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

    The processor decrements the contents of the register by the size
    of the  operand data type; then the register contains the address
    of the operand.  The processor decrements the register by 1, 2, 4,
    8, or  16 for   byte,   word,   longword,   quadword,   or
    octaword  operands, respectively.  Autodecrement mode can be used
    with index mode.

       -(Rn)
       -(AP)
       -(FP)
       -(SP)

    n

    A number in the range of 0 through 12.