Abstract:
A hardware/software system is provided for allocating memory in the form of a buffer zone surrounding a data packet to be stored in the memory. The hardware/software system comprises, first and second registers for storing separate values representing in one register, an amount of memory preceding the first line of the data packet to be stored and in the other the amount succeeding the last line of the packet to be stored, a hardware mechanism for allocating the memory according to computational results computed using the register values and the size of a data packet to be stored, and software for processing stored data packet and for writing any new growth data into the designated buffer zones surrounding the data packet.

Description:
CROSS-REFERENCE TO RELATED DOCUMENTS 
     This application is a continuation of U.S. application Ser. No. 09/933,934, filed Aug. 20, 2001, now U.S. Pat. No. 7,065,096, which is a continuation in part (CIP) of U.S. application Ser. No. 09/737,375, filed Dec. 14, 2000, now U.S. Pat. No. 7,058,064, each of which are incorporated herein in their entirety by reference. In addition, U.S. application Ser. No. 09/737,375 claims priority benefit under 35 U.S.C. 119(e) of Provisional Application Ser. No. 60/181,364, filed on Feb. 8, 2000, which is also incorporated in its entirety by reference. U.S. application Ser. No. 09/933,934 is also a CIP of U.S. application Ser. No. 09/602,279, filed on Jun. 23, 2000, and is incorporated herein in its entirety by reference. The inventors have also participated in the document disclosure program and claim priority to document disclosure number 492429. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the field of digital processing and pertains to apparatus and methods for processing packets in routers for packet networks, and more particularly to apparatus and methods for allocating enough memory to packet storage for enabling packet growth during processing in dynamic multistreaming processors. 
     BACKGROUND OF THE INVENTION 
     The well-known Internet network is a notoriously well-known publicly-accessible communication network at the time of filing the present patent application, and arguably the most robust information and communication source ever made available. The Internet is used as a prime example in the present application of a data-packet-network which will benefit from the apparatus and methods taught in the present patent application, but is just one such network, following a particular standardized protocol. As is also very well known, the Internet (and related networks) are always a work in progress. That is, many researchers and developers are competing at all times to provide new and better apparatus and methods, including software, for enhancing the operation of such networks. 
     In general the most sought-after improvements in data packet networks are those that provide higher speed in routing (more packets per unit time) and better reliability and fidelity in messaging. What is generally needed are router apparatus and methods increasing the rates at which packets may be processed in a router. 
     As is welt-known in the art, packet routers are computerized machines wherein data packets are received at any one or more of typically multiple ports, processed in some fashion, and sent out at the same or other ports of the router to continue on to downstream destinations. As an example of such computerized operations, keeping in mind that the Internet is a vast interconnected network of individual routers, individual routers have to keep track of which external routers to which they are connected by communication ports, and of which of alternate routes through the network are the best routes for incoming packets. Individual routers must also accomplish flow accounting, with a flow generally meaning a stream of packets with a common source and end destination. A general desire is that individual flows follow a common path. The skilled artisan will be aware of many such requirements for computerized processing. 
     Typically a router in the Internet network will have one or more Central Processing Units (CPUs) as dedicated microprocessors for accomplishing the many computing tasks required. In the current art at the time of the present application, these are single-streaming processors; that is, each processor is capable of processing a single stream of instructions. In some cases developers are applying multiprocessor technology to such routing operations. The present inventors have been involved for some time in development of dynamic multistreaming (DMS) processors, which processors are capable of simultaneously processing multiple instruction streams. One preferred application for such processors is in the processing of packets in packet networks like the Internet. 
     In a data-packet processor, a configurable queuing system for packet accounting during processing is known to the inventor and disclosure for same is referenced herein as Ser. No. 09/737,375 in the Cross-Reference section of this specification. The queuing and accounting system has a plurality of queues arranged in one or more clusters, an identification mechanism for creating a packet identifier for arriving packets, insertion logic for inserting packet identifiers into queues and for determining into which queue to insert a packet identifier, and selection logic for selecting packet identifiers from queues to initiate processing of identified packets, downloading of completed packets, or for re-queuing of the selected packet identifiers. 
     A portion of the memory in the above-described system is called packet memory. The packet memory is the memory where data packets reside before they can be downloaded by a packet management unit (PMU) to an output network interface (ONI) during packet processing and routing. A portion of the packet memory described above is called the local packet memory (LPM), and it is directly managed by hardware in the PMU instead of by software. 
     Whenever a data packet has been processed and is ready to be downloaded from LPM, the processing core or streaming processor unit (SPU) sends a command PKTDONE to the PMU. This command contains, among other information, a packet identifier (typically a number) of the packet that is ready to be downloaded. The PMU will then proceed with the download of this packet if it resides in LPM. If not, software operating through a system interface unit (SIUJ) will download the packet upon request. 
     Data packet processing occurs using context registers. In a dynamic multistreaming processor running eight streams, there are at least eight context registers for processing data packets. The context registers are located physically within the streaming processor core having associated therewith functional units required for processing. In this example, each context can have a state of either PMU-owned or SPU-owned. When information is being preloaded into a context the context is PMU-owned. When a stream is running within a context, the context is SPU-owned. 
     During packet processing wherein the SPU is executing instructions to process a data packet, it is possible that the data packet in process will be caused to grow (become larger) in terms of header data and/or tail data. Packet growth during SPU processing may cause the process to overwrite consecutively stored data packets information in memory. The problem presented by this possible growth phenomena is that in order to avoid a possible overwrite and data corruption issue, a packet that has outgrown its memory space must be moved during processing to a new memory space that is large enough (larger than before) to hold it in its expanded form. Repeated instances of this requirement over time tend to add more latency in packet processing. Therefore, what is clearly needed is a method for allocating enough memory space for a new packet to account for a possible growth in packet size during processing. A method such as this would sharply reduce packet move operations, and avoid overwrites in memory. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention a hardware/software system for storing and downloading a data packet is provided, comprising a first configuration register for storing an offset value representing an amount of memory preceding the first line of the data packet to be stored, a second configuration register for storing an offset value representing an amount of memory following the end of the last line of the data packet to be stored, and a mechanism for allocating a portion of memory for storage of data packet. The system is characterized in that the offsets indicated by the first and second configuration registers are added to size of the data packet to be stored, resulting in a new size for memory allocation. 
     In a preferred embodiment the system is implemented within a data packet router connected to a data packet network, and the network may be the Internet network. In some embodiments the hardware accesses the first and second configuration registers, computes the required size of memory allocation, and allocates the memory for packet storage. In some other embodiments the software accesses the first and second registers, computes the required size of memory allocation, and allocates the memory for packet storage. In still other embodiments the hardware accesses the first and second registers and passes the values to the software to complete the computation and allocation of the memory to store the data packet. 
     In some embodiments of the invention there are further one or more additional sets of registers for storing memory values applicable and specific to data packet types, each set associated with the type of data packet other than a type associated with the first and second registers. In some other embodiments the first and second registers are configurable during a boot operation of the system. There may also be one or more additional sets of registers that are configurable during a boot operation of the system. In still other embodiments change values for the first and second registers may be entered into the registers during operation of the system. In the cases where there are additional configuration registers, the change values for the one or more additional sets of registers may be entered into the registers during operation of the system. 
     In some embodiments any changes in packet size during processing are tracked, and the first configuration register is consulted before download of the packet, to determine the actual starting point of the processed packet in memory. 
     In another aspect of the system method for storing and downloading a data packet is provided, comprising the steps of (a) providing an offset value in a first configuration register representing an amount of memory preceding the first line of the data packet to be stored; (b) providing an offset value in a second configuration register representing an amount of memory following the last line of the data packet to be stored (c) upon arrival of a data packet to be stored, accessing the first and second configuration registers for the offset values; (d) summing the acquired values with the original size of the data packet to be stored; and (e) allocating memory for the packet according to the sum in step (d). 
     In some preferred embodiments the system comprises a data packet router connected to a data packet network, and the network may be the well-known Internet network. 
     In some embodiments, in steps (a) and (b) the registers are accessible by hardware responsible for storing the data packet. In other embodiments in steps (a) and (b) the values are configured into each register during a boot operation of a host system. 
     In some embodiments in step (c) access is performed by hardware, and in some other in step (c) access is performed by software. There may further be an additional step between steps (c) and (d) for passing the values to software. In some embodiments steps (c) through (e) are performed entirely by hardware, or step (c) is performed by hardware and steps (d) and (e) are performed by software. 
     In some embodiments in steps (a) and (b) the registers are configured during boot operation, and the memory is local packet memory that is hardware controlled. Further, in other embodiments, in steps (a) and (b) there are a plurality of register sets, each set dedicated to a different type of data packet. 
     In yet other embodiments there are further steps for tracking growth and shrink of a data packet during processing, and for consulting the first configuration register after processing and before download to determine the actual starting point of the processed packet in memory. 
     The embodiments of the invention described in enabling detail below, for the first time provide an ability in a packet processor to allocate memory in a manner allowing for packet growth and shrinkage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram showing relationship of functional areas of a DMS processor in a preferred embodiment of the present invention. 
         FIG. 2  is a block diagram of the DMS processor of  FIG. 1  showing additional detail. 
         FIG. 3  is a block diagram illustrating uploading of data into the LPM or EPM in an embodiment of the invention. 
         FIG. 4   a  is a diagram illustrating determination and allocation for data uploading in an embodiment of the invention. 
         FIG. 4   b  is a diagram showing the state that needs to be maintained for each of the four 64 KB blocks. 
         FIGS. 5   a  and  5   b  illustrate an example of how atomic pages are allocated in an embodiment of the present invention. 
         FIGS. 6   a  and  6   b  illustrate how memory space is efficiently utilized in an embodiment of the invention. 
         FIG. 7  is a top-level schematic of the blocks of the XCaliber PMU unit involved in the downloading of a packet. 
         FIG. 8  is a diagram illustrating the phenomenon of packet growth and shrink. 
         FIG. 9  is a block diagram showing high-level communication between the QS and other blocks in the PMU and SPU in an embodiment of the present invention. 
         FIG. 10  is a table illustrating six different modes in an embodiment of the invention into which the QS can be configured. 
         FIG. 11  is a diagram illustrating generic architecture of the QS of  FIGS. 2 and 7  in an embodiment of the present invention. 
         FIG. 12  is a table indicating coding of the outbound DeviceId field in an embodiment of the invention. 
         FIG. 13  is a table illustrating priority mapping for RTU transfers in an embodiment of the invention. 
         FIG. 14  is a table showing allowed combinations of Active, Completed, and Probed bits for a valid packet in an embodiment of the invention. 
         FIG. 15  is a Pattern Matching Table in an embodiment of the present invention. 
         FIG. 16  illustrates the format of a mask in an embodiment of the invention. 
         FIG. 17  shows an example of a pre-load operation using the mask in  FIG. 16 . 
         FIG. 18  illustrates shows the PMU Configuration Space in an embodiment of the present invention. 
         FIGS. 19   a ,  19   b  and  19   c  are a table of Configuration register Mapping. 
         FIG. 20  is an illustration of a PreloadMaskNumber configuration register. 
         FIG. 21  illustrates a PatternMatchingTable in a preferred embodiment of the present invention. 
         FIG. 22  illustrates a VirtualPageEnable configuration register in an embodiment of the invention. 
         FIG. 23  illustrates a ContextSpecificPatternMatchingMask configuration register in an embodiment of the invention. 
         FIG. 24  illustrates the MaxActivePackets configuration register in an embodiment of the present invention. 
         FIG. 25  illustrates the TimeCounter configuration register in an embodiment of the present invention. 
         FIG. 26  illustrates the StatusRegister configuration register in an embodiment of the invention. 
         FIG. 27  is a schematic of a Command Unit and command queues in an embodiment of the present invention. 
         FIG. 28  is a table showing the format of command inserted in command queues in an embodiment of the present invention. 
         FIG. 29  is a table showing the format for responses that different blocks generate back to the CU in an embodiment of the invention. 
         FIG. 30  shows a performance counter interface between the PMU and the SIU in an embodiment of the invention. 
         FIG. 31  shows a possible implementation of internal interfaces among the different units in the PMU in an embodiment of the present invention. 
         FIG. 32  is a diagram of a BypassHooks configuration register in an embodiment of the invention. 
         FIG. 33  is a diagram of an InternalStateWrite configuration register in an embodiment of the invention. 
         FIGS. 34–39  comprise a table listing events related to performance counters in an embodiment of the invention. 
         FIG. 40  is a table illustrating the different bypass hooks implemented in the PMU in an embodiment of the invention. 
         FIG. 41  is a table relating architecture and hardware blocks in an embodiment of the present invention. 
         FIGS. 42–45  comprise a table showing SPU-PMU Interface in an embodiment of the invention. 
         FIGS. 46–49  comprise a table showing SIU-PMU Interface in an embodiment of the invention. 
         FIG. 50  is a block diagram illustrating a packet overwrite problem due to head and tail growth of the packet during processing. 
         FIG. 51  is a block diagram illustrating insulative packet queuing according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the provisional patent application Ser. No. 60/181,364 referenced above there is disclosure as to the architecture of a DMS processor, termed by the inventors the XCaliber processor, which is dedicated to packet processing in packet networks. Two extensive diagrams are provided in the referenced disclosure, one, labeled NIO Block Diagram, shows the overall architecture of the XCaliber processor, with input and output ports to and from a packet-handling ASIC, and the other illustrates numerous aspects of the Generic Queue shown in the NIO diagram. The NIO system in the priority document equates to the Packet Management Unit (PMU) in the present specification. It is to the several aspects of the generic queue that the present application is directed. 
       FIG. 1  is a simplified block diagram of an XCaliber DMS processor  101  with a higher-level subdivision of functional units than that shown in the NIO diagram of the priority document. In  FIG. 1  XCaliber DMS processor  101  is shown as organized into three functional areas. An outside System Interface Unit (SIU) area  107  provides communication with outside devices, that is, external to the XCaliber processor, typically for receiving and sending packets. Inside, processor  101  is divided into two broad functional units, a Packet Management Unit (PMU)  103 , equating to the NIO system in the priority document mentioned above, and a Stream Processor Unit (SPU)  107 . The functions of the PMU include accounting for and managing all packets received and processed. The SPU is responsible for all computational tasks. 
     The PMU is a part of the XCaliber processor that offloads the SPU from performing costly packet header accesses and packet sorting and management tasks, which would otherwise seriously degrade performance of the overall processor. 
     Packet management is achieved by (a) Managing on-chip memory allocated for packet storage, (b) Uploading, in the background, packet header information from incoming packets into different contexts (context registers, described further below) of the XCaliber processor, (c) Maintaining, in a flexible queuing system, packet identifiers of the packets currently in process in the XCaliber. 
     The described packet management and accounting tasks performed by the PMU are performed in parallel with processing of packets by the SPU core. To implement this functionality, the PMU has a set of hardware structures to buffer packets incoming from the network, provide them to the SPU core and, if needed, send them out to the network when the processing is completed. The PMU features a high degree of programmability of several of its functions, such as configuration of its internal packet memory storage and a queuing system, which is a focus of the present patent application. 
       FIG. 2  is a block diagram of the XCaliber processor of  FIG. 1  showing additional detail. SIU  107  and SPU  105  are shown in  FIG. 2  as single blocks with the same element numbers used in  FIG. 1 . The PMU is shown in considerably expanded detail, however, with communication lines shown between elements. 
     In  FIG. 2  there is shown a Network/Switching Fabric Interface  203  which is in some cases an Application Specific Integrated Circuit (ASIC) dedicated for interfacing directly to a network, such as the Internet for example, or to switching fabric in a packet router, for example, receiving and transmitting packets, and transacting the packets with the XCaliber processor. In this particular instance there are two in ports and two out ports communicating with processor  201 . Network in and out interface circuitry  205  and  215  handle packet traffic onto and off the processor, and these two interfaces are properly a part of SIU  107 , although they are shown separately in  FIG. 2  for convenience. 
     Also at the network interface within the PMU there are, in processor  201 , input and output buffers  207  and  217  which serve to buffer the flow of packets into and out of processor  201 . 
     Referring again to  FIG. 1 , there is shown a Packet Management Unit (PMU)  103 , which has been described as a unit that offloads the requirement for packet management and accounting from the Stream Processing Unit. This is in particular the unit that has been expanded in  FIG. 2 , and consists substantially of Input Buffer (IB)  207 , Output Buffer (OB)  217 , Paging Memory Management Unit (PMNU)  209 , Local Packet Memory (LPM)  219 , Command Unit (CU)  213 , Queueing System (QS)  211 , Configuration Registers  221 , and Register Transfer Unit (RTU)  227 . The communication paths between elements of the PMU are indicated by arrows in  FIG. 2 , and further description of the elements of the PMU is provided below, including especially QS  211 , which is a particular focus of the present patent application. 
     Overview of PMU 
