Abstract: An interconnect has coherency control circuitry for performing coherency control operations and a snoop filter for identifying which devices coupled to the interconnect have cached data from a given address. When an address is looked up in the snoop filter and misses, and there is no spare snoop filter entry available, then the snoop filter selects a victim entry corresponding to a victim address, and issues an invalidate transaction for invalidating locally cached copies of the data identified by the victim. The coherency control circuitry for performing coherency checking operations for data access transactions is reused for performing coherency control operations for the invalidate transaction issued by the snoop filter. This greatly reduces the circuitry complexity of the snoop filter.

Abstract: An interconnect has transaction tracking circuitry for enforcing ordering of a set of data access transactions so that they are issued to slave devices in an order in which they are received from master devices. The transaction tracking circuitry is reused for also enforcing ordering of snoop transactions which are triggered by the set of data access transactions, for snooping master devices identified by a snoop filter as holding cache data for the target address of the transactions.

Abstract: Arbitrating and multiplexing circuitry 28 comprises arbitrating tree circuitry having X arbitrating levels and multiplexing tree circuitry having Y multiplexing levels. The Y multiplexing levels comprise a first set of multiplexing levels upstream of a second set of multiplexing levels. The first set of multiplexing levels operate in parallel with at least some of the arbitrating levels. The second set of multiplexing levels operate in series with the X arbitrating levels such that the second set of multiplexing levels completes the required selection to provide the final output following completion of, and in dependence upon, the arbitration by the arbitrating tree circuitry.

Abstract: A data processing apparatus is provided with a master device and a slave device which communicate via communication circuitry. The slave device is associated with a predetermined number of permission tokens that is equal to a maximum number of currently pending messages that can be accepted for processing from the communication circuitry by that slave device. The slave device transmits these permission tokens to the master device. The master device takes exclusive temporary possession of the permission tokens that it receives such that the permission tokens are then no longer available to any other master device. A master device initiates a message to a slave device when the master device has exclusive temporary possession of a permission token for that slave device. When the master device has initiated its message, then it relinquishes the exclusive temporary possession of the permission token such that it is then available for other devices.

Abstract: A data store has a data array for storing data values and a tag array for storing tag values for tracking which data values are stored in the data array. The associativity of the data array is greater than the associativity of the tag array. This means that fewer tag entries need to be accessed on each data access than in a conventional data store, reducing power consumption.

Abstract: An integrated circuit includes one or more transaction data sources and one or more transaction data destinations connected via interconnect circuitry comprising a plurality of interconnect nodes. Within the interconnect nodes there are one or more converging interconnect nodes. A converging interconnect node includes prediction data generation circuitry for reading characteristics of a current item of transaction data from the converging interconnect node and generating associated prediction data for a future item of transaction data which will be returned to the converging interconnect node at a predetermined time in the future. This prediction data is stored within prediction data storage circuitry and is read by prediction data evaluation circuitry to control processing of a future item of transaction data corresponding to that prediction data when it is returned to the converging interconnect node. The interconnect circuitry may have a branching network topology or recirculating ring based topology.

Abstract: A system-on-chip integrated circuitry includes interconnect circuitry for connecting transaction sources with transaction destinations. A buffer circuit buffers a plurality of access transactions received from the transaction sources before they are passed on to respective transaction destinations. Hazard checking circuitry, such as identifier reuse circuitry, performs hazard checks for access transactions in parallel with snoop operations performed by snoop circuitry for managing coherence between data values stored within the plurality of cache memories. The snoop circuitry includes snoop reordering circuitry for permitting reordering of snoop responses. The snoop circuitry may issue a snoop request for a given access transaction in parallel with the hazard checking circuitry performing one or more hazard checks for that transaction.

Abstract: A system-on-check integrated circuit 2 includes interconnect circuitry 4 connecting a plurality of transaction sources to a plurality of transaction destinations. The interconnect circuitry 4 includes a reorder buffer for buffering access transactions and hazard checking circuitry 46, 48, 50, 52 for performing hazard checks, such as point-of-serialisation checks and identifier reuse checks. Check suppression circuitry 62, 64, 66, 68 serves to suppress one or more hazard checks depending upon one or more state variables that themselves depend upon access transactions other than the access transaction for which the hazard checking is or is not to be suppressed. As an example, hazard checking may be suppressed if it is known that there are no other access transactions currently buffered within the reorder buffer 26 or alternatively no other access transactions from the same transaction source buffered within the reorder buffer 26.

Abstract: A system-on-chip integrated circuit 2 includes interconnect circuitry 4 for communicating transactions between transaction sources and transaction destinations. A reorder buffer 26 serves to buffer and permit reordering of access transactions received from the transaction sources. Processing circuitry performs processing operations in parallel upon a given access transaction taken from the reorder buffer. Hazard detection and repair circuitry serves to detect an ordering hazard arising between the processing operations and if necessary cancel and repeat that processing operation. The access transactions and the reorder buffer are such that access transactions other than the access transaction for which a hazard has been detected may proceed independently of the necessity to cancel and repair that transaction thereby reducing the cost associated with cancelling and repair.

