IBM and Microsoft signed a software agreement.

Microsoft, which develops software for personal computers, and IBM said they agreed to work more closely in developing software for IBM machines. The agreement appeared to mark IBM’s endorsement of a disk operating system created by Microsoft that has become the standard used in most personal computers. Rivals have worried that IBM might create its own operating system and shut them out of the market. Bellevue, Wash.-based Microsoft, a privately held firm, got about 10% of its $140 million in revenue last year from IBM.