Operating System (OS)
The Memory Address Register

is a hardware memory device which denotes the location of the current instruction being executed.
contains the address of the memory location that is to be read from or stored into.
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
None of these
contains a copy of the designated memory location specified by the MAR after a "read" or the new contents of the memory prior to a "write".