Abstract: A data store has a data array for storing data values and a tag array for storing tag values for tracking which data values are stored in the data array. The associativity of the data array is greater than the associativity of the tag array. This means that fewer tag entries need to be accessed on each data access than in a conventional data store, reducing power consumption.

Abstract: A data processing apparatus is provided with a master device and a slave device which communicate via communication circuitry. The slave device is associated with a predetermined number of permission tokens that is equal to a maximum number of currently pending messages that can be accepted for processing from the communication circuitry by that slave device. The slave device transmits these permission tokens to the master device. The master device takes exclusive temporary possession of the permission tokens that it receives such that the permission tokens are then no longer available to any other master device. A master device initiates a message to a slave device when the master device has exclusive temporary possession of a permission token for that slave device. When the master device has initiated its message, then it relinquishes the exclusive temporary possession of the permission token such that it is then available for other devices.

Abstract: A data array has multiple ways, each way having entries for storing data values. In response to a write request, an updated data value having a target address may be stored in any of a corresponding set of entries comprising an entry selected from each way based on the target address. An update queue stores update information representing pending write requests. Update information is selected from the update queue for a group of pending write requests corresponding to different ways, and these write requests are performed in parallel so that updated values are written to entries of different ways.

Abstract: Arbitration circuitry for arbitrating between a plurality W of requests R for access to a shared resource. Included are state bits storage storing I state bits Q and generating 2I output bits comprising the true and compliment values of each stored state bit and routing circuitry for generating a set of mask signals M from the output bits. Grant circuitry receives the set of mask signals and the plurality of requests, and grants access to the shared resource to an asserted request having regard to the priority ordering encoded by the set of mask signals. State bit update circuitry is responsive to a trigger condition to perform an update causing a change in the priority ordering encoded by the set of mask signals. The routing circuitry provides a pattern of connections such that each mask signal in the set is directly connected to one of said output bits.

Abstract: A data processing system 3 employing a coherent memory system comprises multiple main cache memories 8. An inclusive snoop directory memory 14 stores directory lines 22. Each directory line includes a directory tag and multiple snoop vectors. Each snoop vector relates to a span of memory addresses corresponding to the cache line size within the main cache memories 8.

Abstract: Buffer circuitry 14 is provided with shared buffer circuitry 20 which stores, in order of reception time, data transaction requests received from one or more data transaction sources. The buffer circuitry 14 operates in either a bypass mode or a non-bypass mode. When operating in the bypass mode, any low latency data transaction requests stored within the shared buffer circuitry are selected in order for output in preference to data transaction requests that are not low latency data transaction requests. In the non-bypass mode, transactions (whether or not they are low latency transactions) are output from the shared buffer circuitry 20 in accordance with the order in which they are received into the shared buffer circuitry 20. The switch between the bypass mode and the non-bypass mode is made in dependence upon comparison of a detected rate of output of low latency data transaction requests compared to a threshold value.

Abstract: An interconnect comprising paths configured to transmit data packets between nodes on a network. The nodes comprise ports for inputting and outputting the data packets to the interconnect. At least two of the paths each have at least a portion configured such that a data packet addressed for output at one of the nodes on one of the paths and not being accepted at the node will continue along the path and on travelling further will return to the node. The at least two paths are balanced paths such that a data packet not accepted at the one of the nodes will return to the node a same predetermined number of clock cycles later whichever of the balanced paths the data packet is traveling along.

Abstract: An arbiter is configured to select one of several contending data packets transmitted from an initiator, the data packets comprising an identifier identifying the initiator and data. The arbiter comprises: a history buffer for storing the identifiers identifying the initiators of a plurality of recently selected data packets; and selection circuitry configured to select one of the contending data packets in dependence upon the initiators of the contending data packets and the initiators identified in the history buffer, such that a probability of a data packet being selected increases with the number of data packets selected since a data packet from the same initiator was selected.

Abstract: Arbitration circuitry 16 is provided to select an output from between multiple inputs each having an associated priority value. A tie-break value is appended to the least significant bits of each of the priority values to form extended priority values before those extended priority values are compared. Thus, if two priority values are equal, then the appended tie-break bits are used to determine which of the two inputs will be selected as having the higher priority.

Abstract: A method of modifying timings of data traffic in a test system by introducing dependencies that would arise in response to data requiring access to a resource. The resource receives the data traffic from at least one initiator and is connected via an interconnect to at least one recipient, the resource comprises a buffer for storing pending data related to an access to the resource that cannot currently complete.

Abstract: An interconnect block for a data processing apparatus, said interconnect block being operable to provide data routes via which one or more initiator devices may access one or more recipient devices, said interconnect block comprising: a first and a second portion; said first portion comprising at least one initiator port for communicating with one of said initiator devices, and at least one recipient port for communicating with one of said recipient devices; said second portion comprising at least two recipient ports for communicating with at least two recipient devices, said second portion being connected to said first portion via at least two parallel connecting routes, said at least two recipient ports being connectable to said at least two parallel connecting routes; wherein in response to a request received from one of said initiator devices at said first portion to perform a transaction accessing one of said at least two recipients in communication with said second portion, said interconnect block is op