     Again,  FIG. 2  shows the elements of the PMU, which are identified briefly above. Packets arrive to the PMU in the present example through a 16-byte network input interface. In this embodiment packet data arrives to the PMU at a rate of 20 Gbps (max). At an operating speed of 300MHz XCaliber core frequency, an average of 8 bytes of packet data are received every XCaliber core cycle. The incoming data from the network input interface is buffered in InBuffer (IB) block  207 . Network interface  205  within XCaliber has the capability of appending to the packet itself the size of the packet being sent, in the event that the external device has not been able to append the size to the packet before sending the packet. Up to 2 devices can send packet data to XCaliber at (10 Gbps per device), and two in ports are shown from an attached ASIC. It is to be understood that the existence and use of the particular ASIC is exemplary, and packets could be received from other devices. Further, there may be in some embodiments more or fewer than the two in ports indicated. 
     Packet Memory Manager Unit (PMMU)  209  decides whether each incoming packet has to be stored into on-chip Local Packet Memory (LPM)  219 , or, in the case that, for example, no space exists in the LPM to store it, may decide to either send the packet out to an External Packet Memory (EPM) not shown through the SIU block, or may decide to drop the packet. In case the packet is to be stored in the LPM, the PMMU decides where to store the packet and generates all the addresses needed to do so. The addresses generated correspond in a preferred embodiment to 16-byte lines in the LPM, and the packet is consecutively stored in this memory. 
     In the (most likely) case that the PMMU does not drop the incoming packet, a packet identifier is created, which includes a pointer (named packetPage) to a fixed-size page in packet memory where the packet has started to be stored. The identifier is created and enqueued into Queuing System (QS) block  211 . The QS assigns a number from 0 to 255 (named packetNumber) to each new packet. The QS sorts the identifiers of the packets alive in XCaliber based on the priority of the packets, and it updates the sorting when the SPU core notifies any change on the status of a packet. The QS selects which packet identifiers will be provided next to the SPU. Again, the QS is a particular focus of the present application. 
     Register Transfer Unit (RTU) block  227 , upon receiving a packet identifier (packetPage and packetNumber) from the QS, searches for an available context ( 229 ,  FIG. 2 ) out of 8 contexts that XCaliber features in a preferred embodiment. For architectural and description purposes the contexts are considered a part of a broader Stream Processing Unit, although the contexts are shown in  FIG. 2  as a separate unit  229 . 
     In the case that no context is available, the RTU has the ability to notify the SPU about this event through a set of interrupts. In the case that a context is available, the RTU loads the packet identifier information and some selected fields of the header of the packet into the context, and afterwards it releases the context (which will at that time come under control of the SPU. The RTU accesses the header information of the packet through the SIU, since the packet could have been stored in the off-chip EPM. 
     Eventually a stream in the SPU core processes the context and notifies the QS of this fact. There are, in a preferred embodiment, eight streams in the DMS core. The QS then updates the status of the packet (to completed), and eventually this packet is selected for downloading (i.e. the packet data of the corresponding packet is sent out of the XCaliber processor to one of the two external devices). 
     When a packet is selected for downloading, the QS sends the packetPage (among other information) to the PMMU block, which generates the corresponding line addresses to read the packet data from the LPM (in case the packet was stored in the on-chip local memory) or it will instruct the SIU to bring the packet from the external packet memory to the PMU. In any case, the lines of packet data read are buffered into the OutBuffer (OB) block, and from there sent out to the device through the 16-byte network output interface. This interface is independent of its input counterpart. The maximum aggregated bandwidth of this interface in a preferred embodiment is also 20 Gbps, 10 Gbps per output device. 
     CommandUnit (CU)  213  receives commands sent by SPU  105 . A command corresponds to a packet instruction, which are in many cases newly defined instructions, dispatched by the SPU core. These commands are divided into three independent types, and the PMU can execute one command per type per cycle (for a total of up to 3 commands per cycle). Commands can be load-like or store-like (depending on whether the PMU provides a response back to the SPU or not respectively). 
     A large number of features of the PMU are configured by the SPU through memory-mapped configuration registers  221 . Some such features have to be programmed at boot time, and the rest can be dynamically changed. For some of the latter, the SPU has to be running in a single-thread mode to properly program the functionality of the feature. The CU block manages the update of these configuration registers. 
     The PMU provides a mechanism to aid in flow control between ASIC  203  and XCaliber DMS processor  201 . Two different interrupts are generated by the PMU to SPU  105  when LPM  219  or QS  211  are becoming full. Software controls how much in advance the interrupt is generated before the corresponding structure becomes completely full. Software can also disable the generation of these interrupts. 
     LPM  219  is also memory mapped, and SPU  105  can access it through the conventional load/store mechanism. Both configuration registers  221  and LPM  219  have a starting address (base address) kept by SrU  107 . Requests from SPU  105  to LPM  219  and the configuration space arrive to the PMU through SIU block  107 . The SIU is also aware of the base address of the external packet memory. 
     In Buffer (IB) 
     Packet data sent by an external device arrives to the PMU through the network input interface  205  at an average rate of 8 bytes every XCaliber core cycle in a preferred embodiment. IB block  207  of the PMU receives this data, buffers it, and provides it, in a FIFO-like fashion, to LPM  219  and in some cases also to the SIU (in case of a packet overflow, as explained elsewhere in this specification. 
     XCaliber DMS processor  201  can potentially send/receive packet data to/from up to 2 independent devices. Each device is tagged in SIU  107  with a device identifier, which is provided along with the packet data. When one device starts sending data from a packet, it will continue to send data from that very same packet until the end of the packet is reached or a bus error is detected by the SIU. 
     In a preferred embodiment the first byte of a packet always starts at byte  0  of the first 16 bytes sent of that packet. The first two bytes of the packet specify the size in bytes of the packet (including these first two bytes). These two bytes are always appended by the SIU if the external device has not appended them. If byte k in the 16-byte chunk is a valid byte, bytes  0  . . . k−1 are also valid bytes. This can be guaranteed since the first byte of a packet always starts at byte  0 . Note that no valid bits are needed to validate each byte since a packet always starts at byte  0  of the 16-byte chunk, and the size of the packet is known up front (in the first two bytes). 
     The network interface provides, at every core clock, a control bit specifying whether the 16-byte chunk contains, at least, one valid byte. The valid data received from the network input interface is organized in buffer  207 . This is an 8-entry buffer, each entry holding the 16-bytes of data plus the control bits associated to each chunk. PMMU  209  looks at the control bits in each entry and determines whether a new packet starts or to which of the (up to) two active packets the data belongs to, and it acts accordingly. 
     The 16-byte chunks in each of the entries in IB  207  are stored in LPM  219  or in the EPM (not shown). It is guaranteed by either the LPM controller or the SIU that the bandwidth to write into the packet memory will at least match the bandwidth of the incoming packet data, and that the writing of the incoming packet data into the packet memory will have higher priority over other accesses to the packet memory. 
     In some cases IB  207  may get full because PMMU  209  may be stalled, and therefore the LPM will not consume any more data of the IB until the stall is resolved. Whenever the IB gets full, a signal is sent to network input interface  205 , which will retransmit the next 16-byte chunk as many times as needed until the IB accepts it. Thus, no packet data is lost due to the IB getting full. 
     Out Buffer (OB) 
     Network output interface  215  also supports a total aggregated bandwith of 20 Gbps (10 Gbps per output device), as does the Input Interface. At 300 MHz XCaliber clock frequency, the network output interface accepts in average 8 bytes of data every XCaliber cycle from the OB block, and sends it to one of the two output devices. The network input and output interfaces are completely independent of each other. 
     Up to 2 packets (one per output device) can be simultaneously sent. The device to which the packet is sent does not need to correspond to the device that sent the packet in. The packet data to be sent out will come from either LPM  219  or the EPM (not shown). 
     For each of the two output devices connected at Network Out interface  215 , PMMU  209  can have a packet ready to start being downloaded, a packet being downloaded, or no packet to download. Every cycle PMMU  209  selects the highest packet across both output devices and initiates the download of 16 bytes of data for that packet. Whenever the PMMU is downloading packet data from a packet to an output device, no data from a different packet will be downloaded to the same device until the current packet is completely downloaded. 
     The 16-byte chunks of packet data read from LPM  219  (along with some associated control information) are fed into one of the two 8-entry buffers (one per device identifier). The contents of the head of one of these buffers is provided to the network output interface whenever this interface requests it. When the head of both buffers is valid, the OB provides the data in a round robin fashion. 
     Differently than the network input interface, in the 16-byte chunk sent to the network output interface it can not be guaranteed that if a byte k is valid, then bytes  0  . . . k−1 are valid as well. The reason for this is that when the packet is being sent out, it does not need to start at byte  0  of the 16-byte chunk in memory. Thus, for each 16-byte chunk of data that contains the start of the packet to be sent out, OB  217  needs to notify the network interface where the first valid byte of the chunk resides. Moreover, since the first two bytes of the packet contain the size of the packet in bytes, the network output interface has the information to figure out where the last valid byte of the packet resides within the last 16-byte chunk of data for that packet. Moreover, OB  217  also provides a control bit that informs SIU  107  whether it needs to compute CRC for the packet, and if so, which type of CRC. This control bit is provided by PMMU  209  to OB  217 . 
     Paging Memory Management Unit (PMMU) 
     The packet memory address space is 16 MB. Out of the 16 MB, the XCaliber processor features 256 KB on-chip. The rest (or a fraction) is implemented using external storage. 
     The packet memory address space can be mapped in the TLB of SPU  105  as user or kernel space, and as cachable or uncachable. In case it is mapped cachable, the packet memory space is cached (write-through) into an L 1  data cache of SPU  105 , but not into an L 2  cache. 
     A goal of PMMU  209  is to store incoming packets (and SPU-generated packets as well) into the packet memory. In case a packet from the network input interface fits into LPM  219 , PMMU  209  decides where to store it and generates the necessary write accesses to LPM  219 ; in case the packet from the network input interface is going to be stored in the EPM, SPU  105  decides where in the EPM the packet needs to be stored and SIU  107  is in charge of storing the packet. In either case, the packet is consecutively stored and a packet identifier is created by PMMU  209  and sent to QS  211 . 
     SPU  105  can configure LPM  219  so packets larger than a given size will never be stored in the LPM. Such packets, as well as packets that do not fit into the LPM because lack of space, are sent by PMMU  209  to the EPM through SIU  107 . This is a mechanism called overflow and is configured by the SPU for the PMU to do so. If no overflow of packets is allowed, then the packet is dropped. In this case, PMMU  209  interrupts the SPU (again, if configured to do so). 
     Uploading a Packet into Packet Memory 
     Whenever there is valid data at the head of IB  205 , the corresponding device identifier bit is used to determine to which packet (out of the two possible packets being received) the data belongs. When the network input interface starts sending data of a new packet with device identifier d, all the rest of the data will eventually arrive with that same device identifier d unless an error is notified by the network interface block. The network input interface can interleave data from two different device identifiers, but in a given cycle only data from one device is received by IB  207 . 
     When a packet needs to be stored into LPM  219 , PMMU block  209  generates all the write addresses and write strobes to LPM  219 . If the packet needs to be stored into the EPM, SIU  107  generates them. 
       FIG. 3  is a diagram illustrating uploading of data into either LPM  219  or the EPM, which is shown in  FIG. 3  as element  305 , but not shown in  FIG. 2 . The write strobe to the LPM or EPM will not be generated unless the header of the IB has valid data. Whenever the write strobe is generated, the 16-byte chunk of data at the head of the IB (which corresponds to a LPM line) is deleted from the IB and stored in the LPM or EPM. The device identifier bit of the head of the IB is used to select the correct write address out of the 2 address generators (one per input device). 
     In the current embodiment only one incoming packet can be simultaneously stored in the EPM by the SIU (i.e. only one overflow packet can be handled by the SIU at a time). Therefore, if a second packet that needs to be overflowed is sent by the network input interface, the data of this packet will be thrown away (i.e. the packet will be dropped). 
     A Two Byte Packet-Size Header 
     The network input interface always appends two bytes to a packet received from the external device (unless this external device already does so, in which case the SIU will be programmed not to append them). This appended data indicates the size in bytes of the total packet, including the two appended bytes. Thus, the maximum size of a packet that is processed by the XCaliber DMS processor is 65535 bytes including the first two bytes. 
     The network output interface expects that, when the packet is returned by the PMU (if not dropped during its processing), the first two bytes also indicate the size of the processed packet. The size of the original packet can change (the packet can increase or shrink) as a result of processing performed by the XCaliber processor. Thus, if the processing results in increasing the size beyond 64K−1 bytes, it is the responsibility of software to chop the packet into two different smaller packets. 
     The PMU is more efficient when the priority of the packet being received is known up front. The third byte of the packet will be used for priority purpose if the external device is capable of providing this information to the PMU. The software programs the PMU to either use the information in this byte or not, which is does through a boot-time configuration register named Log2InQueues. 
     Dropping a Packet 
     A packet completely stored in either LPM  219  or EPM  305  will be dropped only if SPU  105  sends an explicit command to the PMU to do so. No automatic dropping of packets already stored in the packet memory can occur. In other words, any dropping algorithm of packets received by the XCaliber DMS processor is implemented in software. 
     There are, however, several situations wherein the PMU may drop an incoming packet. These are (a) The packet does not fit in the LPM and the overflow of packets is disabled, (b) The total amount of bytes received for the packet is not the same as the number of bytes specified by the ASIC in the first two bytes of the ASIC-specific header, or (c) A transmission error has occurred between the external device and the network input interface block of the SIU. The PMMU block is notified about such an error. 
     For each of the cases (a), (b) and (c) above, an interrupt is generated to the SPU. The software can disable the generation of these interrupts using AutomaticPacketDropIntEnable, PacketErrorIntEnable on-the-fly configuration flags. 
     Virtual Pages 
     An important process of PMMU  209  is to provide an efficient way to consecutively store packets into LPM  219  with as little memory fragmentation as possible. The architecture in the preferred embodiment provides SPU  105  with a capability of grouping, as much as possible, packets of similar size in the same region of LPM  219 . This reduces overall memory fragmentation. 
     To implement the low-fragmentation feature, LPM  219  is logically divided into 4 blocks of 64 KB bytes each. Each block is divided into fixed atomic pages of 256 bytes. However, every block has virtual pages that range from 256 bytes up to 64 KB, in power-of-2 increments. Software can enable/disable the different sizes of the virtual pages for each of the 4 blocks using an on-the-fly configuration register named VirtualPageEnable. This allows configuring some blocks to store packets of up to a certain size. 
     The organization and features of the PMU assure that a packet of size s will never be stored in a block with a maximum virtual page size less than s. However, a block with a minimum virtual page size of r will accept packets of size smaller than r. This will usually be the case, for example, in which another block or blocks are configured to store these smaller packets, but is full. 
     Software can get ownership of any of the four blocks of the LPM, which implies that the corresponding 64 KB of memory will become software managed. A configuration flag exists per block (SoftwareOwned) for this purpose. The PMMU block will not store any incoming packet from the network input interface into a block in the LPM with the associated SoftwareOwned flag asserted. Similarly, the PMMU will not satisfy a GetSpace operation (described elsewhere) with memory of a block with its SoftwareOwned flag asserted. The PMMU, however, is able to download any packet stored by software in a software-owned block. 
     The PMMU logic determines whether an incoming packet fits in any of the blocks of the LPM. If a packet fits, the PMMU decides in which of the four blocks (since the packet may fit in more than one block), and the first and last atomic page that the packet will use in the selected block. The atomic pages are allocated for the incoming packet. When packet data stored in an atomic page has been safely sent out of the XCaliber processor through the network output interface, the corresponding space in the LPM can be de-allocated (i.e. made available for other incoming packets). 
     The EPM, like the LPM is also logically divided into atomic pages of 256 bytes. However, the PMMU does not maintain the allocation status of these pages. The allocation status of these pages is managed by software. Regardless of where the packet is stored, the PMMU generates an offset (in atomic pages) within the packet memory to where the first data of the packet is stored. This offset is named henceforth packetPage. Since the maximum size of the packet memory is 16 MB, the packetPage is a 16-bit value. 
     As soon as the PMMU safely stores the packet in the LPM, or receives acknowledgement from SIU  107  that the last byte of the packet has been safely stored in the EPM, the packetPage created for that packet is sent to the QS. Operations of the QS are described in enabling detail below. 
     Generating the packetPage Offset 
     The PMMU always monitors the device identifier (deviceId) associated to the packet data at the head of the IB. If the deviceId is not currently active (i.e. the previous packet sent by that deviceId has been completely received), that indicates that the head of the IB contains the first data of a new packet. In this case, the first two bytes (byte 0  and byte 1  in the 16-byte chunk) specify the size of the packet in bytes. With the information of the size of the new incoming packet, the PMMU determines whether the packet fits into LPM  219  and, if it does, in which of the four blocks it will be stored, plus the starting and ending atomic pages within that block. 
     The required throughput in the current embodiment of the PMMU to determine whether a packet fits in LPM  219  and, if so, which atomic pages are needed, is one packet every two cycles. One possible two-cycle implementation is as follows: (a) The determination happens in one cycle, and only one determination happens at a time (b) In the cycle following the determination, the atomic pages needed to store the packet are allocated and the new state (allocated/de-allocated) of the virtual pages are computed. In this cycle, no determination is allowed. 
       FIG. 4   a  is a diagram illustrating determination and allocation in parallel for local packet memory. The determination logic is performed in parallel for all of the four 64 KB blocks as shown. 
       FIG. 4   b  shows the state that needs to be maintained for each of the four 64 KB blocks. This state, named AllocationMatrix, is recomputed every time one or more atomic pages are allocated or de-allocated, and it is an input for the determination logic. The FitsVector and IndexVector contain information computed from the AllocationMatrix. 
     AllocationMatrix[VPSize][VPIndex] indicates whether virtual page number VPIndex of size VPSize in bytes is already allocated or not. FitsVector[VPSize] indicates whether the block has at least one non-allocated virtual page of size VPSize. If FitsVector[VPSize] is asserted, IndexVector[VPSize] vector contains the index of a non-allocated virtual page of size VPSize. 
     The SPU programs which virtual page sizes are enabled for each of the blocks. The EnableVector[VPSize] contains this information. This configuration is performed using the VirtualPageEnable on-the-fly configuration register. Note that the AllocationMatrix[ ][ ], FitsVector[ ], IndexVector[ ] and EnableVector[ ] are don&#39;t cares if the corresponding SoftwareOwned flag is asserted. 
     In this example the algorithm for the determination logic (for a packet of size s bytes) is as follows: 
     
