Operating System (OS)
The advantage of a command processor running only built-in commands is:

the processing is much faster than would other-wise be the case when user defined commands are used
users can create system programs and run them as commands
the command set being common across different hardware configurations
None of these
flexibility to the users in running lists of commands by simply collecting them in named batch command files

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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.
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
None of these
contains the address of the memory location that is to be read from or stored into.

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