• In multiprocessor system, there are more than one processors, which requires control of the system bus at a time.
• Hence an appropriate priority resolving mechanism is required, to decide which processor should get control of bus.
• This is called bus arbitration.
• Following three methods are used to resolve bus arbitration.

1) Daisy chain method:-

• All bus master use the same line for bus request.
• If the bus busy line is inactive, the bus controller gives the bus grant signal.
• Bus grant signal is propagated serially through all masters starting from nearest one.

• The bus master which requires system bus, stops this signal, activates the bus busy line and takes control of system bus.

  • Advantages:

    a) Simple design

    b) Less no. of control lines.

  • Disadvantages:

    a) Priority depends on the physical location of master.

    b) Propagation delay due to serially granting of bus.

    c) Failure of one of the devices may fail entire system.

polling Method:-

• Here all bus masters use the same line for bus request.
• Here controller generate binary address for the master. (To connect 8 bus master we need 3 address lines $2^3$ = 8).
• In response to a bus request, the controller “polls” the bus masters by sending a sequence of bus master address on address lines.

• When requesting master recognizes its address, it activates the bus busylines and takes control of the bus.

• Advantage:

  a) Priority flexible.

  b) One module fails, entire system does not fail.

• Disadvantage:

  a) Adding bus masters is different as increases the number of address lines of the circuit.

2) Independent request method:-

• All bus masters have their individual bus request and bus grant lines.
• The controller thus knows which master has requested, so bus is granted t that master.
• Priorities of the masters are predefined so on simultaneous bus requests, the bus is granted based on the priority, provided the bus busy line is not active.

• The controller consists of encoder and decoder logic for priorities.

• Advantages:

  a) Bus arbitration is fast.

  b) Speed independent of no. of devices connected.

• Disadvantages:

  a) No. of control lines required is more. Hence connecting a large number of bus masters is difficult.