Von Neumann Architecture

The golden rule of modern computing: Both data and instructions are stored in the same memory. Master the Registers.

Concept Explorer: Register Inspector

Registers are super-fast memory slots built right into the CPU. Click on a register to inspect its contents. Notice the difference between storing an Address Location (like a house number) and storing the actual Data (the people inside the house).

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The Stored Program Concept

Data & Instructions in RAM

Before Von Neumann, computers had to be physically re-wired across a room to change programs. The Von Neumann architecture proposed that instructions (the program) and data (the files) should both be stored in the same main memory (RAM) in binary format.

The Specific Registers

Program Counter (PC)

Stores Address

It stores the memory address of the next instruction that needs to be fetched from memory. It increments by 1 during every fetch stage.

Memory Address Register (MAR)

Stores Address

It stores the memory address of the instruction or data that is currently being fetched from (or written to) memory.

Memory Data Register (MDR)

Stores Data

It holds the actual data or instruction itself that has just been fetched from RAM, or is waiting to be saved to RAM.

Accumulator (ACC)

Stores Data (Results)

It acts as a temporary notepad for the ALU, storing the results of any logical or arithmetic calculations.

Check Your Understanding

Examiner's Eye Trap

1. What is the explicit purpose of the Memory Address Register (MAR)?

2. The ALU has just calculated `5 + 10`. Where is the result (`15`) immediately stored?

Written Exam Scenario (AO2/AO3)

1 / 3

"A student states: 'The MAR stores the data being fetched from memory'. Explain why this statement is incorrect and will result in zero marks." (3 marks)