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Systems architecture 3: Von Neumann architecture and registers

The Von Neumann architecture provides the model used for this topic. Program instructions and data are stored in memory, and the CPU fetches and processes the instructions. Inside the CPU, registers temporarily hold particular values needed during processing.

The abbreviations may look unfamiliar at first. Keep asking one simple question: what does this register store? You are learning four short roles, not a long hidden process.

OCR J277 names four registers:

Program Counter (PC)
Memory Address Register (MAR)
Memory Data Register (MDR)
accumulator (ACC)

By the end, you should be able to state what each register stores, distinguish data from an address and apply the registers to processing scenarios.

Multiple choice1 mark

Which CPU component coordinates the fetch-execute cycle and decodes instructions?

AAccumulator
BArithmetic logic unit
CCache
DControl unit

1. Address or data?

An address identifies a location in memory. It tells the computer where an item can be found or where it should be stored.

Data is the content held at a location. In this topic, data can include a value or an instruction being transferred or processed.

Imagine numbered lockers:

locker number 42 is an address
the instruction inside locker 42 is the data or content at that address

This distinction is central to the register questions. The PC and MAR store addresses. The MDR and accumulator store data or instructions, not addresses.

Always describe a register using the verb stores. A register holds a value; another component carries out the action.

Fill in the blanks6 marks

An ______ identifies a ______ in memory. ______ is the ______ stored at that location. The ______ store addresses, while the ______ store content rather than memory addresses.

address
data
location
content
PC and MAR
MDR and accumulator

2. The four required registers

Program Counter (PC)

The Program Counter stores the memory address of the next instruction to be fetched. It is not a count of how many programs or instructions have finished.

Memory Address Register (MAR)

The Memory Address Register stores the address in memory that is currently being accessed. This may be the address of data or an instruction to be fetched, or the address where data will be stored.

Memory Data Register (MDR)

The Memory Data Register stores data or an instruction being transferred between the CPU and memory. It can hold an item fetched from memory or an item that is about to be stored in memory.

Accumulator (ACC)

The accumulator stores data currently being processed and results produced by arithmetic or logical operations in the ALU.

Notice the repeated precision: a register stores something.

Fill in the blanks6 marks

The ______ stores the address of the ______ to be fetched. The ______ stores the address in memory currently being accessed. The ______ stores data or an instruction being transferred between memory and the CPU. The ______ can store a ______ produced by the ALU.

Program Counter
Memory Address Register
Memory Data Register
accumulator
next instruction
calculation result

Multiple choice, several answers4 marks

Which four register statements are correct? Select all that apply.

AThe PC stores the address of the next instruction to be fetched
BThe PC counts how many programs have been opened
CThe MAR stores the address in memory currently being accessed
DThe MDR can store data or an instruction transferred between the CPU and memory
EThe MDR stores only memory addresses
FThe accumulator can store a result produced by the ALU

Written answer8 marks

Describe the purpose of the PC, MAR, MDR and accumulator.

Write one sentence per register. Begin each sentence with “stores” and specify whether the value is an address, data or an instruction.

Students type their answer here.

3. Read a processing snapshot

A CPU is about to fetch the instruction stored at memory address 208. The instruction found there is ADD 7. After execution, the ALU produces the result 19.

At different moments, the relevant values could be described as:

208 is an address
ADD 7 is an instruction
19 is processed data or a result

You do not need to memorise the exact movement of these values from one register to another at every stage. OCR requires you to know the purpose of each register, what it stores and the difference between an address and data.

Written answer8 marks

Using the snapshot above, identify a suitable register for each item and explain why.

A. The address of the next instruction, 208

B. The address currently being accessed, 208

C. The instruction ADD 7 while it is transferred from memory

D. The result 19 after the calculation

Write four pairs: register, then what that register stores.

Students type their answer here.

4. The Von Neumann model

In a Von Neumann system, program instructions and data are stored in the same memory. The CPU can fetch an instruction from memory, decode it and execute it, then continue with the next instruction.

For OCR J277, your assessed register set is PC, MAR, MDR and accumulator. Some older resources show additional registers, but these four are the registers listed in the current 1.1 specification.

You are not required to learn a detailed register-transfer script for every stage. Concentrate on the stable purpose of each named register.

Written answer4 marks

Explain the difference between storing data and storing an address. Use the MAR and MDR as examples.

Define an address as a location and data as the content. Then link one definition to each register.

Students type their answer here.

Written answer8 marks

A student writes: “The PC fetches the next instruction, the MAR stores the instruction, the MDR stores its address, and the accumulator does every calculation.” Correct all four statements.

Use “stores” for every register. Remember that the ALU performs calculations; the accumulator holds data or results.

Students type their answer here.

Written answer8 marks

Challenge: use all four registers together. Explain why confusing an address with data would make an answer about registers inaccurate.

First define address and data. Then group the registers into address holders and content holders, explain the distinct purpose of each, and finish by linking the distinction back to the question.

Students type their answer here.

Closed-book review

Draw two headings: stores an address and stores data or an instruction. Place PC, MAR, MDR and accumulator under the correct heading. Then write the exact purpose of each register.

Finally, answer:

Which register is often wrongly described as counting programs?
Which component performs a calculation?
Which register stores its result?

Check that every register description says what it stores.