Learn: MDR - Storing the Data

Welcome!You've already learned how the CPU processes instructions using registers like the MAR and the Program Counter. Today, we'll dive deeper into the MDR, which is responsible for handling data during the fetch-decode-execute cycle. Let's get started!

What is the MDR?The Memory Data Register (MDR) is a register in the CPU that temporarily stores data being transferred to or from memory. It holds the actual data that the CPU will process or write to memory.The MDR works closely with the Memory Address Register (MAR), which specifies the location of the data in memory.

Why is the MDR important?Without the MDR, the CPU would not be able to process or store data effectively. It acts as a middleman between the CPU and memory, ensuring that data is stored and retrieved correctly during the fetch-decode-execute cycle.For example, when the CPU needs to perform an operation like addition, the MDR will hold the numbers being added together as well as the result before sending it back to memory.

How does the MDR work?When the CPU needs to read data from memory, the MAR holds the memory address where the data is stored. The MDR then retrieves the data from that address and temporarily stores it.Similarly, when writing data to memory, the MDR holds the data until it is written to the location specified by the MAR.

How does the MDR support the fetch-decode-execute cycle?During the fetch phase, the CPU uses the MAR to locate the memory address of the instruction or data it needs. The MDR then stores the data retrieved from that address.Once the CPU has decoded the instruction, the MDR may store new data that the CPU generates during the execute phase, ready to be sent back to memory.

Example of MDR in ActionImagine you are using a calculator on a computer to add two numbers: 3 + 5. The CPU reads the first number (3) from the memory location using the MAR, and the MDR stores the value. The same process happens for the second number (5).After the addition is performed by the ALU, the result (8) is stored in the MDR before being written back to memory.

Review Time!Great work! You've learned about the MDR and its role in storing and transferring data during the fetch-decode-execute cycle. Let's review with a few questions.