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"Unable to stand it any longer, I burst in on them and shouted, 'This is unbelievable! It shows your true character, once and for all.' I was speaking to both of them, but staring straight at Bill," writes Mr. Allen in the book, a part of which was published by Vanity Fair magazine online this week.
The book, Idea Man, will be published April 19 by Portfolio/Penguin Group.
In the published excerpt, Mr. Allen chronicles the first time he and Mr. Gates met, in 1968. Mr. Allen was in the 10th grade and Mr. Gates in the eighth.
"You could tell three things about Bill Gates pretty quickly.
"He was really smart. He was really competitive; he wanted to show you how smart he was. And he was really, really persistent," writes Mr. Allen, who left Microsoft in 1983.
From there on, Mr. Allen paints Mr. Gates as an ultrafocused micromanager who expects nothing short of total dedication from his employees.
Mr. Allen's memoir says Mr. Gates slowly tried to lower Mr. Allen's stake in the firm while increasing his own, which may remind readers of a film about Facebook, a more recent founding story.
"From the time we'd started together in Massachusetts, I'd assumed that our partnership would be a 50-50 proposition. But Bill had another idea," writes Mr. Allen, adding that he later agreed to a 60-40 split of Microsoft.
Not long after, Mr. Allen writes that changed to 64-36.
In a statement, Mr. Gates said, "While my recollection of many of these events may differ from Paul's, I value his friendship and the important contributions he made to the world of technology and at Microsoft."
Microsoft, where Mr. Ballmer is CEO, declined to comment.