Compaq 21264 - Manual (Page 59)

Alpha 21264/EV67 Hardware Reference Manual Internal Architecture 2–31
Replay Traps
The 21264/EV67 maintains the default memory data instruction ordering as shown in
Table 2–10 (assume address X and address Y are different).
The 21264/EV67 maintains the default I/O instruction ordering as shown in Table 2–11
(assume address X and address Y are different).
2.11 Replay Traps
There are some situations in which a load or store instruction cannot be executed due to
a condition that occurs after that instruction issues from the IQ or FQ. The instruction is
aborted (along with all newer instructions) and restarted from the fetch stage of the
pipeline. This mechanism is called a replay trap.
2.11.1 Mbox Order Traps
Load and store instructions may be issued from the IQ in a different order than they
were fetched from the Icache, while the architecture dictates that Dstream memory
transactions to the same physical bytes must be completed in order. Usually, the Mbox
manages the memory reference stream by itself to achieve architecturally correct
behavior, but the two cases in which the Mbox uses replay traps to manage the memory
stream are load-load and store-load order traps.
Table 2–10 Memory Reference Ordering
First Instruction in Pair Second Instruction In Pair Reference Order
Load memory to address X Load memory to address X Maintained (litmus test 1)
Load memory to address X Load memory to address Y Not maintained
Store memory to address X Store memory to address X Maintained
Store memory to address X Store memory to address Y Maintained
Load memory to address X Store memory to address X Maintained
Load memory to address X Store memory to address Y Not maintained
Store memory to address X Load memory to address X Maintained
Store memory to address X Load memory to address Y Not maintained
Table 2–11 I/O Reference Ordering
First Instruction in Pair Second Instruction in Pair Reference Order
Load I/O to address X Load I/O to address X Maintained
Load I/O to address X Load I/O to address Y Maintained
Store I/O to address X Store I/O to address X Maintained
Store I/O to address X Store I/O to address Y Maintained
Load I/O to address X Store I/O to address X Maintained
Load I/O to address X Store I/O to address Y Not maintained
Store I/O to address X Load I/O to address X Maintained
Store I/O to address X Load I/O to address Y Not maintained