 During development of its IBM PC compatible, Compaq engineers found that "Microsoft Flight Simulator" would not run because of what subLOGIC's Bruce Artwick described as "a bug in one of Intel's chips", forcing them to make their computer bug-compatible with the IBM PC.Another hardware example is found in the design of the IBM Personal Computer/AT A20 address line to emulate the behaviour in older processors.Microsoft Excel has always had a deliberate leap year bug, which falsely treats February 29, 1900 as an actual date, to ensure backward compatibility with Lotus 1-2-3.