Shown below is a bus system with a central arbiter that handles four devices. Th

Question

Shown below is a bus system with a central arbiter that handles four devices. The device priorities range from highest for Device 4 down to the lowest for Device 1. The arbiter grants access to the highest present request. Each device has a separate request and grant line. The total available bus bandwidth is 80 MB/s. Each device wants to use 30MB/s of bus bandwidth. If a device is not granted access to the bus, it tries again as soon as the bus is not busy and keeps trying until it gets access. How much bus bandwidth will Device 1 be able to use and how much bus bandwith will Device 3 be able to use if:

a) Bus access is granted strictly on a priority basis?

Device 1 bandwidth = ------------- Device 3 bandwidth = ------------------

b) The bus arbiter grants access based on a fairness basis?

Device 1 bandwidth = ------------- Device 3 bandwidth = ------------------

Explanation / Answer

Priority Based:

Device 3 -> 30 MB/s first device 3 will get, now device 1 wants 30 MB/s it will check whether any other priority devices are avaialble or not, no other devices are avaiabled so it will give 30MB/s to Device 1.

Fairness Basis:

in this case it will divide the max bandwidth it has i.e 80MB/s to 4 devices so 20MB/s per device, hence device 3 will get 20MB/s, device 1 will get 20MB/s.

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