Rn

    One of the base registers, R0 through R30, FP, and SP.

 base_expression

    The base address, which is optional, and can be one of the
    following:

    o  An absolute expression

    o  A relocatable expression

    o  An external expression

    An expression must not contain forward references or implicit
    external symbols. An implicitly defined external symbol is a
    symbol that the assembler defaults to an external symbol. This
    occurs when the assembler encounters references to a symbol, but
    does not encounter a definition for the symbol or an .EXTERNAL
    directive that declares the symbol.