       
         
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 1) 
                 Fits logic: check, for each of the blocks, whether the packet fits in or 
               
               
                   
                 not. If it fits, remember the virtual page size and the number of the 
               
               
                   
                 first virtual page of that size. 
               
             
          
           
               
                     For All Block j Do (can be done in parallel): 
               
               
                       Fits[j] = (s &lt;= VPSize) AND FitsVector[VPSize] 
               
               
                 AND 
               
               
                           Not SoftwareOwned 
               
               
                         where VPSize is the smallest possible page size. 
               
               
                       If (Fits[j]) 
               
               
                         VPIndex[j] = IndexVector[VPSize] 
               
               
                         MinVPS[j] = VPSize 
               
               
                       Else 
               
               
                         MinVPS[j] = &lt;Infinity&gt; 
               
             
          
           
               
                 2) 
                 Block selection: the blocks with the smallest virtual page (enabled or 
               
               
                   
                 not) that is able to fit the packet in are candidates. The block with the 
               
               
                   
                 smallest enabled virtual page is selected. 
               
             
          
           
               
                     If Fits[j] = FALSE for all j Then 
               
               
                       &lt;Packet does not fit in LPM&gt; 
               
               
                       packetPage = OverflowAddress &gt;&gt; 8 
               
               
                     Else 
               
               
                       C = set of blocks with smallest MinVPS AND 
               
               
                       Fits[MinVPS] 
               
               
                       B = block# in C with the smallest enabled virtual 
               
               
                       page 
               
               
                       (if more than one exists, pick the smallest block number) 
               
               
                       If one or more blocks in C have virtual pages enabled 
               
               
                       Then 
               
               
                         Index = VIPIndex[B] 
               
               
                         VPSize = MinVPS[B] 
               
               
                         NumAPs = ceil(S/256) 
               
               
                         packetPage = (B*64KB + Index*VPSize) &gt;&gt; 
               
               
                       Else 
               
               
                         &lt;Packet does not fit in LPM&gt; 
               
               
                         packetPage = OverflowAddress &gt;&gt; 8 
               
               
                   
               
             
          
         
       
     
     If the packet fits in the LPM, the packetPage created is then the atomic page number within the LPM (there are up to 1K different atomic pages in the LPM) into which the first data of the packet is stored. If the packet does not fit, then the packetPage is the contents of the configuration register OverflowAddress right-shifted 8 bits. The packet overflow mechanism is described elsewhere in this specification, with a subheader “Packet overflow”. 
     In the cycle following the determination of where the packet will be stored, the new values of the AllocationMatrix, FitsVector and IndexVector must be recomputed for the selected block. If FitsVector[VPSize] is asserted, then IndexVector[VPSize] is the index of the largest non-allocated virtual page possible for the corresponding virtual page size. If FitsVector[VPSize] is de-asserted, then IndexVector[VpSize] is undefined. 
     The number of atomic pages needed to store the packet is calculated (NumAPs) and the corresponding atomic pages are allocated. The allocation of the atomic pages for the selected block (B) is done as follows:
     1. The allocation status of the atomic pages in AllocationMatrix[APsize][j . . . k], j being the first atomic page and k the last one (k−j+1=NumAPs), are set to allocated.   2. The allocation status of the virtual pages in AllocationMatrix[r][s] are updated following the mesh structure in  FIG. 4   b . (a 2 k+1 -byte virtual page will be allocated if any of the two 2 k -byte virtual pages that it is composed of is allocated).   

