SANTA CLARA, Calif. — National Semiconductor Corp. and IBM Corp. have agreed to end the wafer manufacturing and marketing agreement IBM has with Cyrix, National's wholly owned subsidiary.

IBM will cease the sale of Cyrix-designed processors before the end of the calendar year, cccording to the termination agreement.

IBM has served as a foundry for Cyrix (Richardson, Texas) since it was founded 10 years ago, and most recently manufactured Cyrix's MII and MediaGX processors using a 0.35-micron process at its fabrication facility in Essex Junction, Vt. IBM was also selling the processors it made for Cyrix directly to OEMs.

Though the arrangement will end by December, IBM has the right to continue manufacturing chips for National as manufacturing of affected chips moves to National's 8-inch, 0.25-micron wafer fab in South Portland, Maine, to ensure a smooth transition.

National will return about $5 million in pre-payments to IBM for chips that were already ordered and for some 0.35-micron process equipment, a National spokesman said. National will take a one-time charge of approximately $50 million to $55 million in its second quarter ending Nov. 29 as a result of the termination and transfers.

The termination of the Cyrix-IBM foundry agreement has been planned since National acquired Cyrix last November. At the time, National said it wanted to bring Cyrix chip manufacturing inside the company to achieve marketing advantages and cost savings. National wanted to wait to move the manufacturing in-house until it had its 0.25-micron process up and running at the South Portland, the National spokesman said.

The Maine fab began churning out its first chips in May and is now in volume production. At it current run rate, the fab can supply 10 million processors per year, the spokesman said.

The addition of the Cyrix chips to the Maine fab will boost the factory's utilization rate, which was below 50 percent in the last quarter, the spokesman said.