By the way, z/OS is not the only way to run IBM mainframes. There's TPF (Transaction Processing Facility) "because a real operating system is too high level and

By the way, z/OS is not the only way to run IBM mainframes. There's TPF (Transaction Processing Facility) "because a real operating system is too high level and therefore too slow for real transaction processing needs". Current users of TPF include Sabre, VISA, American Airlines, American Express, HP SHARES (formerly EDS), Holiday Inn, Alitalia, KLM, Amtrak, Marriott International, Travelport, Citibank, Citifinancial, Air Canada, Delta Air Lines, Japan Airlines and many others.