     When the packetPage has been generated, it is sent to the QS for enqueueing. If the QS is full (very rare), it will not be able to accept the packetPage being provided by the PMMU. In this case, the PMMU will not be able to generate a new packetPage for the next new packet. This puts pressure on the IB, which might get full if the QS remains full for several cycles. 
     The PMMU block also sends the queue number into which the QS has to store the packetPage. How the PMMU generates this queue number is described below in sections specifically allocated to the QS. 
     Page Allocation Example 
       FIGS. 5   a  and  5   b  illustrate an example of how atomic pages are allocated. For simplicity, the example assumes 2 blocks (0 and 1) of 2 KB each, with an Atomic page size of 256 bytes, and both blocks have their SoftwareOwned flag de-asserted. Single and double cross-hatched areas represent allocated virtual pages (single cross-hatched pages correspond to the pages being allocated in the current cycle). The example shows how the pages get allocated for a sequence of packet sizes of 256, 512, 1K and 512 bytes. Note that, after this sequence, a 2K-byte packet, for example, will not fit in the example LPM. 
     Whenever the FitsVector[VPSize] is asserted, the IndexVector[VPSize] contains the largest non-allocated virtual page index for virtual page size VPSize. The reason for choosing the largest index is that the memory space is better utilized. This is shown in  FIGS. 6   a  and  6   b , where two 256-byte packets are stored in a block. In scenario A, the 256-byte virtual page is randomly chosen, whereas in scenario B, the largest index is always chosen. As can be seen, the block in scenario A only allows two 512-byte virtual pages, whereas the block in scenario B allows three. Both, however, allow the same number of 256-byte packets since this is the smallest allocation unit. Note that the same effect is obtained by choosing the smallest virtual page index number all the time. 
     Packet Overflow 
     The only two reasons why a packet cannot be stored in the LPM are (a) that the size of the packet is larger than the maximum virtual page enabled across all 4 blocks; or (b) that the size of the packet is smaller than or equal to the maximum virtual page enabled but no space could be found in the LPM. 
     When a packet does not fit into the LPM, the PMMU will overflow the packet through the SIU into the EPM. To do so, the PMMU provides the initial address to the SIU (16-byte offset within the packet memory) to where the packet will be stored. This 20-bit address is obtained as follows: (a) The 16 MSB bits correspond to the 16 MSB bits of the OverflowAddress configuration register (i.e. the atomic page number within the packet memory). (b) The 4 LSB bits correspond to the HeaderGrowthOffset configuration register. The packetPage value (which will be sent to the QS) for this overflowed packet is then the 16 MSB bits of the OverflowAddress configuration register. 
     If the on-the-fly configuration flag OverflowEnable is asserted, the PMMU will generate an OverflowStartedInt interrupt. When the OverflowStartedInt interrupt is generated, the size in bytes of the packet to overflow is written by the PMMU into the SPU-read-only configuration register SizeOfOverflowedPacket. At this point, the PMMU sets an internal lock flag that will prevent a new packet from overflowing. This lock flag is reset when the software writes into the on-the-fly configuration register OverflowAddress. If a packet needs to be overflowed but the lock flag is set, the packet will be dropped. 
     With this mechanism, it is guaranteed that only one interrupt will be generated and serviced per packet that is overflowed. This also creates a platform for software to decide where the starting address into which the next packet that will be overflowed will be stored is visible to the interrupt service routine through the SizeOfOverflowedPacket register. In other words, software manages the EPM. 
     If software writes the OverflowAddress multiple times in between two OverflowStartedInt interrupts, the results are undefined. Moreover, if software sets the 16 MSB bits of OverflowAddress to 0 . . . 1023, results are also undefined since the first 1K atomic pages in the packet memory correspond to the LPM. 
     Downloading a Packet from Packet Memory 
     Eventually the SPU will complete the processing of a packet and will inform the QS of the fact. At this point the packet may be downloaded from memory, either LPM or EPM, and sent, via the OB to one of the connected devices.  FIG. 7  is a top-level schematic of the blocks of the XCaliber DMS processor involved in the downloading of a packet, and the elements in  FIG. 7  are numbered the same as in  FIG. 2 . The downloading process may be followed in  FIG. 7  with the aid of the following descriptions. 
     When QS  211  is informed that processing of a packet is complete, the QS marks this packet as completed and, a few cycles later (depending on the priority of the packet), the QS provides to PMMU  209  (as long as the PMMU has requested it) the following information regarding the packet: 
     (a) the packetPage 
     (b) the priority (cluster number from which it was extracted) 
     (c) the tail growth/shrink information (described later in spec) 
     (d) the outbound device identifier bit 
     (e) the CRC type field (described later in spec) 
     (f) the KeepSpace bit 
     The device identifier sent to PMMU block  209  is a 1-bit value that specifies the external device to which the packet will be sent. This outbound device identifier is provided by software to QS  211  as a 2-bit value. 
     If the packet was stored in LPM  219 , PMMU  209  generates all of the (16-byte line) read addresses and read strobes to LPM  219 . The read strobes are generated as soon as the read address is computed and there is enough space in OB  217  to buffer the line read from LPM  219 . Buffer d in the OB is associated to device identifier d. This buffer may become full for either two reasons: (a) The external device d temporarily does not accept data from XCaliber; or (b) The rate of reading data from the OB is lower than the rate of writing data into it. 
     As soon as the packet data within an atomic page has all been downloaded and sent to the OB, that atomic page can be de-allocated. The de-allocation of one or more atomic pages follows the same procedure as described above. However, no de-allocation of atomic pages occurs if the LPM bit is de-asserted. The KeepSpace bit is a don&#39;t care if the packet resides in EPM  701 . 
     If the packet was stored in EPM  701 , PMMU  209  provides to SIU  107  the address within the EPM where the first byte of the packet resides. The SIU performs the downloading of the packet from the EPM. The SIU also monitors the buffer space in the corresponding buffer in OB  217  to determine whether it has space to write the 16-byte chunk read from EPM  701 . When the packet is fully downloaded, the SIU informs the PMMU of the fact so that the PMMU can download the next packet with the same device identifier. 
     When two packets (one per device) are being simultaneously sent, data from the packet with highest priority is read out of the memory first. This preemption can happen at a 16-byte boundary or when the packet finishes its transmission. If both packets have the same priority (provided by the QS), a round-robin method is used to select the packet from which data will be downloaded next. This selection logic also takes into account how full the two buffers in the OB are. If buffer d is full, for example, no packet with a device identifier d will be selected in the PMMU for downloading the next 16-byte chunk of data. 
     When a packet starts to be downloaded from the packet memory (local or external), the PMMU knows where the first valid byte of the packet resides. However, the packet&#39;s size is not known until the first line (or the first two lines in some cases) of packet data is read from the packet memory, since the size of the packet resides in the first two bytes of the packet data. Therefore, the processing of downloading a packet first generates the necessary line addresses to determine the size of the packet, and then, if needed, generates the rest of the accesses. 
     This logic takes into account that the first two bytes that specify the size of the packet can reside in any position in the 16-byte line of data. A particular case is when the first two bytes span two consecutive lines (which will occur when the first byte is the 16th byte of a line, and second byte is the 1 st  byte of next line. 
     As soon as the PMMU finishes downloading a packet (all the data of that packet has been read from packet memory and sent to OB), the PMMU notifies the QS of this event. The QS then invalidates the corresponding packet from its queuing system. 
     When a packet starts to be downloaded, it cannot be preempted, i.e. the packet will finish its transmission. Other packets that become ready to be downloaded with the same outbound device identifier while the previous packet is being transmitted cannot be transmitted until the previous packet is fully transmitted. 
     Packet Growth/Shrink 
     As a result of processing a packet, the size of a network packet can grow, shrink or remain the same size. If the size varies, the SPU has to write the new size of the packet in the same first two bytes of the packet. The phenomenon of packet growth and shrink is illustrated in  FIG. 8 . 
     Both the header and the tail of the packet can grow or shrink. When a packet grows, the added data can overwrite the data of another packet that may have been stored right above the packet experiencing header growth, or that was stored right below in the case of tail growth. To avoid this problem the PMU can be configured so that an empty space is allocated at the front and at the end of every packet when it is stored in the packet memory. These empty spaces are specified with HeaderGrowthOffset and TailGrowthOffset boot-time configuration registers, respectively, and their granularity is 16 bytes. The maximum HeaderGrowthOffset is 240 bytes (15 16-byte chunks), and the maximum TailGrowthOffset is 1008 bytes (63 16-byte chunks). The minimum in both cases is 0 bytes. Note that these growth offsets apply to all incoming packets, that is, there is no mechanism to apply different growth offsets to different packets. 
     When the PMMU searches for space in the LPM, it will look for contiguous space of Size(packet)+((HeaderGrowthOffset+TailGrowthOffset)&lt;&lt;4). Thus, the first byte of the packet (first byte of the ASIC-specific header) will really start at offset ((packetPage&lt;&lt;8)+(HeaderGrowthOffset&lt;&lt;4)) within the packet memory. 
     The software knows what the default offsets are, and, therefore, knows how much the packet can safely grow at both the head and the tail. In case the packet needs to grow more than the maximum offsets, the software has to explicitly move the packet to a new location in the packet memory. The steps to do this are as follows:
     1) The software requests the PMU for a chunk of contiguous space of the new size. The PMU will return a new packetPage that identifies (points to) this new space.   2) The software writes the data into the new memory space.   3) The software renames the old packetPage with the new packetPage.   4) The software requests the PMU to de-allocate the space associated to the old packetPage.   

