Cementing their October agreement under which AT&T will use the Sun Microsystems SPARC RISC chip set as the processor for its future high-end Unix systems, AT&T and Sun yesterday announced an arrangement under which AT&T Data Systems president Vittorio Cassoni gets a seat on the Sun board in exchange for a commitment that AT&T will subscribe for new Sun shares at a premium over the ruling price in the market whenever Sun needs new capital. Under the agreement, Sun may require AT&T to buy shares equivalent to 15% in stages over the next three years, and may also buy up to another 5% in the open market. Sun says that at its current rate of growth, it will need new cash over the next 18 months, and that the AT&T deal is by far the most attractive of the options open to it. AT&T expects its investment in Sun under the pact to be about $300m.