
54
Functional Operation
Chapter 5
AMD-751
System Controller Data Sheet
21910D
—
August 1999
Preliminary Information
n
The probe queue (PQ) stores probes from the system to the
processor. To maintain coherency, the PQ is snoopable.
The processor system data and control queue (CSQ) stores
system data control commands in three separate read and
write buffers for data movement in and out of the processor
caused by commands generated by the processor itself.
The probe system data and control queue (PSQ) stores
system data control commands for data movement in and out
of the processor for probes generated by the system.
n
n
BIU Functional Units
The BIU contains the following functional units:
The
probe response alert agent
(PRA) broadcasts probe
responses it receives from the processor. The probe response
can
come
as
a
no-data-movement-required commands, or it can come as a
long probe-response command.
The transaction combiner agent (XCA) is responsible for
creating a command packet from the system to the processor
containing SysAddOut command format and probe
information. The XCA intelligently combines, when
possible, the CSQ or PSQ entry and the PQ entry, and
transmits it to the processor using the four-cycle SysAddOut
command format. When there is either no PQ entry, CSQ
entry, or PSQ entry ready for dispatching, a NOP is inserted
by the combiner logic. The acknowledge (A-bit) information
is also packed in the four-cycle SysAddOut command format.
n
quick
probe
miss
or
n
5.2.2
BIU Start-Up
The AMD Athlon system bus is a unique, source-synchronous,
channel that uses protocol and fixed delivery windows to
provide maximum performance.
An AMD Athlon system bus is synchronous. However, the use of
multiple PLLs in the system and the use of source-synchronous
clocking create different clock domains from the processor to
the system logic and from the processor to a backside L2 cache.
A clock domain is the component or set of components running
on one clock signal for its logic operation. For example, the
AMD-751 system controller is one clock domain and operates on
its own PLL. Although this PLL is driven from the same clock
generator as the rest of the system, drifts and skews in the