### TechNote

Number: 04509-01
Date: April 1994

## Arbitration

Arbitration is specific to Micro Channel computers. It is the process that determines which device gains control of the bus when multiple devices simultaneously compete for control of the bus. Each device is assigned a priority level, which is known as an arbitration level. The higher the arbitration level, the greater the priority. This determines which device should control the bus next. The arbitration level for each device is contained in a software file called an Adapter Description File (ADF). Devices that simultaneously contend for control of the bus must not have the same arbitration level. Occasionally, arbitration-level conflicts occur between devices, usually if multiple adapters are installed that require the same resources.

Micro Channel computers have a fairness feature, which ensures that each device gets a turn to control the bus, even if it has a low arbitration level. The fairness feature guarantees that none of the devices is locked out of the bus and that each device can gain control of the bus within a given amount of time. When you configure your Micro Channel computer, you can disable the fairness feature for a device so that it controls the bus more than other devices. A device for which the fairness feature has been disabled can monopolize the bus. If you disable the fairness feature for more than one device, it is especially risky, because you could cause some devices, for which the fairness feature is enabled, including the microprocessor, to be completely locked out of the bus. Therefore, unless you are using your computer for a specific purpose, such as a server, it is best to leave the fairness feature enabled for all devices.

The central arbitration control point is a location in the system master where contending devices send their arbitration signals. The system master is a device integrated into the main circuit board that assigns resources, manages the system configuration, and issues the commands of the primary operating system. It does not actually decide which device should control the Micro Channel bus; the contending devices determine this among themselves using the arbitration logic that is programmed into the devices. However, the central arbitration control point actually grants control after the decision is reached.