System Support

The PCI bus supports a bus master/target arbitration scheme. A bus master is a device that has been granted control of the bus for the purpose of initiating a transaction. A target is a device that is the recipient of a transaction. The Request (REQ), Grant (GNT), and FRAME signals are used by PCI bus masters for gaining access to the PCI bus. When a PCI device needs access to the PCI bus (and does not already own it), the PCI device asserts its REQn signal to the PCI bus arbiter (a function of the system controller component). If the bus is available, the arbiter asserts the GNTn signal to the requesting device, which then asserts FRAME and conducts the address phase of the transaction with a target. If the PCI device already owns the bus, a request is not needed and the device can simply assert FRAME and conduct the transaction. Table 4-1 shows the grant and request signals assignments for the devices on the PCI bus.

Table 4-1.

PCI Bus Mastering Devices

Device

REQ/GNT Line

 

 

PCI Connector Slot 1

REQ0/GNT0

 

 

PCI bus arbitration is based on a round-robin scheme that complies with the fairness algorithm specified by the PCI specification. The bus parking policy allows for the current PCI bus owner (excepting the PCI/ISA bridge) to maintain ownership of the bus as long as no request is asserted by another agent. Note that most CPU-to-DRAM accesses can occur concurrently with PCI traffic, therefore reducing the need for the Host/PCI bridge to compete for PCI bus ownership.

4.2.2 PCI Express Bus Operation

The PCI Express (PCIe) v1.1 bus is a high-performace extension of the legacy PCI bus specification. The PCI Express bus uses the following layers:

■Software/driver layer

■Transaction protocol layer

■Link layer

■Physical layer

Software/Driver Layer

The PCI Express bus maintains software compatibility with PCI 2.3 and earlier versions so that there is no impact on existing operating systems and drivers. During system intialization, the PCI Express bus uses the same methods of device discovery and resource allocation that legacy PCI-based operating systems and drivers are designed to use.

Transaction Protocol Layer

The transaction protocol layer processes read and write requests from the software/driver layer and generates request packets for the link layer. Each packet includes an identifier allowing any required responcse packets to be directed to the originator.

 

4-2

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