MAR, MDR, Program Counter, Accumulator

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

1.1.1 Systems Architecture

Systems Architecture — Note 3 of 3

What you need to know

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.

What to know

Von Neumann architecture stores both instructions AND data in the SAME memory — this is a key characteristic.

Key terms

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.
MARMemory Address Register: Holds the ADDRESS of the memory location to be read from or written to.
MDRMemory 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.
Program CounterPC: The PC holds the ADDRESS of the NEXT instruction to be fetched. It is incremented each cycle.
Accumulator: Stores the RESULTS of calculations performed by the ALU. Also temporarily holds data being processed.

Exam technique

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.
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.

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'.

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