VMS Help MACRO, VAX MACRO Assembler, Operand Formats, Indexed  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Index mode is a general register  mode  that  can  be  used  only
    in combination  with another mode (the base mode).  The base mode
    can be any addressing mode except register, immediate,  literal,
    index,  or branch.   The assembler first evaluates the base mode to
    get the base address.  To get the operand address, the  assembler
    multiplies  the contents  of the index register by the number of
    bytes of the operand data type, then adds the result to the base
    address.

       base-mode[Rx]
       base-mode[AP]
       base-mode[FP]
       base-mode[SP]

    base-mode

    Any addressing mode except register, immediate,  literal,  index,
    or branch, specifying the base address.

    x

    A number in the range 0 through 12, specifying the index register.