Bus Master Mode of PCI-Bus. Part II

The main parameter that controls the allocation of access priorities to PCI-bus is “PCI Bus Arbitration” (Arbitration Priority, PCI Arbiter Mode, PCI Arbitration Mode, PCI Arbit. Rotate Priority). Each of these parameters may have two values. For example, the parameter “PCI Bus Arbitration” has got values “Rotating” and “Fixed” that identify the operating mode of devices with PCI-bus, Favor CPU or Favor PCI and also specify which of devices would receive an access priority to a bus. It is sure that to optimize the computer operation, it is necessary to set an access priority to a bus for the processor but in some cases selection of a value “Rotating” (or Favor PCI) enables to speed up its operation.

To set the access priorities to a bus, you may use separate parameters, for example, the parameter “CPU priority” which enables to set the rank of central processor in the hierarchy of the access priorities of all possible devices that contain a function of master-devices in a system. This parameter may have the following values; Always Last – a processor access to a bus immediately after finishing its operation with the current device; CPU 2nd – omit the current and the next device by a processor; CPU 3rd – omit three devices; CPU 4th – omit four devices. In this case the recommendations are still the same: if you carry out a normal operation, it is optimal to set as low delays as possible for access to a bus; but if you carry out the operation connected with direct interaction of two devices (expansion cards), it is possible to set the delays.