AS · Practice questions · The Von Neumann model and registers

Name the register.

Six original Cambridge-style questions on the Von Neumann model and the processor's registers, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Where and what.

Addresses: PC (next instruction), MAR (location being accessed now).
Contents: MDR (data or instruction to or from memory), CIR (current instruction), ACC (data and results).
Helpers: IX (added to an address), status register (flags such as carry, zero, negative, overflow).
Stored program: instructions and data together in one memory, executed one at a time.

Explain

[3 marks]

Describe the stored program concept in the Von Neumann model.

Program instructions and data are stored together in the same (main) memory. ✓
A program must be loaded into main memory before it is run. ✓
Instructions are fetched from memory and executed one at a time, in sequence. ✓

Recall

[4 marks]

State the purpose of each register.

(a) Current instruction register (CIR)
(b) Memory address register (MAR)
(c) Index register (IX)
(d) Status register (SR)

(a) Holds the instruction currently being decoded and executed. ✓
(b) Holds the address of the memory location about to be read from or written to. ✓
(c) Holds a value that is added to an address to form the address used (indexed addressing). ✓
(d) Holds flags that show the outcome of an operation, for example carry, zero, negative or overflow. ✓

Explain

[2 marks]

Explain the difference between a general purpose register and a special purpose register. Give one example of each.

A general purpose register holds any data being processed, e.g. the accumulator. ✓
A special purpose register has one specific, fixed role, e.g. the program counter. ✓

Recall

[3 marks]

Describe the role of the arithmetic and logic unit (ALU), the control unit (CU) and the system clock.

ALU: carries out arithmetic, logic operations and comparisons. ✓
CU: decodes instructions and sends control signals to coordinate the other components. ✓
System clock: sends regular timing pulses to synchronise the components. ✓

Apply

[2 marks]

The instruction ADD 150 adds the contents of address 150 to the accumulator. Address 150 holds 37 and the accumulator holds 5. Identify the register that holds 37 just before the addition, and the register that holds the result.

The memory data register (MDR) holds 37 after it is read from address 150. ✓
The accumulator (ACC) holds the result, 42. ✓

Analysis

[2 marks]

A student says: "The program counter holds the instruction that is about to be executed." Explain the two mistakes.

The PC holds an address, not an instruction. ✓
It is the address of the next instruction to be fetched; the instruction being executed is in the CIR. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.