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Pre-emptive multitasking with job control assures that a system operates in a timely manner ''most of the time''. In some environments (for instance, operating expensive or dangerous machinery), a strong design constraint of the system is the delivery of timely results in all circumstances. In such circumstances, job control is more complex and the role of scheduling is more important.
Since real-time systems do event-driven scheduling for all real-time operations, "the sequence of these real-time operations is not under the immediate control of a computer operator or programmer."<ref name=X23>{{cite book
|url=http://www.bitsavers.org/pdf/sds/sigma/16-bit/rbm/901785A_Real-Time_Batch_Monitor_Users_Guide_Jan72.pdf
|title=Xerox Real-Time Batch Monitor (RBM), Sigma 2/3 Computers, User's Guide
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However, a system may have the ability to interleave Real-time and other, less time-critical tasks, where the dividing line might for example be response required within one tenth of a second.<ref name=X23/>{{rp|p.1}} In the case of the Xerox RBM (Real-time/Batch Monitor) systems,<ref>a family: Scientific Data Systems SDS Sigma 2 & 3, renamed/combined as Xerox's acquired Xerox Data Systems, Xerox 530.</ref><ref>The SDS Sigma 5, 6 & 7 became the Xerox 560</ref><ref>{{cite book
|url=https://computerarchive.org/files/mirror/www.bitsavers.org/pdf/sds/sigma/rbm/901581A_Sigma5_RBM-2_Nov69.pdf
|title=XOs SIGMR 5/7 REAL-TIME BATCH MONITOR (RBM-2)|accessdate=2017-02-16}}</ref> for example, two other capabilities existed:<ref name=X23/>{{rp|p.2}}
* Computer operator commands ("unsolicited key-in")
* Background job streams ([[#Batch processing|batch jobs]])
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