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21264/EV68A Hardware Reference Manual
Internal Architecture 2–29
Memory and I/O Address Space Instructions
SQ entry data that has not been transferred to the Dcache may source data to newer load
instructions. The Mbox compares the virtual Dcache index bits of incoming load
instructions to queued SQ entries, and sources the data from the SQ, bypassing the
Dcache, when necessary.
2.8.4 I/O Address Space Store Instructions
The Mbox begins processing I/O space store instructions, like memory space store
instructions, by translating the virtual address and placing the state associated with the
store instruction into the SQ.
The Mbox replays retired I/O space store entries from the SQ to the IOWB in program
order at a rate of one per GCLK cycle. The Mbox never allows queued I/O space store
instructions to source data to subsequent load instructions.
The Cbox maximizes I/O bandwidth when it allocates a new IOWB entry to an I/O
store instruction by attempting to merge I/O store instructions in a merge register. Table
2–8 shows the rules for I/O space store instruction data merging. The columns represent
the load instructions replayed to the IOWB while the rows represent the size of the store
in the merge register.
Table 2–8 shows some of the following rules:
• Byte/word store instructions and different size store instructions are not allowed to
merge.
• A stream of ascending non-overlapping, but not necessarily consecutive, longword
store instructions are allowed to merge into naturally aligned 32-byte blocks.
• A stream of ascending non-overlapping, but not necessarily consecutive, quadword
store instructions are allowed to merge into naturally aligned 64-byte blocks.
• Merging of quadwords can be limited to naturally-aligned 32-byte blocks based on
the Cbox WRITE_ONCE chain 32_BYTE_IO field.
• Issued MB, WMB, and I/O load instructions close the I/O register merge window.
To minimize latency, the merge window is also closed when a timer detects no I/O
store instruction activity for 1024 cycles.
After the IOWB merge register has closed its merge window, the Cbox sends I/O space
store requests offchip in the order that they were received from the Mbox.
Table 2–8 Rules for I/O Address Space Store Instruction Data Merging
Merge Register/
Replayed Instruction
Store
Byte/Word Store Longword Store Quadword
Byte/Word Nomerge Nomerge Nomerge
Longword No merge Merge up to 32 bytes No merge
Quadword Nomerge Nomerge Mergeupto64bytes