He didn't know when sparrows fell on the White House lawn after all. And all those legends about his having been a "strong man" with an iron grip on the White House staff, the all-powerful guardian at the president's door, aren't right either. That, at least, is Donald T. Regan's latest account of his service as Ronald Reagan's chief of staff.

The self-portrait Regan drew yesterday before the Iran-contra committees showed someone who knew a little about a lot of things but not much about the dirty details as they applied to the Iran-contra affair. The erstwhile Wall Street wizard, the tough, take-charge, decorated former Marine, the author of a book on the absolute necessity for proper record-keeping and controlling a big organization's paper-work flow couldn't even get a grip on the documents being presented to the president or see that they were preserved, according to his testimony. And when he attempted to assert control, he failed.

He found out, he testified, that what he believed to have been notes taken during the president's daily intelligence briefings weren't notes at all. He knew almost nothing about the National Security Council's secret operational activities to keep the contras forces alive "body and soul." He knew nothing about the secret network of covert operators -- retired Air Force major general Richard V. Secord, Albert A. Hakim and company -- who worked directly under the guidance of Lt. Col. Oliver L. North to ship arms to Iran and resupply the contras with arms and materiel. He lamented that he never even saw an NSC organization chart, had no idea how many people worked for it out of the White House or what its budget was. And far from supervising the activities of national security adviser John M. Poindexter, he took pains to say that Poindexter had an independent relationship with the president. His job with regard to the NSC, he said, was mostly confined to providing the proper administrative support.

"Again I'm trying to make that distinction that he didn't report to me," Regan said, referring to Poindexter. "So you keep using the word 'supervision.' I would say 'liaison' . . . . would be a closer descriptive word, or 'sharing' or something of that nature, rather than 'supervision.' And during his first days on the job, I would occasionally say to him, you know, 'Is everything all right? Is there anything we can do?' You see, from an administrative point of view we had to give them support -- office space, White House passes, parking spaces, all that folderol of administration and wanting to make sure that all of that was being handled."

This picture of Regan, the benign keeper of White House passes and parking spaces, differs dramatically from the description stamped in the public mind by the Tower report earlier this year and in the private accounts about him that have circulated for years among political leaders of both parties in Washington.

The Tower report, drafted by a presidential commission appointed to investigate the causes of the Iran-contra scandal, described Regan as the strongest White House chief of staff in recent memory, an executive who "asserted personal control over the White House staff and sought to extend this control to the national security adviser." The report primarily blamed him: "He must bear primary responsibility for the chaos that descended upon the White House when such disclosure {of the arms sales to Iran and diversion of profits to Nicaraguan contra forces} did occur."

During his time as President Reagan's chief of staff, Regan's brusque and peremptory manner angered many members of Congress, not the least of them among the Republican hierarchy, especially in the Senate, whose members he is said to have called "my board of directors."

That Don Regan was absent yesterday on Capitol Hill. Instead of arrogance, he presented a picture of amiability. He was breezy, wisecracking, self-deprecating. Sen. William S. Cohen (R-Maine) suggested this was a "new Don Regan," one who had "slipped the surly bonds of public service."

Regan's wry asides, Irish-style banter and barbed quips made the old Senate Caucus Room repeatedly ring with laughter. "I don't mind spears in the breast," he said at one point, "it's knives in the back that concern me." In describing how Reagan advisers' conflicting counsel caused the president to "botch" answers during his disastrous Nov. 19 news conference on the Iran arms sales, he snapped: "I think this sorta confused the presidential mind as to what he could say and couldn't say, and what he should say and what he shouldn't say."

And Regan, at least among the parade of committee witnesses, was possessed of a keen recall of critical events.

Unlike Adm. Poindexter, who could not recall much about the crucial conversations in his final hours in the White House, Regan had vivid recollections of them. He described how he burst into Poindexter's NSC office to confront him about the discovery of the scheme to divert Iran arms sales profits to the contras, and found him sitting alone in his conference room, eating breakfast from a tray. He asked Poindexter, "What the heck happened here?" and remembers precisely Poindexter's reaction:

"He adjusted his glasses, he dabbed at his mouth with his napkin, put it down. He said, 'Well, I guess I should have looked into it more, but I didn't.' He said, 'I knew that Ollie was up to something but . . . I didn't know what.' "

In Regan's congressional testimony, the Iran-contra committees are confronted with a familiar problem to resolve, if they can. How credible is he? His description of his role is so diametrically opposed to views previously held about him that the committees are left pondering a remark made yesterday by Cohen, who recalled a line in a novel as being applicable to the hearings: "Everything here is plausible, because nothing here is real."

Based on his experience on the committee, Cohen told Regan, "I find much of what I've heard is implausible," thus perhaps forcing him to conclude "it was real."