Bus Master Mode of PCI-Bus. Part I

All devices connected with a bus are subjected to a particular operating mode which is called “bus master”. A device that controls the bus operation is a host or the dial of cycles.

If you need to transmit data, each device sends a request and either continues waiting or takes a bus for its needs after received a request. The allocation of bus resources depends on the priority or/and a regular request for this device. After finishing its operation, a device sends an appropriate signal and relieves a bus. All devices that operate with this bus undergo this standard procedure.

You can check the opportunity to control a bus by using the parameter “Enabled Master”. This parameter has got two values: Enabled and Disabled.

As a rule, all devices on PCI-bus have equal priority. There are two methods of access to the memory. The first one is the rotation mode where the device priority is periodically changed. In this mode a low priority is given to a device that has a control over a bus, and any other device moves one step up in the priority queue. The second method is the fixed operating mode in which the devices with dominant priority appear.

In modern systems most control functions and functions of allocation of access to a bus are flashed and are not displayed in the appropriate parameters of BIOS. Much older versions of BIOS may contain some of parameters described above.