Compaq 850 manual PCI BUS Master Arbitration, PCI Bus Mastering Devices, REQ/GNT Line Device

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 4-2 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
REQ3/GNT3	PCI Connector Slot 4
REQ4/GNT4	PCI Connector Slot 5
REQ5/GNT5	ES1373 Audio Controller
GREQ/GGNT	AGP Slot

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.

4-6Compaq Deskpro EXS and Workstation 300 Personal Computers Featuring the Intel Pentium 4 Processor

First Edition - December 2000