The address field references a main memory address, and the reference register contains a positive displacement from that address. In this case also the register reference is sometimes explicit and sometimes implicit.
Generally index register are used for iterative tasks, it is typical that there is a need to increment or decrement the index register after each reference to it. Because this is such a common operation, some system will automatically do this as part of the same instruction cycle.
This is known as auto-indexing. We may get two types of auto-indexing:
-- one is auto-incrementing and the other one is
-- auto-decrementing.
If certain registers are devoted exclusively to indexing, then auto-indexing can be invoked implicitly and automatically. If general purpose register are used, the autoindex operation may need to be signaled by a bit in the instruction.
Auto-indexing using increment can be depicted as follows:
EA = A + (R)
R = (R) + 1