CS355 Sylabus

Bus Arbitration

The System Bus is a bunch of electrical paths that are used to convey electrical signals between various devices inside the computer and the bus cannot be used by more than one device at one time due to electrical properties of the devices (outputs will be connected together and will cause damage).
So before any master device is allowed to perform a rea/write operation using the system bus, it must first obtain permission
There is always a bus arbitration period preceeding a bus utilization period:
The two operations (bus arbitration and bus utilization) can be performed in parallel:
- 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.
Therefore, bus arbitration periods and bus utilization periods usually overlap:

There are 2 types of bus arbitration techniques:
- Centralized (commonly used)
- Distributed (rarely used - only instance I know is DEC)
Example centralized aribitration techniques:
- Daisy-chained (cheap - requires no special hardware)
  - The daisy chain consists of wires connecting the devices in a pre-defined order
  - This ordering defines the "pecking order" of the devices.
- Explicit centralized bus arbiter (used in PCI bus):
  - 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. The following table shows the logic values when the highest numbered device receives the highest priority:
```
   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
```
Example distributed bus arbitration:

Advantage of distributed bus arbiter: no single point of failure...
Note:
- The attentive student will have notice that the request outputs of multiple devices are tied together - which I have said it is dangerous to do so because they will burn each other out.
- What I mentioned about tying outputs together is true, but only when the resistance of the outputs are low.
- You normally want low outputs because these outputs will be able to switch faster (i.e., the signal strength changes from 0 to 1 very fast.
- There is a transistor wiring technique called "Open Collector" output that results in outputs with very high resistance.
- 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 !)