Like all System[[OS/360 operating software]], initial releases of DOS could run only one program at a time. Later versions of "real" DOS were able to run up to three programs concurrently, in separate memory partitions, supported by the same hardware memory protection features of the more scalable OS/360 operating system. These were identified as BG (''background''), F1 (''foreground 1'') and F2 (''foreground 2''). Multiprogramming was an optional feature of DOS/360, selectable at [[system generation]].<ref name=sysgen>{{cite book|last=IBM Corporation|title=IBM System/360 Disk Operating System: System Generation and Maintenance|year=1969|url=http://www.bitsavers.org/pdf/ibm/360/dos/C24-5033-7_Disk_Operating_System_System_Generation_and_Maintenance_Apr69.pdf}}</ref>{{rp|p.34}} Another SYSGEN option allowed [[Batch processing|batch]] operation run in either FG partition. Otherwise foreground programs had to be manually started by the [[computer operator]].
DOS/VS allowed up to seven concurrent programs, although five or six was a more common number due to the smaller scale of the hardware usually hosting DOS systems. Both DOS and DOS/VS allow the number of partitions to be set at [[Initial Program Load#IBM Initial Program Load|IPL]] (Initial Program Load), the IBM term for Boot load.
