OS/360 was developed a family of three control programs, which increased in size as well as functionality. Initially, the single task PCP (Primary Control Program) processed jobs sequentially; the next, MFT (Multiprogramming with a Fixed number of Tasks) added multitasking, but only allowed a fixed number of tasks to execute at once, with a memory allocation for each job that was set before it was started. Finally MVT (Multiprogramming with a Variable number of Tasks) allowed tasks to grow or shrink their memory as they executed. OS/360 also introduced JCL (Job control language).
OS/360 was late being delivered, due to a combination of organizational disarray inside IBM and lack of experience with the pitfalls of large software projects, as well as the significant technical challenges. Originally scheduled for delivery in 1965 (for the simplest versions) and 1966 (for the more complex ones), it was not finally first released until 1966 (for PCP) and 1967 (for MVT).
A family of simple alternative systems, BOS (Basic Operating System, for the smallest machines), TOS (Tape Operating System, for machines with only tape drives), and DOS (Disk Operating System), was hurriedly put together to allow customers to use the new machines. Although intended as a temporary interim tool, DOS survived until 1971.
After his experience running the System/360 project, and later specifically in charge of the (by then long overdue) OS/360, Frederick P. Brooks wrote his famous book, The Mythical Man-Month, giving OS/360 as an example of the second-system effect.
When virtual addressing hardware was later developed for the System/370, the systems were upgraded and renamed; MFT became OS/VS1 and MVT became OS/VS2. After further development, OS/VS2 was later renamed MVS (Multiple Virtual Storage).
This article (or an earlier version of it) contains material from FOLDOC's article on OS/360, used with permission.