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In a computer, the memory address register is the CPU register that either stores the memory address from which data will be fetched to the CPU, or the address to which data will be sent and stored. When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes data from MDR to the memory location whose address is stored in MAR.
https://en.wikipedia.org/wiki/Memory_address_register