Von Neumann Architecture (OCR GCSE Computer Science): Revision Notes

Von Neumann Architecture

The Von Neumann architecture describes a computer system where both data and instructions (programmes) are stored in the same memory. This design is widely used in modern computers.

MAR and MDR

MAR (Memory Address Register)

• Stores the memory address of the data or instruction to be fetched from or sent to memory.
• Its purpose is to point to the location in memory where the CPU needs to read or write data.

MDR (Memory Data Register)

• Temporarily holds the data that has been fetched from or is about to be written to memory.
• Its purpose is to act as a buffer between the CPU and memory, storing data being transferred.

Purpose of Each Register

• MAR stores an address, telling the CPU where in memory to fetch or send data.
• MDR stores data, either coming from or going to memory.

Difference Between Storing Data and an Address

• Storing data means holding the actual information being processed (e.g., a number or a letter).
• Storing an address means holding a reference to a specific location in memory where data is stored.

Programme Counter and Accumulator

Program Counter (PC)

• Holds the address of the next instruction to be executed by the CPU.
• It automatically increments after each instruction is fetched, ensuring instructions are executed in sequence.

Accumulator (ACC)

• A special register is used to store intermediate results of arithmetic and logic operations.
• It temporarily holds data being processed by the CPU, particularly the outcome of calculations.