American Telephone & Telegraph Co. yesterday announced a major reorganization of its computer division, for the first time giving direct control over sales to the unit's senior executive.

The move was a new effort to bring daylight to AT&T's troubled venture into computers, which industry analysts say lost up to $1 billion last year.

The plan calls for creating a new Data Systems Group, which will be responsible for product management and development, as well as computer sales and software services. At the helm as group president will be Vittorio Cassoni, 44, who was brought in to bail out the computer operations last November.

Cassoni has been shaking up his division since then, and yesterday's move was widely expected. With control of sales, "he's finally got the last piece of the puzzle to be a more effective player in the computer market," said Charles Nichols, first vice president in equity research at E.F. Hutton & Co. Inc.

At present, computer division sales people report to an overall sales group in AT&T. The company's Customer Programming Services Centers, part of another group, provide software support.

The consolidation should help give Cassoni a more professional sales force, said Frank Governali, an assistant vice president at Kidder Peabody & Co. who follows AT&T. "It's very difficult for anyone with the background or training on the telephone side to be an effective salesman for computer systems," he said.

A major reason behind AT&T's acceptance of its break-up in 1984 was that it expected a huge business in computers and networks, which tie them together. Executives reasoned that AT&T's control of the country's main long-distance network would put the company in a unique position in this information-age sector.

With the intention of fighting such computer industry giants as IBM Corp. and Digital Equipment Corp. head-on, AT&T bought about 26 percent of Italian computer maker Ing. C. Olivetti & Co. to firm up its position in the field.

However, the company has repeatedly stumbled. Industry analysts have blamed much of the failure on AT&T's unfamiliarity with the computer business, a lack of clear strategy and a sales lethargy bred by lack of competition in the era before the break-up.

Cassoni headed Olivetti's North American operations before being brought into AT&T last fall.

He has consolidated 10 computer units into three, shedding hundreds of employes. He has tried to give a new, sales-oriented focus to the division.

Cassoni has said that his operations will reach the break-even point in 1989.

"They are significantly ahead of their plan so far this year," said AT&T vice chairman Randall L. Tobias yesterday. "That's one of the things that has enabled us to move ahead with something that we really consider here to be a logical, evolutionary step in our structure.