The identifier you specify as the first argument to .DEFINE_IREG
    becomes an integer register symbol. Thereafter, it cannot be
    used as a MACRO-64 identifier. Specifically, the user-defined
    register symbol is only allowed where a register is allowed. In
    this sense, the user-defined register symbol is reserved until
    the end of the assembly unit or until you delete its definition
    (see .UNDEFINE_REG), whichever occurs first.

    The second argument to .DEFINE_IREG can be either an integer
    expression or a currently defined integer register symbol.
    An integer expression indicates the register number to assign
    to the register symbol. The expression can contain no forward
    or external references and must be in the range of 0 to 31.
    Alternatively, you can define a register symbol in terms of
    another, currently defined register symbol. A currently defined
    register symbol is a predefined register symbol or a register
    symbol that you have previously defined. In this case, the new
    register symbol you specify with the first argument receives the
    current value of the register symbol you specify with the second
    argument.