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The Library Search
The Library Search
Imagine you're in a massive library looking for a specific book. First, you write down the shelf number on a slip of paper (that's the MAR - it holds the ADDRESS of where to look). You take that slip to the librarian, who finds the shelf, grabs the book, and hands it to you. You now hold the actual book (that's the MDR - it holds the actual DATA that was found). The MAR says WHERE, the MDR holds WHAT. Simple as that!
Deep Dive: How MAR and MDR Work Together
When the CPU needs data from memory, here's exactly what happens:
Earn the mark scheme marks
Exam Tips - MAR and MDR
The golden rule:
- MAR = Address = WHERE = Address Bus
- MDR = Data = WHAT = Data Bus
Common mistakes to avoid:
- Mixing up MAR and MDR - this loses ALL marks for that question!
- Forgetting which bus each connects to
- Saying MAR holds data or MDR holds addresses
Top tip: If you're ever unsure, think "Address before Data" - MAR works first (says where), then MDR receives the result (the data).
Now try it yourself
Quiz · Question 1 of 15
What does MAR stand for?
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This topic in real past papers
Every real exam question we've found on mar, mdr & registers, with a full worked answer.