A CPU has two modes - privileged and non-privileged. In

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A CPU has two modes - privileged and non-privileged. In order to change the mode from privileged to non-privileged

1. a hardware interrupt is needed
2. a software interrupt is needed
3. a privileged instruction (which does not generate an interrtupt) is needed
4. a non-privileged instruction (which does not generate an interrtupt) is needed

Correct Option: B

A software interrupt is initiated by some program module which need some CPU services, at that time the two modes can be interchanged. Hence (b) is correct option.