Cirrus Logic Inc., once a leading computer graphics chip company, said it will spin off a unit that makes chips for computer modems as part of a restructuring effort. About 40 Cirrus workers would become part of a new closely held company, Ambient Technologies Inc., which Cirrus said would acquire its modem products and license its patents. Fremont-based Cirrus would hold an undisclosed stake in Ambient. More details will be disclosed when the transaction is completed by year-end, the company said. Cirrus wants to focus on its more profitable chips for computer disk drives, audio equipment and industrial devices. Cirrus' modem chip line accounted for about 6% of its $954.3-million revenue last year. Cirrus shares rose 74 cents to close at $11.44 on Nasdaq.