Pci Express Bus Operation - Compaq dc5850 - Microtower PC Technical Reference Manual

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.
PCI Bus Mastering Devices
Device
PCI Connector Slot 1
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
Table 4-1.
REQ/GNT Line
REQ0/GNT0
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