Introduction to Computer Architecture - Bus Arbitration 1. An arbiter controls a

Question

Introduction to Computer Architecture - Bus Arbitration

1. An arbiter controls access to a common resource. It uses a rotating-priority scheme in responding to requests on lines R1 through R4. Initially, R1 has the highest priority and R4 the lowest priority. After a request on one of the lines receives service, that lines drops to the lowest priority, and the next line in sequence becomes the highest-priority line. For example, after R2 has been services, the priority order, starting with the highest becomes R3, R4, R1, R2. What will be the sequence of grants fo the following sequence of requests: R3, R1, R4, R2? Assume that the last three requests arrive while the first one is being serviced.

2. Consider an arbiter that uses the priority scheme described in problem 1. What happens if one device requests service repeatedly? Compare the behavior of this arbiter to one that uses a fixedpriority scheme.

Explanation / Answer

1. As it is mentioned that the last 3 requests arrive simultaneously, it means that the first request arrived before them and was all alone. As there is no requests to decide from which to execute, the only request present will execute for R3.

After the request being serviced, the priority order becomes: R4,R1,R2,R3.

Now, the next 3 request comes together, but as R4 at the time has highest priority, hence its request will be serviced next and the new priority order would become:R1,R2,R3,R4.

Now we have only 2 requests left for resource 1 and 2, out of which the current priority of R1 is higher after service of R4 request, hence it will execute next and R2 request will execute last.

So, the complete order of requests would be:

R3 -> R4 -> R1 -> R2.

2. In such a case this type of priority scheme would fail as the resource requiring fast request response would always find itself at the bottom of the priority queue every time one of its request is serviced, making it very difficult to service multiple requests of the same resource, let alone frequent repeated requests.

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