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Chapter 8. Input/Output Organization > 8.11 BUS ARBITRATION

8.11 BUS ARBITRATION

When multiple devices place the request for communication through the same bus at same time, the situation must be solved by arbitration as at any given time only one device can use the bus. In SCSI, its controller itself completes this arbitration. However, for other buses, like ISA, a bus arbitrator is necessary.

The situation of bus arbitration is very much similar to the situation of multiple interrupts encountered by the processor at the same time. One can recollect the case of 8085 with its five interrupt inputs. If all interrupts are acknowledged at the same time, then TRAP would be serviced first as it is assigned the highest priority. The next default priority is assigned to RST7.5 interrupt. Then it is RST6.5, RST5.5 and finally the INTR, the interrupt with lowest priority. So, we see from this analogy that some priority to be assigned for different devices attached to any bus for the purpose of arbitration.


  

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