Operating System (OS)
The instruction register

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".
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.
is a hardware memory device which denotes the location of the current instruction being executed.

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Operating System (OS)
System maintenance:

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

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