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4–28
Clocks, Cache, and External Interface
29 September 1997 – Subject To Change
21164PC-Initiated System Transactions
•
If there is a tag mismatch or the valid bit is clear, a Bcache miss has been
detected. If the block to be replaced is clean, the Bcache continues operation
while the READ MISS request is sent to the system. If the block to be replaced is
dirty, the 21164PC waits for all outstanding probes in flight to complete, and
then starts an external READ MISS with VICTIM PENDING transaction that
instructs the system logic to access and return data.
•
System logic acknowledges acceptance of the command from the 21164PC by
asserting cack_h.
•
Because the transaction is a read operation, requiring a fill operation, the transac-
tion is broken (pended) while system logic obtains the fill data. The Bcache is
nonblocking and allows other transactions to use the Bcache while a miss is
being serviced.
•
Prior to the fill data arriving, the system asserts idle_bc_h back to the 21164PC
to arbitrate for the shared 128-bit data bus. Any private read or write operations
in progress are allowed to complete before the fill data arrives from the system.
•
At a later time, the system asserts fill_h.
•
The 21164PC asserts the tag and tag control bits, and controls the write action
during the fill operation.
•
The system logic provides the data. As each of the two (or four) data cycles
becomes valid, the system logic asserts dack_h to cause the 21164PC to sample
the data and write it into the Bcache.
Interface commands from the 21164PC to the system are driven on the cmd_h<3:0>
signals. Table 4–8 lists and describes the set of interface commands.
Table 4–8 21164PC-Initiated Interface Commands
(Sheet 1 of 2)
Command
cmd_h
<3:0>
Description
NOP
0000
The NOP command is driven by the owner of the cmd_h bus
when it has no tasks queued.
—
0001
Reserved.
—
0010
Reserved.
—
0011
Reserved.
—
0100
Reserved.
—
0101
Reserved.