Oldest software system in continuous use

The oldest software system in use is hard to identify with absolute certainty, but it is likely either the SABRE Airline Reservation System (introduced in 1960), or the IRS Individual Master File and Business Master File (introduced in 1962–63). The uncertainty lies in the difficulty in proving continuity from the earliest versions of these programs to their present-day equivalents. The IRS system is slightly newer, but is also less likely to have been stripped of all its 1960s legacy code, making it perhaps the more probable candidate.

The Individual Master File systems are reportedly written in a mixture of COBOL and IBM Assembler. As assembly code written for the Internal Revenue Service's pre-System/360 mainframes would not work with any subsequent machine, the oldest IBM Assembler in the IMF's codebase would have to have been written after the IRS transition to System/360 architecture in 1967. COBOL is a more easily ported high-level language, however, so it's possible that some elements of the pre-1967 code could have been carried over.

The IMF's persistence is a consequence of the notorious complexity of the United States tax code, the need for full backwards compatibility, and the low level of funding the IRS has had for modernization. As much as 20 million lines of the IMF's code is reportedly written in Assembler – a major obstacle to any modernization, as this code is specific to the System/360 Architecture, and so cannot be run on anything other than an IBM mainframe.