Operating System (OS)
The Memory Address Register

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.
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".

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Operating System (OS)
Semaphores

synchronize critical resources to prevent contention
synchronize critical resources to prevent deadlock
are used for memory management
are used to do I/O
None of these

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