Following the failure of OS, IBM designed DOS for the low end machines, able to run in 16KB(?) and 64KB memory.
DOS/360 used three memory partitions, but it had no serious memory protection. The three partitions were not specialised, but frequently one was used for spooling punched cards to disk, another one for batch job execution and another for spooling disk to printers.
With DOS/VS, introduced in 1970, the number of partitions was increased, virtual memory was introduced and the minimum memory requirements increased.
Later they released DOS/VSE and ESA/VSE. DOS/360 successors are still alive today (1997) though not as popular as in the late 1960s.