Inter Processor Arbitration
The
processor, main memory and I/O devices can be interconnected by means of a
common bus. A bus is set of lines (wires) defined to transfer all bits of a
word from a specified source to a specified destination. Thus, bus provides a
communication path for the transfer of data.
The bus includes data lines, address lines and control lines. Such a bus known as system bus. Different types of arbitration: Serial (Daisy Chain) arbitration, Parallel arbitration, Dynamic arbitration.
Serial (Daisy Chain) arbitration
In this
type of arbitration, processors can access bus based on priority. In serial
arbitration, bus access priority resolving based on the serial connection of
the processors. This technique is obtained from daisy chain (serial) connection
of processors. The serial priority resolving technique is obtained from
daisy-chain connection similar to the daisy chain priority interrupt logic. The
processors connected to the system bus are assigned priority according to their
position along the priority control line.
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| Figure : Serial (Daisy Chain) Arbitration |
When
multiple devices concurrently request the use of the bus, the device with the
highest priority is granted access to it. Each processor has its own bus
arbiter logic with priority-in and priority-out lines. The priority out (PO) of
each arbiter is connected to the priority in (PI) of the next-lower-priority
arbiter. The PI of the highest-priority unit is maintained at a logic value 1. The
highest-priority unit in the system will always receive access to the system
bus when it requests it. The processor whose arbiter has a PI = 1 and PO = 0.
That processor accesses the system bus.
Advantages
Simple and
cheaper method
Least
number of lines.
Disadvantages
Higher
delay
Priority
of the processor is fixed
Not
reliable
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