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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 it’s 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 4-3 shows the grant and

request signals assignments for the devices on the PCI bus.

Table 4-3. PCI Bus Mastering Devices

Table 4-3.

PCI Bus Mastering Devices

Device REQ/GNT Line Note

PCI Connector Slot 1 REQ0/GNT0

PCI Connector Slot 2 REQ1/GNT1 [1]

PCI Connector Slot 3 REQ2/GNT2 [2]

PCI Connector Slot 4 REQ3/GNT3 [3]

PCI Connector Slot 5 REQ4/GNT4 [3]

lot GREQ/GGNT AGP S

OTE: N

only. [1] ST, SFF, DT, MT, & CMT form factors

rm factors only [2] ST, DT, MT, & CMT fo

[3] CMT form factor only

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.

hp compaq d330 and d530 Series of Personal Computers

Featuring the Intel Pentium 4 Processor

First Edition – June 2003

4-6

HP Compaq d330 DT User Manual

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