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Thursday, December 6, 2012

Introduction
Bus is very important in order for the computer equipment to function properly. Bus seems like a cable that carries data signals from one place to another. Without Bus computer equipment could not send a signal to the processor to execute the next instruction. In other words, the bus acts as an intermediary between the processor instruction equipment for the action to be carried out. According to pccomputernotes.com, they define bus as “a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer.” Computer bus is divided into two types, namely Internal and external bus. Generally Internal buses connect the equipment available in the computer chassis such as system memory, processors and other components connected to the main board. While the external bus is also known as the expansion bus. External bus connecting the different equipment with input-output ports, expansions lots and connection to another drive. In general, external buss lower than the system bus.

Advantages of bus
2.1 Simple & versatile:
The new device can be added easily when needed. For example, we could add input or output device if required. Bus system will act to bring instructions or data sent by the new equipment added earlier.

2.2 The device can be moved when necessary
The device can be transferred between computers using the same standard bus system. Bus system will read the same device even if the device has been transferred to another computer. These advantages save money as could avoid spending more to buy a lot of devices to meet the needs of computer users.

2.3 Low Cost
A single set of wires is shared in multiple ways - This explains that the cost used is very low compared with other methods for the device to send / receive signals from the processor. Manage complexity by partitioning the design – If many devices need a bus at the same time, then it will be broken up into several sections that allow processors to receive and send commands required by the device.

3.0 Disadvantages of bus
3.1 Communication bottleneck
The bandwidth of that bus can limit the maximum I/O throughput. In this case, if the bus bandwidth is insufficient or can not meet demand device, the failure to deliver the next instruction will occur. In other words, the device is notable to use the bus when it is needed. This will affect the actual input output required by the device. The maximum bus speed is largely limited by:

  • The length of the bus.
  • The number of devices on the bus.
  •  The need to support a range of devices.
3.2 No Preference
In centralized Arbitration, the device closest to the arbitrator has a higher priority than the remote. Indirectly bus devices that require fast action but away from the arbitrator is unable to perform the desired direction. If this happens, it can slow down the process of receiving and transmitting from the device.

3.3 Priorities are fixed
In Decentralized Arbitration, it is possible that the device has a low priority do not get a bus for a long time. Total device is also limited to the bus line.

4.0 Bus arbitration problem
Before we go further on bus arbitration, we need to under stand the true meaning of bus arbitration. Bus Arbitration is a method to determine the priority of the access signal transmitted from computer equipment to the motherboard through a bus. According to answers.com, “In a single bus architecture when more than one device requests the bus, a controller called bus arbiter decides who gets the bus, this is called the bus arbitration”. Bus Arbitration has several problems that can not be avoided when several devices simultaneously execute instructions. 2 problems that often occur in the bus Arbitration is:

(i) It will happen when all the bus wants to be the leader of the other buses. So when no priority is given, then the congestion to get the bus going to happen and in turn will slow down the process of getting and sending bus requested by devices.
(ii) The more difficult if each device has a bus requester and sender. Each device will want a fast bus. This situation requires a quick arbitrator to deal with this problem.

To overcome the problems mentioned above, I will describe two types of bus Arbitration: Centralized Arbitration scheme and Decentralized Arbitration scheme.

5.0 Centralized Arbitration scheme
Bus centralized require hardware or arbitrator authorized the bus to perform the tasks required by the device. In this case, the hardware is probably from the CPU itself or devices available on the motherboard. There are three types of centralized Arbitration.
(a) A single shared bus request Line







Figure 1:A centralized one-level bus arbiter using daisy chaining

Just use a line bus. Arbiter function to receive signals from the device and the transmission path to the input-output bus closest to the arbitrator. If the device is denied bus received, then it will be given to the device I / O to another inline to receive the bus.

(b) Multiple shared bus request but with different priority levels



Figure 2 :A centralized two-level bus arbiter using daisy chaining

Each level has priority needs bus and the bus grant. Each device fitted to the bus at every level where it was needed bus devices will be given high priority level. If many requests are required by the devices simultaneously, arbitrator will provide a bus to the devices really need. But if all devices have interests / priorities are equal, the daisy chaining is used (The bus grant line - from one device to the next).
For example, based on the diagram above, if level 2 has a higher priority than level 1, then the order of priority is 3 -> 5 -> 1 -> 2 -> 4.

(c) Independent bus request lines



Figure 3: A centralized bus arbitration with independent bus request and grant lines

Such configuration used in the PCI bus. Each device has a bus request and bus grant. Arbitrator shall give preference to several bus. Arbitration faster because the bus is not shared. Arbiter also consider the policy priorities of the requesting device. However, it would take a high cost due to more complex and a lot of hose needed to be implemented. Based on Figure 3.0, all devices share the same arbitrator centrally. All devices also do not share the bus, then this allows bus request and bus grant can occur more rapidly.

 6.0 Decentralized arbitration scheme

Decentralized Arbitration does not require arbitrator. So the device should make sure who goes first. This condition causes the device task becomes more complex and complicated. Indirectly, the bus trip to be quite slow. However, this saves cost because it has no arbitrator as happened with centralized bus.






 Figure 4.0 : Decentralized arbitration

Based on Figure 4.0 above, the requester bus line is usually given priority according to numbers 0 to 3. So if the device requires bus simultaneously, then the devices will know which are the highest priority will get first bus. And so on happen to other devices.

7.0 Conclusion

Based on a written paragraph above, all types of buses has its own advantages and disadvantages. Respectively, were adjusted according to the need of bus technology that can respond immediately. For example, for banking hardware used must be made up of the best device and the bus. While the computer for home use may only use the device and mediumspeed bus. All this depends on the engineers who built the device and the bus for any hardware to be used.

Refferences
http://fcit.usf.edu/network/chap5/chap5.htm#LinearBusnetwork
http://www.pccomputernotes.com/system_bus/bus01.htm
http://www.sm.luth.se/csee/courses/smd/137/slides/F12/bus.pdf
http://www.answer.com









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