CS355 Sylabus

Bus Arbitration

Accessing the system bus

How devices are connected inside a compute:

There are always 3 sets of wires (= bus) in a computer:

Address bus use to convey the value of the address
Data bus use to convey the information (data) sent between the devices
The control bus use for control signals (such as MREQ, Read/Write, etc)

The collection of address bus, data baus and control bus is called the system bus

The System Bus is a bunch of electrical paths that are used to convey electrical signals between various devices inside the computer.
Important fact:
Solution: arbitration

Communication jargon
- Master device:
- Slave device:

Bus "cycle"

Bus cycle:

What happens in a bus cycle:

The master device sends out the address of the slave device
The master device sends out the operation (READ or WRITE) to the slave device
If the operation = READ, then:
If the operation = WRITE, then:

How to determine the master in a bus cycle

A bus cycle consists of 2 periods:
There is always a bus arbitration period preceeding a bus utilization period:

Because the bus arbitration operation and the bus utilization operation uses different set of wires:

The bus arbitration period and the bus utilization period can overlap in execution:

In other words:

While the current bus utilization period is going on, devices that want to use the bus in the next cycle can decide among themselves who will get to use the system bus in the next period.

Techniques used to implement bus arbitration
- There are 2 types of bus arbitration techniques:

Centralized arbitration technique

Daisy chaining:

The daisy chain consists of wires connecting the devices in a pre-defined order:

This ordering defines the "pecking (=priority) order" of the devices:

The arbiter sends out a grant signal = 1
A device that does not want to use the system bus will pass the grant signal = 1 to the next device in the chain
Example: device 1
A device that wants to use the system bus will remove the grant signal and sends grant = 0 to the next device in the chain
Example: device 2

The device that has:

Incoming grant = 1
Outgoing grant = 0

is the winner in the arbitration procedure

Advance of daisy chaining:

Cheap !!!
(The personal computers around 1980 use daisy chaining !!!)
Nowadays, hardware is so cheap that daisy-chaining is not used anymore....

Note:
- The attentive student will have notice that the request outputs of multiple devices are tied together in the daisy chaining arbitraton scheme:
  Of which I have said it is not allowed because they will burn each other out.
- What I said about connecting outputs together is true, but only when the resistance of the outputs are low (so the current will be high and burn each other).
- You normally want low resistive outputs because then the outputs will be able to switch faster (i.e., the signal strength changes from 0 to 1 very fast) because of the low resistance.
- There is a transistor wiring technique called "Open Collector" output that results in outputs with very high resistance (the signals on these output will change slower than normal digital circuit outputs).
- You can connect open collector outputs together without having them burning each other out, but (there is always a but...), these outputs will not switch as fast:
- Requests outputs are made of open-collector, because they don't need to switch very fast (they have a whole memory read/write cycle time to do their work !)

Explicit centralized bus arbiter (e.g.: the PCI bus arbiter):

Such an explicit centralized bus arbiter is used in the popular PCI bus.

The logic of the bus arbiter is simple and and depends on how the priorities are assigned.

Example: The following table shows the logic values when priority_req2 > priority_req1 > priority_req0

   Req2  Req1   Req0  |  Grant2  Grant2   Grant0
 ---------------------+----------------------------
    0     0      0    |    0       0        0
    0     0      1    |    0       0        1
    0     1      0    |    0       1        0
    0     1      1    |    0       1        0
    1     0      0    |    1       0        0
    1     0      1    |    1       0        0
    1     1      0    |    1       0        0
    1     1      1    |    1       0        0

You can program the arbiter chip with a different priority

Distributed bus arbitration:

Advantage of distributed bus arbiter: no single point of failure...