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
22
Section 3: System Overview
Document
1250_1125-UM100CB-R
A
DDRESS
P
HASE
In the address phase the requesting agent puts the TID, address, byte-enables, command and cache attributes
onto the address bus. All agents will examine the request and respond by setting their R_SHD and R_EXC bits
in the R-phase.
The address has cache block granularity, and is therefore bits [39:5] of the physical address being accessed.
Thirty-two byte enables indicate which bytes within the block are to be transferred.
The bus commands are in
. Uncached and non-coherent accesses use the same commands, but will
be responded to by the agent that owns the address. The default owner of a block is the L2 or memory
controller for all addresses in the memory address space, the generic bus is the default owner for addresses
in the generic bus range.
Table 3: ZBbus Commands
A_CMD[2:0]
Command
Action
Data Bus
READ_SHD
000
Read
Shared
The block is provided by the current owner and
becomes shared. If the current owner is L2/memory
and no other agent has a shared copy then the
acquiring agent may take exclusive ownership (with
no other action).
Data will be supplied by the
exclusive owner if one is
identified in the response
phase, otherwise the
default owner will supply it.
READ_EXC
001
Read
Exclusive
The block is provided by the current owner and
becomes exclusively owned by the acquiring agent.
All other copies of the block are invalidated.
Data will be supplied by the
exclusive owner if one is
identified in the response
phase, otherwise the
default owner will supply it.
WRITE
010
Write
The block is written back to L2/memory.
Data will be supplied by the
requester.
WRITE_INV
011
Write
Invalidate
The requesting agent acquires ownership of the
block and invalidates all copies (even if they are
dirty), it then releases ownership and writes the block
back to L2/memory. This is an optimization that
avoids the need for a READ_EXC and transfer of
data that will be discarded, it is used when the
requesting agent is overwriting a complete block with
new data (for example when a packet is DMAed from
the network).
Data will be supplied by the
requester.
INVALIDATE
100
Invalidate
Block
The requesting agent acquires ownership of the
block and all copies are invalidated (even if they are
dirty). It is used when an agent upgrades a shared
line to exclusive (in which case none of the copies
will be dirty), or if the agent needs to become
exclusive owner and guarantees to overwrite the
complete line.
There is no data phase for
this command.
101
Reserved
110
Reserved
NOP
111
No
Operation
This command is used when the agent arbitrated for
the bus and was granted, but is unable to complete
the transaction. This will happen occasionally if the
access is blocked by a buffer becoming full between
the time the agent requested the bus and the
address phase. It can also happen as a result of an
exception in the requesting agent.
There is no data phase for
this command.
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