Address 000 specifies that no index register is to be used with the instructions.
The index register address of an instruction is indicated by x.
The indirect address can be acted on by an index register and deferred again if desired.
Since they operate on the index register location, x, they cannot be indexed.
These digits address an index register for memory-type instructions.
The contents of octal digits 0-5 of the index register location are unaffected by this instruction.