     In the case of header growth or shrinkage, the packet data will no longer start at ((packetpage&lt;&lt;8)+(HeaderGrowthOffset&lt;&lt;4)). The new starting location is provided to the PMU with a special instruction executed by the SPU when the processing of the packet is completed. This information is provided to the PMMU by the QS block. 
     Time Stamp 
     The QS block of the PMU (described in detail in a following section) guarantees the order of the incoming packets by keeping the packetPage identifiers of the packets in process in the XCaliber processor in FIFO-like queues. However, software may break this ordering by explicitly extracting identifiers from the QS, and inserting them at the tail of any of the queues. 
     To help software in guaranteeing the relative order of packets, the PMU can be configured to time stamp every packet that arrives to the PMMU block using an on-the-fly configuration flag TimeStampEnabled. The time stamp is an 8-byte value, obtained from a 64-bit counter that is incremented every core clock cycle. 
     When the time stamp feature is on, the PMMU appends the 8-byte time stamp value in front of each packet, and the time stamp is stripped off when the packet is sent to the network output interface. The time stamp value always occupies the 8 MSB bytes of the (k−1)th 16-byte chunk of the packet memory, where k is the 16-byte line offset where the data of the packet starts (k&gt;0). In the case that HeaderGrowthOffset is 0, the time stamp value will not be appended, even if TimeStampEnabled is asserted. 
     The full 64-bit time counter value is provided to software through a read-only configuration register (TimeCounter). 
     Software Operations on the PMMU 
     Software has access to the PMMU to request or free a chunk of contiguous space. In particular, there are two operations that software can perform on the PMMU. Firstly the software, through an operation GetSpace(size), may try to find a contiguous space in the LPM for size bytes. The PMU replies with the atomic page number where the contiguous space that has been found starts (i.e. the packetPage), and a success bit. If the PMU was able to find space, the success bit is set to ‘1’, otherwise it is set to ‘0’. GetSpace will not be satisfied with memory of a block that has its SoftwareOwned configuration bit asserted. Thus, software explicitly manages the memory space of software-owned LPM blocks. 
     The PMMU allocates the atomic pages needed for the requested space. The EnableVector set of bits used in the allocation of atomic pages for incoming packets is a don&#39;t care for the GetSpace operation. In other words, as long as sufficient consecutive non-allocated atomic pages exist in a particular block to cover size bytes, the GetSpace(size) operation will succeed even if all the virtual pages in that block are disabled. Moreover, among non-software-owned blocks, a GetSpace operation will be served first using a block that has all its virtual pages disabled. If more than such a block exists, the smallest block number is chosen. If size is 0, GetSpace(size) returns ‘0’. 
     The second operation software can perform on the PMMU is FreeSpace(packetPage). In this operation the PMU de-allocates atomic pages that were previously allocated (starting at packetPage). This space might have been either automatically allocated by the PMMU as a result of an incoming packet, or as a result of a GetSpace command. FreeSpace does not return any result to the software. A FreeSpace operation on a block with its SoftwareOwned bit asserted is disregarded (nothing is done and no result will be provided to the SPU). 
     Local Packet Memory 
     Local Packet Memory (LPM), illustrated as element  219  in  FIGS. 2 and 7 , has in the instant embodiment a size of 256 KB, 16-byte line width with byte enables, 2 banks (even/odd), one Read and one Write port per bank, is fully pipelined, and has one cycle latency 
     The LPM in packet processing receives read and write requests from both the PMMU and the SIU. An LPM controller guarantees that requests from the PMMU have the highest priority. The PMMU reads at most one packet while writing another one. The LPM controller guarantees that the PMMU will always have dedicated ports to the LPM. 
     Malicious software could read/write the same data that is being written/read by the PMMU. Thus, there is no guarantee that the read and write accesses in the same cycle are performed to different 16-byte line addresses. 
     A request to the LPM is defined in this example as a single access (either read or write) of 16-bytes. The SIU generates several requests for a masked load or store, which are new instructions known to the inventors and the subject of at least one separate patent application. Therefore, a masked load/store operation can be stalled in the middle of these multiple requests if the highest priority PMMU access needs the same port. 
     When the PMMU reads or writes, the byte enable signals are assumed to be set (i.e. all 16 bytes in the line are either read or written). When the SIU drives the reads or writes, the byte enable signals are meaningful and are provided by the SIU. 
     When the SPU reads a single byte/word in the LPM, the SIU reads the corresponding 16-byte line and performs the extraction and right alignment of the desired byte/word. When the SPU writes a single byte/word, the SIU generates a 16-byte line with the byte/word in the correct location, plus the valid bytes signals. 
     Prioritization Among Operations 
     The PMMU may receive up to three requests from three different sources (IB, QS and software) to perform operations. For example, requests may come from the IB and/or Software: to perform a search for a contiguous chunk of space, to allocate the corresponding atomic page sizes and to provide the generated packetPage. Requests may also come from the QS and/or Software to perform the de-allocation of the atomic pages associated to a given packetPage. 
     It is required that the first of these operations takes no more than 2 cycles, and the second no more than one. The PMMU executes only one operation at a time. From highest to lowest, the PMMU block will give priority to requests from: IB, QS and Software. 
     Early Full-PMMU Detection 
     The PMU implements a mechanism to aid in flow control between any external device and the XCaliber processor. Part of this mechanism is to detect that the LPM is becoming full and, in this case, a NoMorePagesOfXsizeInt interrupt is generated to the SPU. The EPM is software controlled and, therefore, its state is not maintained by the PMMU hardware. 
     The software can enable the NoMorePagesOfXsizeInt interrupt by specifying a virtual page size s. Whenever the PMMU detects that no more available virtual pages of that size are available (i.e. FitsVector[s] is de-asserted for all the blocks), the interrupt is generated. The larger the virtual page size selected, the sooner the interrupt will be generated. The size of the virtual page will be indicated with a 4-bit value (0:256 bytes, 1:512 bytes, . . . , 8:64 KB) in an on-the-fly configuration register IntIfNoMoreThanXsizePages. When this value is greater than 8, the interrupt is never generated. 
     If the smallest virtual page size is selected (256 bytes), the NoMorePagesOfXsizeInt interrupt is generated when the LPM is completely full (i.e. no more packets are accepted, not even a 1-byte packet). 
     In general, if the IntIfNoMoreThanXsizePages is X, the soonest the interrupt will be generated is when the local packet memory is (100/2 X )% full. Note that, because of the atomic pages being 256 bytes, the LPM could become full with only 3 K-bytes of packet data (3 byte per packet, each packet using an atomic page). 
     Packet Size Mismatch 
     The PMMU keeps track of how many bytes are being uploaded into the LPM or EPM. If this size is different from the size specified in the first two bytes, a PacketErrorInt interrupt is generated to the SPU. In this case the packet with the mismatch packet size is dropped (the already allocated atomic pages will be de-allocated and no packetPage will be created). No AutomaticDropInt interrupt is generated in this case. If the actual size is more than the size specified in the first two bytes, the remaining packet data being received from the ASIC is gracefully discarded. 
     When a packet size mismatch is detected on an inbound device identifier D (D=0, 1), the following packets received from that same device identifier are dropped until software writes (any value) into a ClearErrorD configuration register. 
     Bus Error Recovering 
     Faulty packet data can arrive to or leave the PMU due to external bus errors. In particular the network input interface may notify that the 16-byte chunk of data sent in has a bus error, or the SIU may notify that the 16-byte chunk of data downloaded from EPM has a bus error. In both cases, the PMMU generates the PacketErrorInt interrupt to notify the SPU about this event. No other information is provided to the SPU. 
     Note that if an error is generated within the LPM, it will not be detected since no error detection mechanism is implemented in this on-chip memory. Whenever a bus error arises, no more data of the affected packet will be received by the PMU. This is done by the SIU in both cases. For the first case the PMMU needs to de-allocate the already allocated atomic pages used for the packet data received previous to the error event. 
     When a bus error is detected on an inbound device identifier D (D=0, 1), the following packets received from that same device identifier are dropped until software writes (any value) into a ClearErrorD (D=0, 1) configuration register. 
     Queuing System (OS) 
     The queueing system (QS) in the PMU of the XCaliber processor has functions of holding packet identifiers and the state of the packets currently in-process in the XCaliber processor, keeping packets sorted by their default or software-provided priority, selecting the packets that need to be pre-loaded (in the background) into one of the available contexts, and selecting those processed packets that are ready to be sent out to an external device. 
       FIG. 9  is a block diagram showing the high-level communication between the QS and other blocks in the PMU and SPU. When the PMMU creates a packetPage, it is sent to the QS along with a queue number and the device identifier. The QS enqueues that packetPage in the corresponding queue and associates a number (packetNumber) to that packet. Eventually, the packet is selected and provided to the RTU, which loads the packetPage, packetNumber and selected fields of the packet header into an available context. Eventually the SPU processes that context and communicates to the PMU, among other information, when the processing of the packet is completed or the packet has been dropped. For this communication, the SPU provides the packetNumber as the packet identifier. The QS marks that packet as completed (in the first case) and the packet is eventually selected for downloading from packet memory. 
     It is a requirement in the instant embodiment (and highly desirable) that packets of the same flow (same source and destination) need to be sent out to the external device in the same order as they arrived to the XCaliber processor (unless software explicitly breaks this ordering). When the SPU begins to process a packet the flow is not known. Keeping track of the ordering of packets within a flow is a costly task because of the amount of processing needed and because the number of active flows can be very large, depending on the application. Thus, the order within a flow is usually kept track by using aggregated-flow queues. In an aggregated-flow queue, packet identifiers from different flows are treated as from the same flow for ordering purposes. 
     The QS offloads the costly task of maintaining aggregated-flow queues by doing it in hardware and in the background. Up to 32 aggregated-flow queues can be maintained in the current embodiment, and each of these queues has an implicit priority. Software can enqueue a packetPage in any of the up to 32 queues, and can move a packetPage identifier from one queue to another (for example, when the priority of that packet is discovered by the software). It is expected that software, if needed, will enqueue all the packetPage identifiers of the packets that belong to the same flow into the same queue. Otherwise, a drop in the performance of the network might occur, since packets will be sent out of order within the same flow. Without software intervention, the QS guarantees the per-flow order of arrival. 
     Generic Queue 
     The QS implements a set of up to 32 FIFO-like queues, which are numbered, in the case of 32 queues, from 0 to 31. Each queue can have up to 256 entries. The addition of all the entries of all the queues, however, cannot exceed 256. Thus, queue sizes are dynamic. A queue entry corresponds to a packetPage identifier plus some other information. Up to 256 packets are therefore allowed to be in process at any given time in the XCaliber processor. This maximum number is not visible to software. 
     Whenever the QS enqueues a packetPage, a number (packetNumber) from 0 to 255 is assigned to the packetPage. This number is provided to the software along with the packetPage value. When the software wants to perform an operation on the QS, it provides the packetNumber identifier. This identifier is used by the QS to locate the packetPage (and other information associated to the corresponding packet) in and among its queues. 
     Software is aware that the maximum number of queues in the XCaliber processor is 32. Queues are disabled unless used. That is, the software does not need to decide how many queues it needs up front. A queue becomes enabled when at least one packet is in residence in that queue. 
     Several packet identifiers from different queues can become candidates for a particular operation to be performed. Therefore, some prioritization mechanism must exist to select the packet identifier to which an operation will be applied first. Software can configure (on-the-fly) the relative priority among the queues using an “on-the-fly” configuration register PriorityClusters. This is a 3-bit value that specifies how the different queues are grouped in clusters. Each cluster has associated a priority (the higher the cluster number, the higher the priority). The six different modes in the instant embodiment into which the QS can be configured are shown in the table of  FIG. 10 . 
     The first column of  FIG. 10  is the value in the “on-the-fly” configuration register PriorityClusters. Software controls this number, which defines the QS configuration. For example, for PriorityClusters=2, the QS is configured into four clusters, with eight queues per cluster. The first of the four clusters will have queues  0  through  7 , the second cluster will have queues  8 – 15 , the third clusters  16  through  23 , and the last of the four clusters has queues  24  through  31 . 
     Queues within a cluster are treated fairly in a round robin fashion. Clusters are treated in a strict priority fashion. Thus, the only mode that guarantees no starvation of any queue is when PriorityClusters is 0, meaning one cluster of 32 queues. 
     Inserting a packetPage/deviceId into the QS 
       FIG. 11  is a diagram illustrating the generic architecture of QS  211  of  FIGS. 2 and 7  in the instant embodiment. Insertion of packetPages and DeviceId information is shown as arrows directed toward the individual queues (in this case 32 queues). The information may be inserted from three possible sources, these being the PMMU, the SPU and re-insertion from the QS. There exists priority logic, illustrated by function element  1101 , for the case in which two or more sources have a packetPage ready to be inserted into the QS. In the instant embodiment the priority is, in descending priority order, the PMMU, the QS, and the SPU (software). 
     Regarding insertion of packets from the SPU (software), the software can create packets on its own. To do so, it first requests a consecutive chunk of free space of a given size (see the SPU documentation) from the PMU, and the PMU returns a packetPage in case the space is found. The software needs to explicitly insert that packetPage for the packet to be eventually sent out. When the QS inserts this packetPage, the packetNumber created is sent to the SPU. Software requests an insertion through the Command Unit (see  FIG. 2 ). 
     In the case of insertion from the QS, an entry residing at the head of a queue may be moved to the tail of another queue. This operation is shown as selection function  1103 . 
     In the case of insertion from the PMU, when a packet arrives to the XCaliber processor, the PMMU assigns a packetpage to the packet, which is sent to the QS as soon as the corresponding packet is safely stored in packet memory. 
     An exemplary entry in a queue is illustrated as element  1105 , and has the following fields: Valid (1) validates the entry. PacketPage (16) is the first atomic page number in memory used by the packet. NextQueue (5) may be different from the queue number the entry currently belongs to, and if so, this number indicates the queue into which the packetPage needs to be inserted next when the entry reaches the head of the queue. Delta (10) contains the number of bytes that the header of the packet has either grown or shrunk. This value is coded in 2&#39;s complement. Completed (1) is a single bit that indicates whether software has finished the processing of the corresponding packet. DeviceId (2) is the device identifier associated to the packet. Before a Complete operation is performed on the packet (described below) the DeviceId field contains the device identifier of the external device that sent the packet in. After the Complete operation, this field contains the device identifier of the device to which the packet will be sent. Active (1) is a single bit that indicates whether the associated packet is currently being processed by the SPU. CRCtype (2) indicates to the network output interface which type of CRC, if any, needs to be computed for the packet. Before the Complete operation is performed on the packet, this field is 0. KeepSpace (1) specifies whether the atomic pages that the packet occupies in the LPM will be de-allocated (KeepSpace de-asserted) by the PMMU or not (KeepSpace asserted). If the packet resides in EPM this bit is disregarded by the PMMU. 
     The QS needs to know the number of the queue to which the packetPage will be inserted. When software inserts the packetPage, the queue number is explicitly provided by an XStream packet instruction, which is a function of the SPU, described elsewhere in this specification. If the packetPage is inserted by the QS itself, the queue number is the value of the NextQueue field of the entry where the packetPage resides. 
     When a packetPage is inserted by the PMMU, the queue number depends on how the software has configured (at boot time) the Log2InputQueues configuration register. If Log2InputQueues is set to 0, all the packetPages for the incoming packets will be enqueued in the same queue, which is specified by the on-the-fly configuration register FirstInputQueue. If Log2InputQueues is set to k (1&lt;=k&lt;=5), then the k MSB bits of the 3rd byte of the packet determine the queue number. Thus an external device (or the network input interface block of the SIU) can assign up to 256 priorities for each of the packets sent into the PMU. The QS maps those 256 priorities into 2 k , and uses queue numbers FirstInputQueue to FirstInputQueue+2 k −1 to insert the packetPages and deviceId information of the incoming packets. 
     It is expected that an external device will send the same 5 MSB bits in the 3 rd  byte for all packets in the same flow. Otherwise, a drop in the performance of the network might occur, since packets may be sent back to the external device out-of-order within the same flow. Software is aware of whether or not the external device (or SIU) can provide the information of the priority of the packet in the 3 rd  byte. 
     When packetPage p is inserted into queue q, the PacketPage field of the entry to be used is set to p and the Valid field to ‘1’. The value for the other fields depend on the source of the insertion. If the source is software (SPU), Completed is ‘0’; NextQueue is provided by SPU; DeviceId is ‘0’; 
     Active is ‘1’; CRCtype is 0; KeepSpace is 0, and Probed is 0. 
     If the source is the QS, the remaining fields are assigned the value they have in the entry in which the to-be-inserted packetPage currently resides. If the source is the PMMU, Completed is ‘0’, NextQueue is q, DeviceId is the device identifier of the external device that sent the packet into XCaliber, Active is ‘0’, CRCtype is 0, KeepSpace is 0, and Probed is 0. 
     Monitoring Logic 
     The QS monitors entries into all of the queues to detect certain conditions and to perform the corresponding operation, such as to re-enqueue an entry, to send a packetPage (plus some other information) to the PMMU for downloading, or to send a packetPage (plus some other information) to the RTU. 
     All detections take place in a single cycle and they are done in parallel. 
     Re-Enqueuing an Entry 
     The QS monitors all the head entities of the queues to determine whether a packet needs to be moved to another queue. Candidate entries to be re-enqueued need to be valid, be at the head of a queue, and have the NextQueue field value different from the queue number of the queue in which the packet currently resides. 
     If more than one candidate exists for re-enqueueing, the chosen entry will be selected following a priority scheme described later in this specification. 
     Sending an Entry to the PMMU for Downloading 
     The QS monitors all the head entities of the queues to determine whether a packet needs to be downloaded from the packet memory. This operation is  1102  in  FIG. 11 . The candidate entries to be sent out of XCaliber need to be valid, be at the head of the queue, have the NextQueue field value the same as the queue number of the queue in which the packet currently resides, and have the Completed flag asserted and the Active flag de-asserted. Moreover the QS needs to guarantee that no pending reads or writes exist from the same context that has issued the download command to the QS. 
     If more than one candidate exists for downloading, the chosen entry will be selected following a priority scheme described later in this specification. 
     A selected candidate will only be sent to the PMMU if the PMMU requested it. If the candidate was requested, the selected packetPage, along with the cluster number from which it is extracted, the tail growth/shrink, the outbound device identifier bit, the CRCtype and the KeepSpace bits are sent to the PMMU. 
       FIG. 12  is a table indicating coding of the Deviceid field. If the Deviceid field is 0, then the Outbound Device Identifier is the same as the Inbound Device Identifier, and so on as per the table. 
     When an entry is sent to the PMMU, the entry is marked as “being transmitted” and it is extracted from the queuing system (so that it does not block other packets that are ready to be transmitted and go to a different outbound device identifier). However, the entry is not invalidated until the PMMU notifies that the corresponding packet has been completely downloaded. Thus, probe-type operations on this entry will be treated as valid, i.e. as still residing in the XCaliber processor. 
     Reincarnation Effect 
     As described above, the QS assigns a packetNumber from 0 to 255 (256 numbers in total) to each packet that comes into XCaliber and is inserted into a queue. This is done by maintaining a table of 256 entries into which packet identifiers are inserted. At this time the Valid bit in the packet identifier is also asserted. Because the overall numbers of packets dealt with by XCaliber far exceeds 256, packet numbers, of course, have to be reused throughout the running of the XCaliber processor. Therefore, when packets are selected for downloading, at some point the packetNumber is no longer associated with a valid packet in process, and the number may be reused. 
     As long as a packet is valid in XCaliber it is associated with the packetNumber originally assigned. The usual way in which a packetNumber becomes available to be reused is that a packet is sent by the QS to the RTU for preloading in a context prior to processing. Then when the packet is fully processed and fully downloaded from memory, the packet identifier in the table associating packetNumbers is marked Invalid by manipulating the Valid bit (see  FIG. 1I  and the text accompanying). 
     In usual operation the system thus far described is perfectly adequate. It has been discovered by the inventors, however, that there are some situations in which the Active and Valid bits are not sufficient to avoid contention between streams. One of these situations has to do with a clean-up process, sometimes termed garbage collection, in which software monitors all packet numbers to determine when packets have remained in the system too long, and discards packets under certain conditions, freeing space in the system for newly-arriving packets. 
     In these special operations, like garbage collection, a stream must gain ownership of a packet, and assure that the operation it is to perform on the packet actually gets performed on the correct packet. As software probes packets, however, and before action may be taken, because there are several streams operating, and because the normal operation of the system may also send packets to the RTU, for example, it is perfectly possible in these special operations that a packet probed may be selected and effected by another stream before the special operation is completed. A packet, for example, may be sent to the RTU, processed, and downloaded, and a new packet may then be assigned to the packetNumber, and the new packet may even be stored at exactly the same packetPage as the original packet. There is a danger, then, that the special operations, such as discarding a packet in the garbage collection process, may discard a new and perfectly valid packet, instead of the packet originally selected to be discarded. This, of course, is just one of potentially many such special operations that might lead to trouble. 
     Considering the above, the inventors have provided a mechanism for assuring that, given two different absolute points in time, time s and time r, for example, that a valid packetNumber at time s and the same packetNumber at time r, still is associated to the same packet. A simple probe operation is not enough, because at some time after s and before time r the associated packet may be downloaded, and another (and different) packet may have arrived, been stored in exactly the same memory location as the previous packet, and been assigned the same packetNumber as the downloaded packet. 
     The mechanism implemented in XCaliber to ensure packetNumber association with a specific packet at different times includes a probe bit in the packet identifier. When a first stream, performing a process such as garbage collection, probes a packet, a special command, called Probe&amp;Set is used. Probe&amp;Set sets (asserts) the probe bit, and the usual information is returned, such as the value for the Valid bit, the Active bit, the packetPage address, and the old value of the probe bit. The first stream then executes a Conditional Activate instruction, described elsewhere in this specification, to gain ownership of the packet. Also, when the queuing system executes this Conditional Activate instruction it asserts the active bit of the packet. Now, at any time after the probe bit is set by the first stream, when a second stream at a later time probes the same packet, the asserted probe bit indicates that the first stream intends to gain control of this packet. The second stream now knows to leave this packet alone. This probe bit is de-asserted when a packet enters the XCaliber processor and a new (non-valid) number is assigned. 
     Sending an Entry to the RTU 
     The RTU uploads in the SPU background to the XCaliber processor some fields of the headers of packets that have arrived, and have been completely stored into packet memory. This uploading of the header of a packet in the background may occur multiple times for the same packet. The QS keeps track of which packets need to be sent to the RTU. The selection operation is illustrated in  FIG. 11  as  1104 . 
     Whenever the RTU has chosen a context to pre-load a packet, it notifies the QS that the corresponding packet is no longer an inactive packet. The QS then marks the packet as active. 
     Candidate entries to be sent to the RTU need to be valid, to be the oldest entry with the Active and Completed bits de-asserted, to have the NextQueue field value the same as the queue number of the queue in which the packet currently resides, and to conform to a limitation that no more than a certain number of packets in the queue in which the candidate resides are currently being processed in the SPU. More detail regarding this limitation is provided later in this specification. When an entry is sent to the RTU for pre-loading, the corresponding Active bit is asserted. 
     A queue can have entries with packet identifiers that already have been presented to the RTU and entries that still have not. Every queue has a pointer (NextPacketForRTU) that points to the oldest entry within that queue that needs to be sent to the RTU. Within a queue, packet identifiers are sent to the RTU in the same order they were inserted in the queue. 
     The candidate packet identifiers to be sent to the RTU are those pointed to by the different NextPacketForRTU pointers associated with the queues. However, some of these pointers might point to a non-existent entry (for example, when the queue is empty or when all the entries have already been sent to the RTU). The hardware that keeps track of the state of each of the queues determines these conditions. Besides being a valid entry pointed to by a NextPacketForRTU pointer, the candidate entry needs to have associated with it an RTU priority (described later in this specification) currently not being used by another entry in the RTU. If more than a single candidate exists, the chosen entry is selected following a priority scheme described later in this specification. 
     As opposed to the case in which an entry is sent to the PMMU for downloading, an entry sent to the RTU is not extracted from its queue. Instead, the corresponding NextPacketForRTU pointer is updated, and the corresponding Active bit is asserted. 
     The QS sends entries to an 8-entry table in the RTU block as long as the entry is a valid candidate and the corresponding slot in the RTU table is empty. The RTU will accept, at most, 8 entries, one per each interrupt that the RTU may generate to the SPU. 
     The QS maps the priority of the entry (given by the queue number where it resides) that it wants to send to the RTU into one of the 8 priorities handled by the RTU (RTU priorities). This mapping is shown in the table of  FIG. 13 , and it depends on the number of clusters into which the different queues are grouped (configuration register PriorityClusters) and the queue number in which the entry resides. 
     The RTU has a table of 8 entries, one for each RTU priority. Every entry contains a packet identifier (packetpage, packetNumber, queue#) and a Valid bit that validates it. The RTU always accepts a packet identifier of RTU priority p if the corresponding Valid bit in entry p of that table is de-asserted. When the RTU receives a packet identifier of RTU priority p from the QS, the Valid bit of entry p in the table is asserted, and the packet identifier is stored. At that time the QS can update the corresponding NextPacketForRTU pointer. 
     Limiting the Packets Sent within a Queue 
     Software can limit the number of packets that can be active (i.e. being processed by any of the streams in the SPU) on a per-queue basis. This is achieved through a MaxActivePackets on-the-fly configuration register, which specifies, for each queue, a value between 1 and 256 that corresponds to the maximum number of packets, within that queue, that can be being processed by any stream. 
     The QS maintains a counter for each queue q which keeps track of the current number of packets active for queue q. This counter is incremented whenever a packet identifier is sent from queue q to the RTU, a Move operation moves a packet into queue q, or an Insert operation inserts a packet identifier into queue q; and decremented when any one the following operations are performed in any valid entry in queue q: a Complete operation, an Extract operation, a Move operation that moves the entry to a different queue, or a MoveAndReactivate operation that moves the entry to any queue (even to the same queue). Move, MoveAndReactivate, Insert, Complete and Extract are operations described elsewhere in this specification. 
     Whenever the value of the counter for queue q is equal to or greater than the corresponding maximum value specified in the MaxActivePackets configuration register, no entry from queue q is allowed to be sent to the RTU. The value of the counter could be greater since software can change the MaxActivePackets configuration register for a queue to a value lower than the counter value at the time of the change, and a queue can receive a burst of moves and inserts. 
     Software Operations on the QS 
     Software executes several instructions that affect the QS. The following is a list of all operations that can be generated to the QS as a result of the dispatch by the SPU core of an XStream packet instruction: 
     Insert(p,q): the packetPage p is inserted into queue q. A ‘1’ will be returned to the SPU if the insertion was successful, and a ‘0’ if not. The insertion will be unsuccessful only when no entries are available (i.e. when all the 256 entries are valid). 
     Move(n,q): asserts to q the NextQueue field of the entry in which packetNumber n resides. 
     MoveAndReactivate(n,q): asserts to q the NextQueue field of the entry in which packetNumber n resides; de-asserts the Active bit. 
     Complete(n,d,e): asserts the Completed flag, the Delta field to d and the deviceId field to e of the entry in which packetNumber n resides. De-asserts the Active bit and de-asserts the KeepSpace bit. 
     CompleteAndKeepSpace(n,d,e): same as Complete( ) but it asserts the KeepSpace bit. 
     Extract(n): resets the Valid flag of the entry in which packetNumber n resides. 
     Replace(n,p): the PacketPage field of the entry in which packetNumber n resides is set to packetPage p. 
     Probe(n): the information whether the packetNumber n exists in the QS or not is returned to the software. In case it exists, it returns the PacketPage, Completed, NextQueue, DeviceId, CRCtype, Active, KeepSpace and Probed fields. 
     ConditionalActivate(n): returns a ‘1’ if the packetNumber n is valid, Probed is asserted, Active is de-asserted, and the packet is not being transmitted. In this case, the Active bit is asserted. 
     The QS queries the RTU to determine whether the packet identifier of the packet to be potentially activated is in the RTU table, waiting to be preloaded, or being preloaded. If the packet identifier is in the table, the RTU invalidates it. If the query happens simultaneously with the start of preloading of that packet, the QS does not activate the packet. 
     ProbeAndSet(n): same as Probe( ) but it asserts the Probed bit (the returned Probed bit is the old Probed bit). 
     Probe(q): provides the size (i.e. number of valid entries) in queue q. 
     A Move( ), MoveAndReactivate( ), Complete( ), CompleteAndKeepSpace( ), Extract( ) and Replace( ) on an invalid (i.e. non-existing) packetNumber is disregarded (no interrupt is generated). 
     A Move, MoveAndReactivate, Complete, CompleteAndKeepSpace, Extract and Replace on a valid packetNumber with the Active bit de-asserted should not happen (guaranteed by software). If it happens, results are undefined. Only the Insert, Probe, ProbeAndSet and ConditionalActivate operations reply back to the SPU. 
     If software issues two move-like operations to the PMU that affect the same packet, results are undefined, since there is no guarantee that the moves will happen as software specified. 
       FIG. 14  is a table showing allowed combinations of Active, Completed, and Probed bits for a valid packet. 
     Basic Operations 
     To support the software operations and the monitoring logic, the QS implements the following basic operations:
     1. Enqueue an entry at the tail of a queue.   2. Dequeue an entry from the queue in which it resides.   3. Move an entry from the head of the queue wherein it currently resides to the tail of another queue.   4. Provide an entry of a queue to the RTU.   5. Provide the size of a queue.   6. Update any of the fields associated to packetNumber.   

