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2–34 Internal Architecture
Alpha 21264/EV67 Hardware Reference Manual
I/O Write Buffer and the WMB Instruction
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.
c. When a probe response has been sent to the system for the marked probe queue
entry, the Cbox considers the WMB to be satisfied.
• If Cbox CSR SYSBUS_MB_ENABLE is set, the Cbox performs the following
actions:
a. Stalls further MAF and IOWB processing.
b. Sends the MB command to the system port.
c. Waits until the MB command is acknowledged by the system with a SysDc
MBDone command, then sends acknowledge and marks the youngest entry in
the probe queue.
d. When a probe response has been sent to the system for the marked probe queue
entry, the Cbox considers the WMB to be satisfied.
2.12.1.3 TB Fill Flow
Load instructions (HW_LDs) to a virtual page table entry (VPTE) are processed by the
21264/EV67 to avoid litmus test problems associated with the ordering of memory
transactions from another processor against loading of a page table entry and the subse-
quent virtual-mode load from this processor.
Consider the sequence shown in Table 2–12. The data could be in the Bcache. Pj should
fetch datai if it is using PTEi.
Table 2–12 TB Fill Flow Example Sequence 1
Pi Pj
Write Datai Load/Store datai
MB <TB miss>
Write PTEi Load-PTE
<write TB>
Load/Store (restart)