Figure 8-7. Address-Bus Arbitration Showing Bus Parking - IBM PowerPC 750GX User Manual

User's Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
External arbiters must allow only one device at a time to be the address-bus master. For implementations in
which no other device can be a master, BG can be grounded (always asserted) to continually grant master-
ship of the address bus to the 750GX.
Note: Arbiter designs must ensure that no more than one address-bus master can be granted the bus at one
time (that is, bus grants must be mutually exclusive).
If the 750GX asserts BR before the external arbiter asserts BG, the 750GX is considered to be unparked, as
shown in Figure 8-6. Figure 8-7 shows the parked case, where a qualified bus grant exists on the clock edge
following a need_bus condition. Notice that the bus clock cycle required for arbitration is eliminated if the
750GX is parked, reducing overall memory latency for a transaction. The 750GX always negates ABB for at
least one bus clock cycle after the address acknowledge (AACK) signal is asserted, even if it is parked and
has another transaction pending.
Typically, bus parking is provided to the device that was the most recent bus master. However, system
designers might choose other schemes, such as providing unrequested bus grants in situations where it is
easy to correctly predict the next device requesting bus mastership.

Figure 8-7. Address-Bus Arbitration Showing Bus Parking

-1 0 1
need_bus
BR
bg
abb
artry
qual BG
ABB
When the 750GX receives a qualified bus grant, it assumes address-bus mastership by asserting ABB and
negating the BR output signal. Meanwhile, the 750GX drives the address for the requested access onto the
address bus and asserts transfer start (TS) to indicate the start of a new transaction.
When designing external bus arbitration logic, note that the 750GX might assert BR without using the bus
after it receives the qualified bus grant. For example, in a system using bus snooping, if the 750GX asserts
BR to perform a replacement copy-back operation, another device can invalidate that line before the 750GX
is granted mastership of the bus. Once the 750GX is granted the bus, it no longer needs to perform the copy-
back operation. Therefore, the 750GX does not assert ABB and does not use the bus for the copy-back oper-
ation. Note that the 750GX asserts BR for at least one clock cycle in these instances.
gx_08.fm.(1.2) Bus Interface Operation
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