start

no

=

t←r

yes
a←b

done

b←t

Figure 5.2: Controller for a  machine.

must be connected. If we view the arrows as wires and the X buons as
switches, the data-path diagram is very like the wiring diagram for a
machine that could be constructed from electrical components.
In order for the data paths to actually compute s, the buons
must be pushed in the correct sequence. We will describe this sequence
in terms of a controller diagram, as illustrated in Figure 5.2. e elements of the controller diagram indicate how the data-path components
should be operated. e rectangular boxes in the controller diagram
identify data-path buons to be pushed, and the arrows describe the
sequencing from one step to the next. e diamond in the diagram represents a decision. One of the two sequencing arrows will be followed,
depending on the value of the data-path test identified in the diamond.
We can interpret the controller in terms of a physical analogy: ink
of the diagram as a maze in which a marble is rolling. When the marble rolls into a box, it pushes the data-path buon that is named by the
box. When the marble rolls into a decision node (such as the test for

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