Operating System (OS)
The instruction 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 hardware memory device which denotes the location of the current instruction being executed.
None of these
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
contains the address of the memory location that is to be read from or stored into.

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Operating System (OS)
System maintenance:

is not required if the system is well written
None of these
always requires several programs
is necessary on all systems, regardless of how good
is usually not necessary

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