Von Neumann architecture: MAR, MDR, Program Counter, Accumulator

Von Neumann architecture is a design model where both instructions and data are stored in the same memory. Most modern CPUs follow this model. The architecture uses key registers — MAR, MDR, Program Counter, and Accumulator — to manage the flow of data and instructions.

Key points

• Definition:Von Neumann Architecture: a computer design where both program instructions and data are stored in the same main memory and share the same pathways to the CPU.
• Definition:MAR (Memory Address Register): holds the ADDRESS of the memory location to be read from or written to.
• Definition:MDR (Memory Data Register): the MDR holds the DATA that has been fetched from memory or is about to be written to memory. Acts as a buffer between the CPU and main memory.
• Definition:Program Counter (PC): the PC holds the ADDRESS of the NEXT instruction to be fetched. It is incremented each cycle.
• Definition:Accumulator: stores the RESULTS of calculations performed by the ALU. Also temporarily holds data being processed.
• Exam Tip:An ADDRESS is a LOCATION in memory. DATA is the actual value or instruction stored at that location. The MAR stores an address; the MDR stores data. Do not confuse these.
• Common Mistake:Saying the Program Counter 'counts programs' or 'tracks how many programs have run'. It stores the ADDRESS of the NEXT instruction — it does not count anything.
• Von Neumann architecture stores both instructions AND data in the SAME memory — this is a key characteristic.
• Exam Example:If asked to name a register not in a table and describe its purpose: Accumulator — 'stores the result of calculations carried out by the ALU'.