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OS/360 was a batch operating system created by IBM for the then-new IBM System/360 line. It is perhaps best known now for being the project that prompted Fred Brooks to write his famous book, The Mythical Man-Month.

OS/360 originaly came in three forms, each with different capabilities, and suitable for different hardware configurations:

  • Primary Control Program (PCP) - intended for the smallest configurations, it only ran one job at a time; dropped in later versions of OS/360
  • Multiprogramming with a Fixed number of Tasks (MFT) - intended as a temporary stop-gap until the last form (MVT) was available, the first version was effectively PCP with support for 4 simultaneous jobs
  • Multiprogramming with a Variable number of Tasks (MVT) - the original target of OS/360, it took several years to get running reasonably well

MVT had issues with main memory fragmentation, caused by the fact that the 360 did not provide each process with its own address space, but instead used protection keys; if a process was swapped out, it had to be reloaded into the exact same location in memory.

Due to the delays in getting OS/360 out, IBM provided several stop-gap OS's for use on 360's:

OS/360 was originally written entirely in assembly language.