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ReadClean transaction (ACE protocol)

ka

Hello everyone,

Can any one explain me why there is a case where a start state in Unique Dirty leads to a final state in Unique Dirty for a readClean transaction in ACE protocol.

This transaction requires a clean copy of the data, so how it can possible to have the (UD-UD) transition possible?

Thanks,

Anya ka,

  • 0 Arm Employee Badge

    Hi Anya,

    This transition is a 'permitted' transaction, as opposed to 'expected' transaction.  This generally means that its a less efficient, or an unexpected use of a transaction.  In the case of a ReadClean, its clear that it not efficient to issue this transaction when the master already has the data.

    If the line starts in a Dirty state, and the transaction does not transfer this Dirty responsibility in some way (for example, by writing the data back to memory) then the line must finish in a Dirty state.  Otherwise, the Dirty responsibility would have been lost. 

    Regardless of the RRESP information indicating that PassDirty is not asserted, the Dirty responsibility cannot be lost.