I get some problems about the Round Robin Scheduling

Actually I am doing some self-study and I got stuck in calculating the waiting time of the RR scheduling.

Here is the slide about RR Scheduling

I am confused by the "Queue" column, at time 6, what's inside the "Queue" is "D<-C". I wo开发者_如何学Cnder how this is inferred? If that is the case, then at time 5, the Queue should be "B<-D" instead of a single "B". I'm confused by how to form the Queue.

Thanks for answering!

The way I see it is that a new item in the queue appears when it wants to start to execute. Look at time 2, there is no B in the queue prior to its execution either.

As the red arrow indicates, D starts (or rather, wants to start) execution at time 6.