SANTA CLARA, Calif. (AP) _ Hitachi Ltd. and General Motors Corp.'s Electronics Data Systems unit said Monday they have formed a joint venture company that will operate National Advanced Systems, which they bought from National Semiconductor Corp. for $398 million.

National Advanced Systems, which markets Hitachi's mainframe computers and peripheral equipment, will operate as a subsidiary of the joint venture, called Hitachi Data Systems Corp.

The agreement to buy NAS was announced two months ago and was completed on Friday.

Gary B. Moore, a 16-year veteran of Dallas-based EDS, was appointed president and chief executive officer of Hitachi Data Systems.

David N. Martin, president of NAS, said he will not be joining the new company.

''Dave Martin has earned our respect for his tenure at the helm of NAS,'' Moore said. ''During that time, NAS grew from a company with slightly more than $300 million in annual revenues to sales approaching the billion dollar mark today.''

Hitachi owns 80 percent and EDS 20 percent of the new joint venture.