Bus Arbitration Techniques in Computer Organization
- 2. Single-bus Figure 1.3. Single-bus structure. Memory Input Output Processor
- 3. There are many ways to connect different parts inside a computer together. A group of lines that serves as a connecting path for several devices is called a bus. Address/data/control
- 5. Reasonably efficient architecture but begins to break down as higher and higher performance is seen in I/O devices. A common approach adopted by industry is to build a high speed bus closely integrated with the rest of the system. It require only a bridge between the processor’s bus and high speed bus.
- 7. The advantage of this arrangement is that the high speed bus brings high-demand devices into closer integration with the processor and at the same time, is independent of the processor. Changes in processor architecture do not affect the high speed bus and vice-versa
- 8. In a computer system, there may be more than one bus masters such as processor, DMA controller, etc, which share the system bus. When current master relinquishes control of the bus, another bus master can acquire the control of the bus. Bus Arbitration is the process by which the next device to become the bus master is selected and bus mastership is transferred to it.
- 9. There are two approaches to bus arbitration: 1. Centralized bus arbitration, and 2. Distributed bus arbitration
- 10. A single bus arbiter performs the required arbitration. The bus arbiter may be the processor or a separate controller connected to the bus. Three different arbiter schemes using centralized arbitration: 1. Daisy Chaining 2. Polling Method 3. Independent Priority
- 11. A simple and cheaper method for a centralized bus arbitration process. All masters make use of the same line for bus request. In response, the controller sends a bus grant if the bus is free. The bus grant serially propagates through each master until it encounters the first one that is requesting the access to the bus.
- 13. This master then blocks the propagation of the bus grant signal, activates the busy line and gains the control of the bus. Advantages: Simple and cheaper method. Requires the least number of lines which is independent of the number of masters in the system.
- 14. Disadvantages: The propagation delay is more which makes arbitration slow and hence limits the number of masters in the system. The priority of the master is fixed by its physical location. Failure of any one master causes the whole system to fail.
- 15. Bus Request Bus Busy Module Address
- 16. In this method the controller is used to generate the addresses for the masters. Number of address lines required depends on the number of masters connected in the system. In response to a bus request, controller generates a sequence of master addresses. When the requesting master recognizes its address, it activates the busy line and begins to use the bus.
- 17. Advantages: The priority can be changed by altering the polling sequence stored in the controller. If one module fails, entire system does not fail.
- 18. Each master has a separate pair of bus request and bus grant lines and each pair has a priority assigned to it. The built-in priority decoder within the controller selects the highest priority request and asserts the corresponding bus grant signal. Advantage: Due to separate pairs of bus request and bus grant signals, arbitration is fast and is independent of the number of masters in the system.
- 20. Disadvantage: It requires more bus request and bus grant signals ( 2*n signals for n modules).
- 21. In Distributed arbitration, all devices participate in the selection of next bus master. Each device in the bus is assigned a 4-bit identification number. Number of bits used for identification number actually depends on the number of devices connected on the bus. When one or more devices request for the control of the bus, they assert the Start-Arbitration signal and place their 4-bit ID numbers on arbitration lines, ARB0 through ARB3.
- 22. All four lines are open collector therefore, more than one devices can place their 4-bit ID number to indicate that they need the control of bus. If one device puts 1 on the bus line and another puts 0 on the same bus line, the bus line status will be 0. In this scheme, the device having the highest ID number has the highest priority.
- 23. When two or more devices place their ID number on bus line then it is necessary to identify the highest ID number from the status of bus line. The connection performs an OR operation in which logic 1 wins.
- 25. Assume that two devices, A and B, having ID numbers 5 and 6 respectively, are requesting the use of the bus. Device A transmits the pattern 0101, while B transmits 0110. The code seen by both the devices is 0111. Each device compares the pattern on the arbitration lines to its own ID, starting from the most significant bit. If it detects a difference at any bit position, it disables its drivers at that bit position and for all lower order bits.
- 26. Advantages: Offers higher reliability because operation of the bus is not dependent on any single device.