The mainframe business, while far smaller than it was, remains crucial for I.B.M. Sales of the machines alone account for only about 5 percent of I.B.M.’s revenue. But all mainframe-related hardware, software and services account for a quarter of its revenue and, more important, about half of I.B.M.’s total operating profit.
Competitors say most of that growth comes from a comparatively small number of big customers like banks, brokerage houses, insurance companies and some government agencies, whose growing computing needs in general require continued investment in mainframes. The number of mainframe computers in use worldwide, analysts say, is about half what it was a decade ago.