Knowledge Organiser: MAR and MDR Registers
Key Terms
- MAR: Memory Address Register — holds the address of the memory location to be accessed
- MDR: Memory Data Register — holds the data that has been read from or is to be written to memory
- Address Bus: Carries the address from MAR to main memory (one-way)
- Data Bus: Carries data between MDR and main memory (two-way)
- Register: Tiny, ultra-fast storage location inside the CPU
Must-Know Facts
- MAR holds the ADDRESS — connected to the Address Bus
- MDR holds the DATA — connected to the Data Bus
- During a memory READ: MAR sends address → memory returns data → MDR receives it
- During a memory WRITE: MAR holds address, MDR holds data to be written
- MAR is filled FIRST (says where), then MDR receives the result (what was there)
- MDR is also called the Memory Buffer Register (MBR) in some specifications
Key Concepts
- MAR = WHERE (address), MDR = WHAT (data) — never mix these up
- In the FDE cycle: PC → MAR → Address Bus → memory → Data Bus → MDR
- MDR acts as a buffer — temporarily holds data before it is used or stored
- Both registers work in every single memory access operation
Common Mistakes
- Swapping MAR and MDR: MAR holds the memory ADDRESS (where to look), MDR holds the actual DATA — remember "Address before Data"
- Saying registers "store" data permanently: Registers are temporary holding areas inside the CPU — they are overwritten with each new operation
- Confusing MDR with RAM: The MDR is inside the CPU; RAM is separate external memory — the MDR is just a transit point for data moving between them
- Forgetting MDR is bidirectional: The MDR can both receive data from memory AND send data to memory — it works in both directions