     Operations 1, 2, 4 and 6 above (applied to different packets at the same time) are completed in 4 cycles in a preferred embodiment of the present invention. This implies a throughput of one operation per cycle. 
     Some prioritization is necessary when two or more operations could start to be executed at the same time. From highest to lowest priority, these events are inserting from the PMMU, dequeuing an entry, moving an entry from one queue to another queue, sending an entry to the RTU for pre-loading, or a software operation. The prioritization among the software operations is provided by design since software operations are always executed in order. 
     Early QS Full Detection 
     The PMU implements a mechanism to aid in flow control between the ASIC (see element  203  in  FIG. 2 ) and the XCaliber processor. Part of this mechanism is to detect that the QS is becoming full and, in this case, a LessThanXpacketIdEntriesInt interrupt is generated to the SPU. The software can enable this interrupt by specifying (in a IntIfLessThanXpacketIdEntries configuration register) a number z larger than 0. An interrupt is generated when 256−y&lt;z, being y the total number of packets currently in process in XCaliber. When z=0, the interrupt will never occur. 
     Register Transfer Unit (RTU) 
     A goal of the RTU block is to pre-load an available context with information of packets alive in XCaliber. This information is the packetPage and packetNumber of the packet and some fields of its header. The selected context is owned by the PMU at the time of the pre-loading, and released to the SPU as soon as it has been pre-loaded. Thus, the SPU does not need to perform the costly load operations to load the header information and, therefore, the overall latency of processing packets is reduced. 
     The RTU receives from the QS a packet identifier (packetPage, packetNumber) and the number of the queue from which the packet comes from) from the QS. This identifier is created partly by the PMMU as a result of a new packet arriving to XCaliber through the network input interface (packetPage), and partly by the QS when the packetPage and device identifier are enqueued (packetNumber). 
     Another function of the RTU is to execute masked load/store instructions dispatched by the SPU core since the logic to execute a masked load/store instruction is similar to the logic to perform a pre-load. Therefore, the hardware can be shared for both operations. For this reason, the RTU performs either a masked load/store or a pre-load, but not both, at a time. The masked load/store instructions arrive to the RTU through the command queue (CU) block. 
     Context States 
     A context can be in one of two states: PMU-owned or SPU-owned. The ownership of a context changes when the current owner releases the context. The PMU releases a context to the SPU in three cases. Firstly, when the RTU has finished pre-loading the information of the packet into the context. Secondly, the PMU releases a context to the SPU when the SPU requests a context to the RTU. In this case, the RTU will release a context if it has one available for releasing. Thirdly, all eight contexts are PMU-owned. Note that a context being pre-loaded is considered to be a PMU-owned context. 
     The SPU releases a context to the RTU when the SPU dispatches an XStream RELEASE instruction. 
     Pre-Loading a Context 
     At boot time, the PMU owns 7 out of the 8 contexts that are available in the embodiment of the invention described in the present example, and the SPU owns one context. The PMU can only pre-load information of a packet to a context that it owns. The process of pre-loading information of a packet into a context is divided into two steps. A first phase to load the address (the offset within the packet memory address space), from where the packet starts. This offset points to the first byte of the two-byte value that codes the size in bytes of the packet. In the case that the packet has been time stamped and HeaderGrowthOffset is not 0, the time stamp value is located at offset-4. The offset address is computed as (packetPage&lt;&lt;8) (HeaderGrowthOffset&lt;&lt;4). This offset is loaded into register number StartLoadingRegister in the selected context. StartLoadingRegister is a boot-time configuration register. The packetNumber value is loaded in register number StartLoadingRegister+1. 
     The second phase is to load the packet header. The packet header is loaded using registers StartLoadingRegister+2, StartLoadingRegister+3, . . . (as many as needed, and as long as there exist GPR registers). The PatternMatchingTable[q] (q being the queue number associated to the packet) mask specifies how the header of the packet will be loaded into the GPR registers of the context. The PatternMatchingTable is an on-the-fly configuration register that contains masks. To obtain the header data, the RTU requests the SIU to read as many 16-byte lines of packet data as needed into the packet memory. The RTU, upon receiving the 16-byte lines from packet memory (either local or external), selects the desired bytes to load into the context using pattern mask to control this operation. 
     The step described immediately above of loading the packet header may be disabled by software on a per-queue basis through the on-the-fly PreloadMaskNumber configuration register. This register specifies, for each of the 32 possible queues in the QS, which mask (from 0 to 23) in the PatternMatchingTable is going to be used for the pre-loading. If a value between 24 and 31 is specified in the configuration register, it is interpreted by the RTU as not to perform. 
     The RTU only loads the GPR registers of a context. The required CP0 registers are initialized by the SPU. Since the context loaded is a PMU-owned context, the RTU has all the available write ports to that context (4 in this embodiment) to perform the loading. 
     Whenever the pre-loading operation starts, the RTU notifies this event to the SPU through a dedicated interface. Similarly, when the pre-loading operation is completed, the RTU also notified the SPU. Thus the SPU expects two notifications (start and end) for each packet pre-load. A special notification is provided to the SPU when the RTU starts and ends a pre-load in the same cycle (which occurs when the step of loading packet header is disabled). In all three cases, the RTU provides the context number and the contents of the CodeEntryPoint configuration register associated to the packet. In the case that the PMU releases a context to the SPU because all eight contexts are PMU-owned, the contents of the CodeEntryPointSpecial are provided to the SPU. The RTU has an 8-entry table (one for each context), each entry having a packet identifier ready to be pre-loaded and a valid bit that validates the entry. The RTU selects always the valid identifier of the highest entry index to do the pre-load. When a context is associated to this identifier, the corresponding valid bit is de-asserted. The RTU pre-loads one context at a time. After loading a context, the context is released to the SPU and becomes a SPU-owned context. At this point the RTU searches its table for the next packet to be pre-loaded into a context (in case there is at leas one PMU-owned context). 
     Pattern-Matching Table 
       FIG. 15  illustrates a Pattern Matching Table which is an on-the-fly configuration register that contains a set of sub-masks. The RTU can use any sub-mask (from 0 to 23) within this table for a pre-loading a context. Sub-masks can also be grouped into a larger mask containing two or more submasks. 
       FIG. 16  illustrates the format of a mask. A mask is a variable number (1 to 8) of sub-masks of 32×2 bits each, as shown. Every sub-mask has an associated bit (EndOfMask) that indicates whether the composite mask finishes with the corresponding sub-mask, or it continues with the next sub-mask. The maximum total number of sub-masks is 32, out of which 24 (sub-mask indexes 0 to 23) are global, which means any stream in the SPU can use and update them, and 8 are per-stream sub-masks. The per-stream sub-masks do not have an EndOfMask bit, which is because no grouping of per-stream sub-masks is allowed. 
     The two 32-bit vectors in each sub-mask are named SelectVector and RegisterVector. The SelectVector indicates which bytes from the header of the packet will be stored into the context. The RegisterVector indicates when to switch to the next consecutive register within the context to keep storing the selected bytes by the SelectVector The bytes are always right aligned in the register. 
       FIG. 17  shows an example of a pre-load operation using the mask in  FIG. 16 . A bit asserted in the SelectVector indicates that the corresponding byte of the header are stored into a register. In the example, bytes  0 ,  1  and  7  of the header are loaded into GPR number StartLoadingRegister+2 in bytes  0 ,  1  and  2 , respectively (i.e. the header bytes are right-aligned when loaded into the register). A bit asserted in the RegisterVector indicates that no more header bytes are loaded into the current GPR register, and that the next header bytes, if any, are loaded into the next (consecutively) GPR register. In the example, bytes  12  and  13  of the header are loaded into GPR number StartLoadingRegister+3. 
     Selecting a PMU-Owned Context 
     There are a total of eight functional units in the PMU core. However, due to complexity-performance tradeoffs, a stream (context) can only issue instructions to a fixed set of 4 functional units. 
     The RTU may own at any given time several contexts. Therefore, logic is provided to select one of the contexts when a pre-load is performed, or when a context has to be provided to the SPU. This logic is defined based on how the different streams (contexts) in the PMU core can potentially dispatch instructions to the different functional units, and the goal of the logic is to balance operations that the functional units in the SPU can potentially receive. 
     The selection logic takes as inputs eight bits, one per context, that indicates whether that context is PMU or SPU-owned. The logic outputs which PMU-owned context(s) that can be selected. 
     
