A register found in some CPUs, whose contents can be added to the address operand to give the effective address. Incrementing the index register then allows the program to access the next location in memory and so on, making it very useful for working with arrays or blocks of memory.
Index registers first appeared around April 1949 in the Manchester Mark I. The Mark I's index register's contents were simply added to the entire instruction, thus potentially changing the opcode (see The story of Mel)!
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.