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

    The contents of the register plus the displacement (sign-extended
    to a  longword)  produce  the address of the operand.  Displacement
    mode can be used with index mode.

       displ(Rn)
       displ(AP)
       displ(FP)
       displ(SP)

    n

    A number in the range of 0 through 12.

    displ

    An expression  specifying  a  displacement;  the  expression  can
    be preceded  by  one  of  the  following displacement length
    specifiers, which indicate the number of bytes needed to store the
    displacement.

          B^    Displacement requires one byte.
          W^    Displacement requires one word (two bytes).
          L^    Displacement requires one longword (four bytes).

    If no displacement length specifier precedes the expression, and
    the value  of the expression is known, the assembler chooses the
    smallest number of bytes (one, two, or four) needed to store the
    displacement. If  no length specifier precedes the expression, and
    the value of the expression is unknown, the assembler reserves one
    word  (two  bytes) for  the  displacement.   Note  that  if  the
    displacement is either relocatable or defined later in the  source
    program,  the  assembler considers it unknown.  If the actual
    displacement does not fit in the memory reserved, the Linker
    displays an error message.