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21264/EV68A Hardware Reference Manual
Internal Architecture 2–33
I/O Write Buffer and the WMB Instruction
2.12.1.1 MB Instruction Processing
When an MB instruction is fetched in the predicted instruction execution path, it stalls
in the map stage of the pipeline. This also stalls all instructions after the MB, and con-
trol of instruction flow is based upon the value in Cbox CSR SYSBUS_MB_ENABLE
as follows:
• If Cbox CSR SYSBUS_MB_ENABLE is clear, the Cbox waits until the IQ is
empty and then performs the following actions:
a. Sends all pending MAF and IOWB entries to the system port.
b. Monitors Cbox CSR MB_CNT[3:0], a 4-bit counter of outstanding committed
events. When the counter decrements from one to zero, the Cbox marks the
youngest probe queue entry.
c. Waits until the MAF contains no more Dstream references and the SQ, LQ, and
IOWB are empty.
When all of the above have occurred and a probe response has been sent to the sys-
tem for the marked probe queue entry, instruction execution continues with the
instruction after the MB.
• If Cbox CSR SYSBUS_MB_ENABLE is set, the Cbox waits until the IQ is empty
and then performs the following actions:
a. Sends all pending MAF and IOWB entries to the system port
b. Sends the MB command to the system port
c. Waits until the MB command is acknowledged, then marks the youngest entry
in the probe queue
d. Waits until the MAF contains no more Dstream references and the SQ, LQ, and
IOWB are empty
When all of the above have occurred and a probe response has been sent to the sys-
tem for the marked probe queue entry, instruction execution continues with the
instruction after the MB.
Because the MB instruction is executed speculatively, MB processing can begin
and the original MB can be killed. In the internal acknowledge case, the MB may
have already been sent to the system interface, and the system is still expected to
respond to the MB.
2.12.1.2 WMB Instruction Processing
Write memory barrier (WMB) instructions are issued into the Mbox store-queue, where
they wait until they are retired and all prior store instructions become writable. The
Mbox then stalls the writable pointer and informs the Cbox. The Cbox closes the IOWB
merge register and responds in one of the following two ways:
• If Cbox CSR SYSBUS_MB_ENABLE is clear, the Cbox performs the following
actions:
a. Stalls further MAF and IOWB processing.
b. Monitors Cbox CSR MB_CNT[3:0], a 4-bit counter of outstanding committed
events. When the counter decrements from one to zero, the Cbox marks the
youngest probe queue entry.