VMS Help MACRO, VAX MACRO Assembler, Directives, .OPDEF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Defines an opcode, which  is  inserted  into  a  user-defined
    opcode table.

      .OPDEF opcode value,operand-descriptor-list

    opcode

    An ASCII string specifying the name of the opcode.  The string can
    be up to 31 characters long and can contain the letters A through
    Z, the digits 0 through 9, and the special characters underline,
    (_), dollar sign  ($),  and period (.).  The string should not
    start with a digit and should not be surrounded by delimiters.

    value

    An expression that specifies the value of the opcode.  The
    expression must  be  absolute  and  must  not contain any undefined
    values.  The value of the expression must be in the range  of  0
    through  decimal 65,535  (hexadecimal FFFF), but you cannot use the
    values 252 through 255 because the architecture specifies these as
    the start of a 2-byte opcode.  The expression is represented as
    follows:

      if 0 < expression < 251     expression is a 1-byte opcode.

      if expression > 255         expression bits 7:0 are the first
                                  byte of the opcode and expression
                                  bits 15:8 are the second byte of
                                  the opcode.

    operand-descriptor-list

    A list of operand descriptors that specifies the number  of
    operands and  the  type  of each.  Up to 16 operand descriptors are
    allowed in the list.