The next generation in mainframe computer technology got its start Wednesday, delivered by the industry's smallest player.

Hitachi Data Systems, headquartered in Santa Clara, introduced three new mainframe computers, including one that's now the most powerful on the market. HDS hopes the machines - along with a new emphasis on customer satisfaction rather than quick sales - will help boost its market share, which by one estimate is now less than 5 percent of all U.S. sales of IBM-compatible mainframes.International Business Machines Corp. of Armonk, N.Y., has about 84 percent of the market, and Amdahl Corp. of Sunnyvale about 14 percent, according to Annex Research in Phoenix.

HDS, which sells Hitachi computers outside of Japan, is a beefed-up successor to National Advanced Systems, a subsidiary of National Semiconductor Corp. that marketed Hitachi computers in this country. Last year, a struggling National sold the unit to a joint venture owned 80 percent by Hitachi and 20 percent by Electronic Data Systems Corp., which financier H. Ross Perot founded and later sold to General Motors Corp.

Since then, the unit was renamed and has strengthened its local operations, increasing its Silicon Valley work force from about 350 to about 500, said Gary B. Moore, president and chief executive. Mr. Moore was brought in from Electronic Data Systems.

While the top local official at the joint venture that owns HDS is from Japan, most of the HDS management is from the United States. Only the American managers were on hand for Wednesday's announcement, though.

The top machine in the new HDS lineup has a maximum performance of about 150 mips, or million instructions per second, about 30 percent higher than the top IBM machine. The large lead will be short-lived, though, as IBM is expected to announce its own next-generation mainframe computer, called Summit, later this year.

The IBM announcement has been awaited for years, and while a current rumor has it coming in a few weeks, most people expect it in the fall. The response from Amdahl, the other major supplier of computers considered "plug- compatible" with IBM's, is expected to follow.