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