Auto-indexing using decrement can be depicted as follows:

EA     =     A    +   (R)
R       =    (R)   -    1

In some machines, both indirect addressing and indexing are provided, and it is possible to employ both in the same instruction. There are two possibilities: The indexing is performed either before or after the indirection.

If indexing is performed after the indirection, it is termed postindexing

EA     =   (A)    +   (R)

First, the contents of the address field are used to access a memory location containing an address. This address is then indexed by the register value.

With preindexing, the indexing is performed before the indirection:

EA    =    ( A    +    (R) )

           An address is calculated, the calculated address contains not the operand, but the address of the operand.