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Revision 3.1

Integrated Functions (Continued)

4.6.8.3

PCI Arbitration

An agent requests the bus by asserting its REQ#. Based

on the arbitration scheme set in the PCI Arbitration Con-

trol 2 Register (Index 44h), the GX PCI arbiter will grant

the request by asserting GNT#. Figure 4-20 illustrates

basic arbitration.

REQ#-a is asserted at clock 1. The PCI GXm processor

arbiter grants access to Agent A by asserting GNT#-a on

clock 2. Agent A must begin a transaction by asserting

FRAME# within 16 clocks, or the GX PCI arbiter will

remove GNT#. Also, it is possible for Agent A to lose bus

ownership sooner if another agent with higher priority

requests the bus. However, in this example, Agent A

starts the transaction on clock 3 by asserting FRAME#

and completes its transaction. Since Agent A requests

another transaction, REQ#-a remains asserted. When

FRAME# is asserted on clock 3, the GXm processor’s PCI

arbiter determines Agent B should go next, asserts

GNT#-b and deasserts GNT#-a on clock 4. Agent B

requires only a single transaction. It completes the trans-

action, then deasserts FRAME# and REQ#-b on clock 6.

The GXm processor’s PCI arbiter can then grant access

to agent A, and does so on clock 7. Note that all buffers

must flush before a grant is given to a new agent.

For additional information refer to Chapter 3.4.1, Arbitra-

tion Signaling Protocol, of the PCI Local Bus Specifica-

tion, Revision 2.1.

4.6.8.4

PCI Halt Command

Halt is a broadcast message from the processor indicating

it has executed a halt instruction. The PCI Special Cycle

command is used to broadcast the message to all agents

on the bus segment. During the address phase of the Halt

Special cycle, C/BE[3:0]# = 0001 and AD[31:0] are driven

to random values. During the data phase, C/BE[3:0]# =

1100 indicating bytes 1 and 0 are valid and AD[15:0] =

0001h.

For additional information, refer to Chapter 3.7.2, Special

Cycle, and Appendix A, Special Cycle Messages, of the

PCI Local Bus Specification, Revision 2.1.

Figure 4-20. Basic Arbitration

CLK

REQ#-a

REQ#-b

GNT#-a

GNT#-b

FRAME#

DATA

ADDR

DATA

ADDR

access-a

access-b

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