       
         
               
             
           
               
                   
               
             
             
               
                 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 3, 20, 5, 6, 7, 24, 9, 10, 11, 12, 
               
               
                 13, 14, 15, 32, 33, 34, 3, 36, 5, 6, 7, 40, 9, 10, 11, 12, 13, 14, 15, 48, 48, 48, 51, 48, 53, 
               
               
                 54, 7, 48, 57, 58, 11, 60, 13, 14, 15, 64, 65, 66, 3, 68, 5, 6, 7, 72, 9, 10, 11, 12, 13, 14, 
               
               
                 15, 80, 80, 80, 83, 80, 85, 86, 7, 80, 89, 90, 11, 92, 13, 14, 15, 96, 96, 96, 99, 96, 101, 
               
               
                 102, 7, 96, 105, 106, 11, 108, 13, 14, 15, 112, 112, 112, 112, 112, 112, 112, 119, 112, 
               
               
                 112, 112, 123, 112, 125, 126, 15, 128, 129, 130, 3, 132, 5, 6, 7, 136, 9, 10, 11, 12, 13, 
               
               
                 14, 15, 144, 144, 144, 147, 144, 149, 150, 7, 144, 153 154, 11, 156, 13, 14, 15, 160, 
               
               
                 160, 160, 163, 160, 165, 166, 7, 160, 169, 170, 11, 172, 13, 14, 15, 176, 176, 176, 176, 
               
               
                 176, 176, 176, 183, 176, 176, 176, 187, 176, 189, 190, 15, 192, 192, 192, 195, 192, 197, 198, 7, 192, 201, 
               
               
                 202, 11, 204, 13, 14, 15, 208, 208, 208, 208, 208, 208, 208, 215, 208, 
               
               
                 208, 208, 219, 208, 221, 222, 15, 224, 224, 224, 224, 224, 224, 224, 231, 224, 224, 224, 235, 224, 237, 
               
               
                 238, 15, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240, 240 
               
               
                   
               
             
          
         
       
     
