Base indexed displacement
  L R1,23(R2,R3) - IBM 360 base (index) displacement.
    R2 is treated as an index
    R3 is the base pointer.
    23 is a hard coded displacement.
    The contents of both registers and the displacement are added
      to arrive at the memory location to access.

    This is a register indirect address by another name.

  MOV [BX+SI+4],AX 
    Intel 8086
    Here there is a base, an index, and a hardcoded displacement. 
    Restricting rule: two base or two indexes invalid.