For two days last month, 150 representatives from every corner of the computer industry assembled in a New York hotel, paying $795 apiece to a consulting firm to hear its seminar on "The Future of IBM."

Curiosity about International Business Machines Corp. isn't exactly a new development in the computer industry. IBM, the industry leader, casts a giant shadow, with worldwide sales of $29 billion last year, an army of 354,936 employes and a enormous pocket of cash from which to finance offensive and defensive strategies.

Now, however, the titan is shifting its weight and moving in new directions, and many eyes are on it.

Within the past year, IBM has:

* Persuaded the Justice Department to drop its monumental, 13-year antitrust suit charging IBM with mantaining a monopoly in large central computers. Announcing his decision Jan. 8, Assistant Attorney General William Baxter called IBM a "good corporate citizen," a blessing that frees IBM "to compete aggressively" without the government looking over its shoulder, says a market analysis by Alex Brown & Sons. Its competitors will be looking harder over their shoulders now.

* Announced a stream of new products and services, like the IBM Personal Computer and an assembly robot for small manufacturing tasks, in areas IBM had previously passed by.

* Reorganized the entire company hierarchy, reshuffling sales forces to deal with the demand for small computer systems for homes and offices. (IBM found that potl buyers were confused by the array of IBM products, sold by competing sales divisions. In response, the company split its sales force into two new divisions, each selling IBM's full product line to different groups of customers.)

The changes go beyond the equipment IBM sells and leases. Francis G. "Buck" Rodgers, IBM's vice president for marketing, says that while hardware technology is still moving ahead--"the next five to 10 years will dwarf the past in achievement"--IBM expects to gain its greatest competitive leverage by making its machines more versatile and simpler to use through advances in software, the intricate instructions for data processing equipment.

Ahead lies a growing competition with American Telephone & Telegraph Co., Xerox Corp., General Telephone & Telegraph Corp., and smaller, high technology companies to build new communications links that will tie together the millions of new terminals, processors, printers and other machines the industry will sell in the coming decade.

And on the horizon may be the first serious challenge to IBM's market supremacy as a coalition of Japanese electronics companies, backed by government support, attempts to develop a new, much faster and sophisticated generation of computers and simpler methods for instructing them.

It was this rapid evolution of the computer industry that prompted Baxter to drop the IBM case, he said. IBM remains, by far, the single greatest power in the industry despite some setbacks in recent years.

In the past five years, it has spent an incredible $29 billion renovating and expanding its production facilities. As a result, costs and expenses have grown faster than revenues, says John R. Opel, IBM's president and chief executive officer.

IBM stock--the bluest of blue chips through most of the 1960s and 1970s--has lost that reputation. IBM was riding high when the stock was split 4 for 1 in mid-1979, bringing the price below 100 for the first time in nearly 50 years. It then fell from near 80 to below 60.

Its weakest position competitively, according market analysts, is in the market for smaller computers that will grow dramatically in the 1980s.

Despite these drawbacks, a number of analysts like IBM's prospects, however.

Although IBM's first quarter results "may not look so pretty" because of the recession, says Ulric Weil of Morgan Stanley, it is headed upward fast. He speculates that IBM may raise its dividend later in 1982 for the first time in three years.

Alex Brown & Sons recently put IBM stock on its "buy" list, saying its growth rate in 1982 could be a phenomenal 20 percent.

To meet those lofty targets, IBM must catch up with smaller competitors who saw the potential for small, smart computer equipment and jumped in, leaving a slower IBM behind, industry analysts say.

"In the 1970s, IBM was insensitive to changing markets and it cost them dearly," says Peter Wright of Gartner Group Inc.

"IBM has decided to become the agent of change instead of reacting to it," says Howard Anderson of the Yankee Group, sponsors of the New York seminar on IBM's future.

An example of IBM's new attitude is its Personal Computer, said Wright. Instead of waiting for the home computer market to grow big enough to justify full-scale production of an all-IBM machine, IBM has jumped in with a hybrid--its computer has a display screen from Japan, semiconductor chips from Intel Corp., memory units made in India, software from Microsoft of Bellevue, Wash. It is being sold through Sears, Roebuck and Computerland outlets as well as IBM's new retail product centers.

IBM's traditional base, the mainframe central processing computer, is shrinking in importance as new kinds of customers look to the cheaper, smaller devices.

According to estimates by International Data Corp., mainframe computers will constitute only 36 percent of worldwide shipment in 1985, compared to a 60 percent share in 1980. The most dramatic gains will be by the "minicomputer" products, the desktop computers, word processors and display terminals that will be filling professional offices, homes and plants.

The new hardware IBM is introducing is one part of its answer to a fast-changing market. IBM is also counting on its strength in producing the complex instructions that run computer devices.

This not-so-secret weapon is IBM's "systems network architecture." Introduced in the mid-1970s, SNA consists of electronic specifications for equipment and programmed operating instructions that enable computers and data processing equipment on the same system to talk to one another.

These rules of conversation also provide backup channels for data flow as insurance against the failure of communications links and coding of data to assure privacy and security in transmission.

Data communications is a weak link for IBM, says Anderson, despite the company's investment in a joint domestic satellite venture with Comsat General Corp. and Aetna Life and Casualty Corp.

"IBM must be prepared to move into communications," says Anderson of the Yankee Group, or see others gain crucial strategic positions. IBM probably has about one-third of the business in such spread-out computer networks, he says, much less than its 60 to 70 percent share of centralized computer systems.

Says IBM's Rodgers, "Today, it's hard to distinguish the difference between data processing and communications. In the 1980s, voice, text, data and image are all coming together" as part of the computer information stream. "The ability to transmit this information at very high speed . . . and at a reasonable cost is very, very key," he said.

But IBM is not alone in seeing the importance of the communications link between computers.

AT&T is preparing a new version of its own communications system for computers and plans to make it the first major product marketed by "Baby Bell," the new high-technology firm to be split off from the Bell System under the Justice Department's divestiture plan for the giant telephone monopoly.

For AT&T, IBM and the other competitors who would control the communications link, software--the painstaking instructions that direct computer operations and applications--is a critical factor.

The software competition will be played in IBM's backyard, says Anderson. Its ability to set technical and performance standards for most of the industry is a great advantage.

Not all in the industry agree that it's position is so solid. George M. Perry, vice president of Investors Diversified Services of Minneapolis, says IBM is too expensive for many in industry.

In any case, there is no question where IBM is headed. "We may be at the midpoint of where hardware technology is going to take us," says Rodgers. But software is in its infancy. "The manufacturer who can offer the greatest variety of those software packages . . . will maintain the leadership role," said Rodgers