Operating System (OS)
The Memory Address Register

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".
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
None of these
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.

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Operating System (OS)
Most of the microcomputer's operating systems like Apple DOS, MS DOS and PC DOS etc. are called disk operating systems because

they are partly in primary memory and partly on disk
they are memory resident
they are initially stored on disk
they are available on magnetic tapes
None of these

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