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 → CIR
- 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
This topic summary covers Knowledge Organiser: MAR and MDR Registers within MAR, MDR & Registers for GCSE Computer Science. Revise MAR, MDR & Registers in 3.4 Computer Systems for GCSE Computer Science with 15 exam-style questions and 10 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for MAR, MDR & Registers
What does MAR stand for?
Explain the difference between the MAR and the MDR.
Quick recall flashcards
What does MDR hold?
The actual data being transferred (WHAT)
What does MAR hold?
The address of data to be read/written (WHERE)