DOS/360

DOS/360, or Disk Operating System/360, is an operating system IBM built for its System/360 mainframes, announced on December 31, 1964 and first delivered in June 1966; it later ran on System/370 and subsequent IBM mainframe families too. IBM aimed it at smaller System/360 installations and wrote it entirely in assembly language so it could run in as little as 16 kilobytes of memory. It began as a single job system, later gaining multiprogramming with up to three concurrent memory partitions, and was originally meant as a stopgap while the more capable OS/360 was delayed. It became, in its time, the most widely used operating system in the world, and many organizations that adopted it stayed with it even after OS/360 became available, because they were already invested in it. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/