The arbiter of Example 7.2 controls access to a common resource. It does not allow preemption. This means that if a high-priority request is received after a lower-priority request has been granted, it must wait until service to the device that is currently using the common resource is completed. In some cases, it is desirable to allow preemption, to provide service to a high-priority device more quickly. Devices in such a system, must be able to stop and relinquish the use of the common resource when asked to do so by the arbiter. This must be done in a safe manner. A device that is using the resource must be allowed to reach a safe point at which service can be terminated. It would then signal to the arbiter that it has stopped using the resource.

(a) Suggest a suitable modification to the signaling protocol that enables the service in progress to be terminated safely.

(b) Modify the state diagram of the arbiter to implement the revised protocol.

Example 7.2

An arbiter receives three request signals, R1, R2, R3, and generates three grant signals, G1, G2, G3. Request R1 has the highest priority and request R3 the lowest priority. An example of the operation of such an arbiter is given in Figure 7.9. Give a state diagram that describes the behavior of this arbiter.