In Section C1.3 Channel Overview of the AMBA_AXI_and_ACE protocol specifications, It is mentioned under "Store operations where the cache line is already cached" as :
The initiating master component requests a unique copy of the cache line by issuing a CleanUniquetransaction on the read address channel. This removes all other copies of the cache line and writes any dirtycopy to main memory.
Consider that our initiating master has a clean, shared copy. There is another master having a dirty, shared copy.
Now the initiating master issues a CleanUnique transaction on the Read Address Channel. Since the snooped master has had a dirty copy, the interconnect constructs a transaction to write the cacheline to the main memory, and provides a response to the initiating master.
Now at this point in time, the initiating master has a copy that is no more clean, since the copy it has with itself is modified relative to the main memory; and the previously dirty cacheline was not provided to the initiating master.
The next step mentioned is that the master performs a store and uses the RACK signal to indicate that the transaction has been completed.
This seems ambiguous since the initiating master performed a store even when it's copy of the cacheline, though unique ; wasn't clean.
Am I missing something?
In case of readunique transaction cache line is copied into the initiating master's cache(whether it is clean or dirty) and invalidated in snooped master's cache and then store operaation is performed in initiating master's cache line.
In case of cleanunique transaction cache line is copied to main memory if it is dirty(and invalidate) and then store operation performed in initiating master's cache.
Why do we need extra write to memory in case of cleanunique transaction?
Now the initiating master issues a CleanUnique transaction o omegle discord xender n the Read Address Channel. Since the snooped master has had a dirty copy, the interconnect constructs a transaction to write the cacheline to the main memory, and provides a response to the initiating master.
That is correct , but why it is required ? Like in readunique case , we also can send copy of cache line directly to initiating master itself right?