Community resourceWorksheet

J277 1.1.1 Von Neumann architecture and registers

Part 3 of 6 · J277 1.1 · Systems architecture

The stored-program model behind section 1.1, and the four registers OCR names by abbreviation.

Students will:

  • distinguish an address from the contents stored at it
  • state what the PC, MAR, MDR and accumulator each hold
  • read a processing snapshot and say what happens next
  • describe the Von Neumann model of stored programs and data
  • apply the registers to processing scenarios

Inside: 6 text sections, 1 multiple-choice activity, 3 completion activities, 6 written answers. 19 marks. Suggested teaching time: 40–60 minutes. 2 diagrams.

Includes a teacher guide with suggested sections, timings and common misconceptions.

Series: J277 1.1 · Systems architecture, part 3 of 6.

Shared by Coding PathwayVerified teacher

  • 16 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 27 Oct 2026

Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

Von Neumann architecture and registers

In a Von Neumann system, program instructions and data share the same memory. The CPU uses registers to hold particular values while processing instructions.

The key question for each register is: what does it store?

Multiple choice1 mark

Which CPU component decodes instructions and coordinates processing?

  • AAccumulator
  • BArithmetic logic unit
  • CCache
  • DControl unit

1. Address or contents?

An address identifies a location in memory. Its contents are the data or instruction stored there. Think of a locker number and the item inside the locker: they are different things.

This small example shows both an instruction and a data value in the same memory. The instruction is written in words to make its meaning clear.

Memory addresses and their contents A single memory contains two illustrated locations. Address 40 contains an instruction, shown in words. Address 41 contains the data value 12. Addresses identify locations; contents are stored at those locations. One memory Address Contents 40 Add 5 to the total 41 12 The instruction is shown in words here. Instructions and data share this memory.
Fill in the blanks2 marks
Use the memory diagram. (a) The value 12 is stored at address Answer 1. (b) 'Add 5 to the total' is an Answer 2.
Written answer2 marks

Explain how the example illustrates a feature of Von Neumann architecture.

Students type their answer here.

2. Four register roles

RegisterWhat it stores
Program Counter (PC)The address of the next instruction to be fetched.
Memory Address Register (MAR)The memory address currently being accessed, for reading or writing.
Memory Data Register (MDR)Data or an instruction being transferred between the CPU and memory.
Accumulator (ACC)Data being processed or a result produced by the ALU.

The PC is not a tally of programs. The ALU performs a calculation; the accumulator can hold its result.

What each register stores The PC and MAR hold addresses. The MDR holds transferred data or instructions. The accumulator holds data being processed or an ALU result. These are categories, not a transfer sequence. PC and MAR Addresses MDR Transferred data or an instruction Accumulator (ACC) Data being processed or an ALU result
Fill in the blanks4 marks
The Answer 1 holds the next instruction address. The Answer 2 holds the address being accessed. The Answer 3 holds transferred data or an instruction. The Answer 4 can hold an ALU result.
  • MDR
  • ALU
  • PC
  • ACC
  • MAR
Written answer1 mark

Describe the purpose of the accumulator.

Students type their answer here.

3. Use the roles

The following questions describe separate moments during processing. Use the register's role to decide your answer; you do not need a sequence of transfers.

Fill in the blanks4 marks
These are separate moments during processing. Give a register name or the requested value. (a) Before the CPU fetches from address 40, the next-instruction address is held in the Answer 1. (b) The CPU is reading location 41, which contains 12. The MAR holds Answer 2. (c) The value 12 is transferred from memory to the CPU in the Answer 3. (d) The ALU adds 5 to 12. The result 17 can be stored in the Answer 4.
Written answer2 marks

In the memory diagram, explain why 41 and 12 have different roles.

Students type their answer here.

4. Diagnose a mistake

Correct the statement by explaining the register's actual role.

Written answer1 mark

Correct this statement: 'The Program Counter counts how many programs have run.'

Students type their answer here.

5. Check from memory

Give the role, rather than just expanding the abbreviation.

Written answer1 mark

(a) What can the MDR store?

Students type their answer here.

Written answer1 mark

(b) State the key memory feature of Von Neumann architecture.

Students type their answer here.