3. Registers in a CPU - MDR and MAR

The diagram below shows the main registers within a CPU.

Note that with the Von Neumann architecture, both data and programs, whilst in use, are located in main memory (RAM).

Memory Data Register.

In order for the CPU to fetch an instruction from main memory (RAM), the instruction is temporarily stored in a special register called the Memory Data Register (MDR).

Data (unlike instructions) goes both ways. Data can be fetched from main memory and brought to the CPU to be processed. After processing, it may be sent back to RAM to be stored (short term). All data must pass through the MDR whether it is on its way to the CPU or RAM.

All data and instructions pass via the data bus.

Memory Address Register

Every instruction or data in main memory (RAM) is located at a specific location. This location has an unique address, just like a house has its own unique address.

The address of the data or instruction being accessed is temporarily stored in the Memory Address Register.

This address is passed to RAM by means of the address bus.