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Chapter 4 System Support
4.2.2 PCI BUS MASTER ARBITRATION
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 4-2 shows the grant and
request signals assignments for the devices on the PCI bus.
Table 4-2. PCI Bus Mastering Devices
Table 4-2.
PCI Bus Mastering Devices
REQ/GNT Line Device
REQ0/GNT0 PCI Connector Slot 1
REQ1/GNT1 PCI Connector Slot 2
REQ2/GNT2 PCI Connector Slot 3
GREQ/GGNT AGP Slot
NOTE:
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 and AGP-to-DRAM accesses can occur
concurrently with PCI traffic, therefore reducing the need for the Host/PCI bridge to compete for
PCI bus ownership.
Compaq D315 and hp d325 Personal Computers
Featuring the AMD Athlon XP Processor
Second Edition – April 2003
4-6