Indexed
  Effective address is found by adding the contents of a index register
    to a base address, either hard coded or in another register.

  Value is usually unsigned.

  Range is often less than possible address range.
    6502: LDA $2000,x  - 8 bit (0-255) index offset.

  Usually part of a more complex addressing mode.

  May be combined with indirect for increased flexibility.

    6502: LDA ($42),y  indirect, indexed.  Get address at $42 and add index.
    6502: LDA ($42,x)  indexed, indirect. Get address at ($42 + x).

    Some ISA architectures allow 2 or more pointer to be combined, calling one 
    the base and the other the index although they may be interchangeable.  

      L R1,23(R2,R3)  (Base displacement, IBM 360)

    Other systems only allow certain pairing.