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
20
Section 3: System Overview
Document
1250_1125-UM100CB-R
O
VERVIEW
OF
THE
ZB
BUS
P
ROTOCOL
This section gives an overview of the ZBbus protocol. It contains the details needed to do system and
performance debugging using the bus trace unit, but is not a full specification for the protocol. The bus is
entirely internal to the part, and it connects the main blocks of the system. Each block that directly connects to
the ZBbus is called an agent.
shows the agents and
lists their four bit bus ID number. The
bus runs at half the CPU clock speed and can transfer a new request and 256 data bits every cycle.
The bus is split into an address section and a data section. These are arbitrated for separately and run
independently. For a given transaction, use of the data bus always follows use of the address bus, but there
is no other ordering imposed. Many transactions can be in progress at a given time. In theory each agent could
have 64 outstanding operations on the bus, In practice, the agents are limited by their internal transaction
tracking buffers to fewer than 64.
lists the signals in each section of the bus.
Table 1: ZBbus Agent IDs
Agent
ID
Description
CPU0
0
SB-1 CPU 0.
CPU1
1
SB-1 CPU 1. (Only in BCM1250)
IOB0
2
I/O Bridge 0, connects PCI and HyperTransport interfaces to the ZBbus.
IOB1
3
I/O Bridge 1, connects the MACs and slow speed peripheral interfaces to the ZBbus.
SCD
4
System Control and Debug Unit.
5
Reserved
L2C
6
L2 Cache.
MC
7
Memory Controller.
Table 2: ZBbus Signals
Address/Control Section
A_AD[39:5]
Address of cache block moved by the request. The byte enables complete the address.
A_BYEN[31:0]
Byte enables. A_BYEN[31] corresponds to D_DA[255:248], down to D_BYEN[0] which corresponds to
D_DA[7:0]. (See
A_ID[9:0]
Transaction ID: [9:6] are requester agent ID (see
). [5:0] are unique for that requester.
A_CMD[2:0]
Command (see
A_L1CA[1:0]
L1 (base) cache attribute (see
A_L2CA
Request that the L2 cache allocate on miss.
R_SHD[5:0]
Response indicating block is shared. Each agent drives the bit corresponding to its ID.
R_EXC[5:0]
Response indicating block is exclusive. Each agent drives the bit corresponding to its ID.
R_L2HIT
Response indicating block is in the L2 cache. This signal is only used following a read command, but
is also valid for most writes. This signal my be incorrect following writes to the L2 registers, or after an
uncorrectable tag ECC error.
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