BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
214
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
A
CCESSES
FROM
THE
S
I
B
YTE
TO
THE
PCI
OR
H
YPER
T
RANSPORT
The data path from the ZBbus to the PCI or HyperTransport is split into three sections. The interface to the
ZBbus includes queues of requests and returning data. Inside the I/O bridge there are additional buffers in the
routing path to the interface block. Finally, there are additional queues and tables of outstanding requests in
the interface blocks.
Figure 44: Buffers Used for Accesses from the ZBbus to PCI and HyperTransport
illustrates these queues. The request queues always maintain transactions in the order that their A-
phase happened on the ZBbus, for uncached accesses this matches the program order of the loads and
stores. Once the requests are on the HyperTransport fabric or PCI bus the normal PCI ordering rules apply,
so writes can pass reads; for uncacheable accesses from the SB-1 cores this is not a problem because the
CPU will not issue an uncacheable write while there are uncacheable reads outstanding. The internal
connections will maintain the order of reads and the SB-1 CPU supports multiple outstanding uncached reads
to allow streaming. There is one case where this can cause problems: in the HyperTransport fabric (due to the
ordering of the chain), or PCI with bridges (using delayed reads) reads to different devices may reach their
destinations in a different order than they were issued. If the reads have side effects that are signalled on a
back-channel between the two devices the second read issued by the CPU may or may not see the effect of
the first (this is always a danger with back-channels). If this rare situation is encountered in a system then the
program must include a SYNC instruction between the reads to ensure they complete in the expected order.
Writes from the ZBbus are always posted and need no reply, they pass through the request queues and are
issued to the I/O bus. Reads also pass through the queues to be issued, but also have associated Read-Data-
Return (RDR) passing back up through the bridge to the ZBbus.
Queue
Queue
ZBbus
Queue
Queue
Queue
Requests
Queue
Requests
If No Reads
Queue 5 Writes
Queue
14 Reads
Outstanding on HT Fabric
Queue
1 Read or Write Outstanding
on PCI Bus
Else Queue 1 Write
4 Requests
2 RDR
2 RDR
2 RDR
4 Posted
4 Non-Posted
2 Reads
1 RDR
Содержание BCM1125
Страница 18: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page xviii Document 1250_1125 UM100CB R ...
Страница 28: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page xxviii Document 1250_1125 UM100CB R ...
Страница 515: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page vii Index Document 1250_1125 UM100CB R ...