Memory and bits If a memory's addressability is 64 bits. What does that tell you about the size of the memory address register (MAR) and memory data register (MDR)?

MAR and MDR are involved and reading memory locations and writing values (numbers, data) to memory locations, respectively. In the case the OP brought up the memory address register (MAR) needs to have 64 bits to be able to accommodate the address of any memory location within the computer's memory. 
To read memory the address of a location is written to the MAR and the value contained in that address is written to the memory's data register (MDR).
To write a value to a memory location its address is first written to MAR, and the value (which in the present example is a 64-bit number) stored in MDR. So both, MDR and MAR contain 64 bits of information.
