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176

Revision 1.0

Integrated Functions (Continued)

4.7.8.3

PCI Arbitration

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

on the arbitration scheme set in the PCI Arbitration Control

2 register (Index 44h), the GX1 processor’sPCI arbiter will

grant the request by asserting GNT#. Figure 4-20 illus-

trates basic arbitration.

REQ#-a is asserted at CLK 1. The PCI arbiter grants

access to Agent A by asserting GNT#-a on CLK 2. Agent A

must begin a transaction by asserting FRAME# within 16

clocks, or the GX1’s 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. How-

ever, in this example, Agent B is of higher priority than

Agent A. When Agent B requests the bus on CLK 2, Agent

A is allowed to proceed per Specification. Agent A starts its

transaction on CLK 3 by asserting FRAME# and completes

its transaction. Since Agent A requests another transac-

tion, REQ#-a remains asserted. When FRAME# is

asserted on CLK 3, the PCI arbiter determines Agent B

should go next, asserts GNT#-b and deasserts GNT#-a on

CLK 4. Agent B requires only a single transaction. It com-

pletes the transaction, then deasserts FRAME# and

REQ#-b on CLK 6. The PCI arbiter can then grant access

to Agent A, and does so on CLK 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, Arbitration

Signaling Protocol, of the PCI Local Bus Specification,

Revision 2.1.

4.7.8.4

PCI Halt Command

Halt is a broadcast message from the GX1 processor indi-

cating 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 arbitrary 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

Agent-A

Agent-B

4567

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