Improving Bus Performance

These improvements mostly come at the cost of increased expense and/or complexity.

1. Hierarchy of buses.
   
   
2. Synchronous instead of asynchronous protocols.
   • Synchronous is actually simplier, but it essentially implies a hierarchy of protocols since not all devices can operate at the same speed.
   
   
3. Wider data path: Use more wires, send more at once.
   
   
4. Separate address and data lines: Same as above.
   
   
5. Block transfers: Permit a single transaction to transfer more than one busload of data. Saves the time to release and acquire the bus, but the protocol is more complex.
   

   

6. Obtaining bus access:
   • The simplest scheme is to permit only one bus master.
     • That is on each bus only one device is permited to initiate a bus transaction.
     • The other devices are slaves that only respond to requests.
     • With a single master, there is no issue of arbitrating among multiple requests.
   • One can have multiple masters with daisy chaining of the grant line.
     • Any device can raise the request line.
     • The device with the request raises the release line when done.
     • The arbiter monitors the request and request lines and raises the grant line.
     • The grant signal is passed from one to the other so the devices near the arbiter have priority and can starve the ones further away.
     • Passing the grant from device to device takes time.
     • Simple but not fair or high performance
   • Centralized parallel arbiter: Separate request lines from each device and separate grant lines. The arbiter decides which device should be granted the bus.
   • Distributed arbitration by self-selection: Requesting processes identify themselves on the bus and decide individually (and consistently) which one get the grant.
   • Distributed arbitration by collision detection: Each device transmits whenever it wants, but detects collisions and retries. Ethernet uses this scheme (but not new switched ethernets).

Do on the board the example on pages 665-666