Patent ID: 7181556

Claim:
A data processing apparatus comprising: a master device; a first slave device; and a common communication bus providing a plurality of communication paths between said master device, said first slave device and at least one further slave device, and for passing transaction requests from said master device to said first slave device; wherein said master device having a transaction annotator for generating a transaction identifier as part of each transaction request passed from said master device to said first slave device, said transaction identifier having a master identifier portion and a priority request portion specifying a priority value for said transaction request; and said first slave device having transaction ordering logic operable to determine an order of service of a plurality of transaction requests having respective transaction identifiers in dependence upon transaction ordering constraints at least partially derived from master identifier portions of said transaction identifiers and in dependence upon said priority values of said transaction identifiers, wherein said master identifier portion specifies one of a plurality of possible master identifier values associated with said master device, wherein said transaction ordering constraints relate only to subsets of transaction request generated by said master device for which said master identifier portion specifies an identical master identifier value.