IBM ACP (Airline Control Program) was originally a S/360 OS optimized for transaction processing that underlies things like the PARS airline reservation system

IBM ACP (Airline Control Program) was originally a S/360 OS optimized for transaction processing that underlies things like the PARS airline reservation system and the card authorization systems at AmEx. It has been under continuous development and evolution since, I believe, 1969. Along the way it became ACP/TPF (Transaction Processing Facility) and then just TPF. It's now called z/TPF and runs on the latest zSeries mainframes. I suspect, due to it's widespread use in embedded systems of all sorts, there will be things still running MS-DOS 50 years from now.