     The selection logic is specified with the previous list of 254 numbers. 
     Each number is associated to a possible combination of SPU/PMU-owned context. For example, the first number corresponds to the combination ‘00000001’, i.e. context number 0 is PMU owned and context numbers 1 to 7 are SPU owned (LSB digit corresponds to context 0, MSB digit to context 7; digit value of 0 means SPU owned, digit value of 1 means PMU owned). The second number corresponds to combination ‘00000010’, the third to combination ‘00000011’, and so forth up to combination ‘11111110’. The 19 th  combination (‘00010011’) has associated number 3 (or ‘00000011’) in the previous list, which means that context 0 and 1 can be selected. Context 4 could also be selected, however it is not the best choice to balance the use of the functional units in the SPU core. 
     Interrupt when No Context is Available 
     The RTU has a table of 8 entries named NewPacketIdTable). Entry p in this table contains a packet identifier (packetPage, packetNumber and queue number) with an RTU-priority of p, and a Valid bit that validates the identifier. When the RTU is not busy pre-loading or executing a masked load/store, it will obtain from this table the valid identifier with the highest RTU-priority. In case it exists and there is at least one PMU-owned context, the RTU will start the pre-loading of a PMU-owned context, and it will reset the Valid bit in the table. 
     In case there is no PMU-owned context, the RTU sits idle (assuming no software operation is pending) until a context is released by the SPU. At that point in time the RTU obtains, again, the highest valid RTU-priority identifier from the NewPacketIdTable (since a new identifier with higher RTU priority could have been sent by the QS while the RTU was waiting for a context to be released by the SPU). The Valid bit is reset and the packet information starts being pre-loaded into the available context. At this point the RTU is able to accept a packet with RTU priority p from the QS. 
     When an identifier with a RTU priority of p is sent by the QS to the RTU, it is loaded in entry p in the NewPacketIdTable, and the Valid bit is set. At this time, if the number of valid identifiers (without counting the incoming one) in the NewPacketIdTable is equal or larger than the current available PMU-owned contexts (without counting the context that the RTU currently might be loading), then a PacketAvailableButNoContextPriorityP Int interrupt is generated to the SPU. P ranges from 0 to 7, and its value is determined by a boot-time configuration flag PacketAvailableButNo ContextIntMapping. If this flag is ‘0’, P is determined by the 3-bit boot-time configuration register DefaultPacketPriority. If this flag is ‘1’, P is the RTU priority. However, the PacketAvailableButNoContextPriorityPint will not be generated if the corresponding configuration flag PacketAvailableButNo ContextPriorityPintEnable is de-asserted. 
     The SPU, upon receiving the interrupt, decides whether or not to release a context that it owns so that the RTU can pre-load the packetPage, packetNumber and header information of the new packet. 
     When the RTU generates a PacketAvailableButNoContext PriorityPInt interrupt, it may receive after a few cycles a context that has been released by the SPU. This context, however, could have been released when, for example, one of the streams finished the processing of a packet. This can happen before the interrupt service routine for the PacketAvailable ButNoContextPriorityPInt interrupt finishes. Thus, when a context is released due to the ISR completion, the packet pre-load that originated the interrupt already might have used the context first released by another stream in the SPU. Thus, the context released due to the interrupt will be used for another (maybe future) packet pre-load. If no other entry is valid in the NewPacketIdTable, the context is be used and sits still until either an identifier arrives to the RTU or the SPU requesting a context to the RTU. 
     Whenever a context becomes SPU-owned, and the RTU has a pre-load pending, the RTU selects the most priority pending pre-load (which corresponds to the highest-valid entry in the NewPacketTable), and will start the preload. If the PacketAvailableButNoContextPriorityint interrupt associated to this level was asserted, it gets de-asserted when the pre-load starts. 
     Software Operations on the RTU 
     Software executes a number of instructions that affect the RTU. Following is a list of all operations that can be generated to the RTU as a result of dispatch by the SPU core of an XStream packet instruction. The operations arrive to the RTU through the command queue (CU), along with the context number associated to the stream that issued the instruction:
     1. Release(c): context number c becomes PMU owned.   2. GetContext: the RTU returns the number of a PMU-owned context number. This context, if it exists, becomes SPU owned and a success flag is returned asserted; otherwise it is return de-asserted, in which case the context number is meaningless.   3. MaskedLoad(r,a,m), MaskedStore(r,a,m): the SPU core uses the RTU as a special functional unit to execute the masked load/store instructions since the logic to execute a masked load/store instruction is similar to the logic to perform a pre-load. Therefore, the hardware can be shared for both operations. For this reason, the RTU performs either a masked load/store or a pre-load, but not both at a time. For either the masked load or masked store, the RTU will receive the following parameters:
       (a) A mask number m that corresponds to the index of the first submask in the PatternMatchingTable to be used by the masked load/store operation.   (b) A 36-bit address a that points to the first byte in (any) memory to which the mask will start to be applied.   (c) A register number r (within the context number provided) that corresponds to the first register involved in the masked load/store operation. Subsequent registers within the same context number will be used according to the selected mask.   
       

     For masked load/store operations, the mask can start to be applied at any byte of the memory, whereas in a pre-load operation (a masked-load like operation) the mask will always be applied starting at a 16-byte boundary address since packet data coming from the network input interface is always stored in packet memory starting at the LSB byte in a 16-byte line. 
     The MaskedLoad, MaskedStore and GetContext operations communicate to the SPU when they complete through a dedicated interface between the RTU and the SPU. The RTU gives more priority to a software operation than packet pre-loads. Pre-loads access the packet memory whereas the masked load/store may access any memory in the system as long as it is not cacheable or write-through. If not, results are undefined. 
     The RTU is able to execute a GetContext or Release command while executing a previous masked load/store command. 
     Programming Model 
     Software can configure, either at boot time or on the fly, several of the features of the PMU. All of the features configurable at boot time only, and some configurable on the fly, must happen only when the SPU is running in a single-stream mode. If not, results are undefined. The PMU does not check in which mode the SPU is running. 
     Software can update some of the information that the PMU maintains for a given packet, and also obtain this information. This is accomplished by software through new XStream packet instructions that are the subject of separate patent applications. These instructions create operations of three different types (depending on which block of the PMU the operation affects, whether PMMU, QS or RTU) that will be executed by the PMU. Some of the operations require a result from the PMU to be sent back to the SPU. 
     The packet memory and configuration space are memory mapped. The SIU maintains a configuration register (16 MB aligned) with the base address of the packet memory, and a second configuration register with the base address of EPM. Software sees the packet memory as a contiguous space. The system, however, allows the EPM portion of the packet memory to be mapped in a different space. 
     The SIU also maintains a third configuration register with the base of the PMU configuration register space. All the load/store accesses to LPM and configuration space performed by the SPU reach the PMU through the SIU. The SIU determines to which space the access belongs, and lets the PMU know whether the access is to LPM or to the PMU configuration space. Accesses to the EPM are transparent to the PMU. 
     The PMU can interrupt the SPU when certain events happen. Software can disable all these interrupts through configuration registers. 
     Configuration Registers 
     The configuration registers of the PMU reside in the PMU Configuration Space of the XCaliber address space. The base address of this space is maintained by the SIU and does not need to be visible by the PMU. The SIU notifies to the PMU with a signal when a read/write access performed by the SPU belongs to this space, along with the information needed to update the particular register on a write access. 
     Some of the PMU configuration registers can be configured only at boot time, and some can be configured on the fly. All boot-time configurable and some on-the-fly configurable registers need to be accessed in single-stream mode. A boot-time configurable register should only be updated if the PMU is in reset mode. Results are undefined otherwise. The PMU will not check whether the SPU is indeed in single-stream mode when a single-stream mode configuration register is updated. All the configuration registers come up with a default value after the reset sequence. 
     In the instant embodiment 4 KB of the XCaliber address space is allocated for the PMU configuration space. In XCaliber&#39;s PMU, some of these configuration registers are either not used or are sparsely used (i.e. only some bits of the 32-bit configuration register word are meaningful). The non-defined bits in the PMU configuration space are reserved for future PMU generations. Software can read or write these reserved bits but their contents, although fully deterministic, are undefined. 
       FIG. 18  shows the PMU Configuration Space, which is logically divided into 32-bit words. Each word or set of words contains a configuration register. 
       FIGS. 19   a  and  19   b  are two parts of a table showing mapping of the different PMU configuration registers into the different words of the configuration space. The block owner of each configuration register is also shown in the table. 
     Following is the list of all configuration registers in this particular embodiment along with a description and the default value (after PMU reset). For each of the configuration registers, the bit width is shown in parenthesis. Unless otherwise specified, the value of the configuration register is right aligned into the corresponding word within the configuration space. 
     Boot-Time Only Configuration Registers: 
     
         
         1. Log2InputQueues (5)
       (a) Default Value: 0   (b) Description: Number of queues in the QS used as input queues (i.e. number of queues in which packetPages/deviceIds from the PMMU will be inserted).   
     
         2. PriorityClustering (3)
       (a) Default Value: 5 (32 clusters)   (b) Description: Specifies how the different queues in the QS are grouped in priority clusters (0: 1 cluster, 1: 2 clusters, 2: 4 clusters, . . . , 5: 32 clusters).   
     
         3. HeaderGrowthOffset (4)
       (a) Default Value: 0   (b) Description: Number of empty 16-byte chunks that will be left in front of the packet when it is stored in packet memory. Maximum value is 15 16-byte chunks. Minimum is 0.   
     
         4. TailGrowthOffset (6)
       (a) Default Value: 0   (b) Description: Number of empty 16-byte chunks that will be left at the end of the packet when it is stored in packet memory. Maximum value is 63 16-byte chunks. Minimum is 0.   
     
         5. PacketAvailableButNoContextIntMapping (1)
       (a) Default Value: 0   (b) Description: Specifies the P in the PacketAvailableButNoContextPriorityPInt interrupt, if enabled. The possible values are:
           (1) 0: P is specified by the DefaultPacketPriority register.   (2) 1: P is the RTU priority.   
           
     
         6. StartLoadingRegister (5)
       (a) Default Value: 1   (b) Description: Determines the first GPR register number to be loaded by the RTU when performing the background load of the packet header on the chosen context. In this register, the value (packetPage&lt;&lt;8) (HeaderGrowthOffset&lt;&lt;4) is loaded. The packetNumber is loaded in the next GPR register. The following GPR registers will be used to pre-load the packet header data following PatternMatchingMask0 mask if this feature is enabled.   
     
         7. PreloadMaskNumber (32×5)
       (a) Default Value: mask  31  for all queues (i.e. pre-load of header is disabled).   (b) Description: It specifies, for each of the 32 possible queues in the QS, which mask in the PatternMatchingTable is going to be used for pre-loading.   
     
       
    
       FIGS. 19   a–c  show a mapping of the PreloadMaskNumber configuration register. 
     The configuration registers described above are the boot-time-only configuration registers in the instant example. Immediately below are listed the On-The-Fly configuration registers. 
     Singles-Stream Configuration Registers 
     
         
         1. OverflowEnable (1)
       (a) Default Value: 0   (b) Description: Enables/disables the overflow of packets in case they do not fit into LPM. When disabled, these packets are dropped.   
     
         2. PatternMatchingTable (24×(32×2+1)
       (a) Default Value (per each of the 24 entries):
           (1) SelectVector: select all bytes   (2) RegisterVector: store 4 consecutive bytes per register   (3) EndOfMask: 1   
           (b) Description: It specifies, for masked load/store operations, which bytes to load/store and in which (consecutive) registers. Mask 0 of this table is used by the RTU to pre-load, in the background, some bytes of the header of the packet in one of the available contexts. There are a total of 24 masks.   (c) Note: Mask 0 needs to be written when the PMU is freezed (see Section 0), otherwise results are undefined.   
     
       
    
       FIG. 21  illustrates the PatternMatchingTable described immediately above.
     3. Freeze (1)
       (a) Default Value: 1   (b) Description: Enables/disables the freeze mode.   
       4. Reset (1)
       (a) Default Value: 0   (b) Description: When set to 1, forces the PMU to perform the reset sequence. All packet data in the PMU will be lost. After the reset sequence all the configuration registers will have the default values.
 
Multi-Stream Configuration Registers
   
       1. ClearErrorD (D=0,1)
       (a) Default Value: 0   (b) Description: When written by software (with any data), the packet error condition detected on device identifier D is cleared.   
       2. PacketAvailableButNoContextPriorityPintEnable (8) [P=0 . . . 7]
       (a) Default Value: 0 (for all levels)   (b) Description: Enables/disables the PacketAvailableButNoContextPriorityPint interrupt.   
       3. AutomaticPacketDropIntEnable (1)
       (a) Default Value: 1   (b) Description: Enables/disables the AutomaticPacketDropInt interrupt.   
       4. TimeStampEnable (1)
       (a) Default Value: 0   (b) Description: Enables/disables the time stamp of packets. When enabled and HeaderGrowthOffset is greater than 0, a 4-byte time stamp is appended to the packet before it is written into the packet memory.   
       5. PacketErrorIntEnable (1)
       (a) Default Value: 0   (b) Description: Enables/disables the PacketErrorInt interrupt.   
       6. VirtualPageEnable (9×4)
       (a) Default Value: all virtual pages enabled for all blocks.   (b) Description: Enables/disables the virtual pages for each of the 4 blocks that the LPM is divided into. There are up to 9 virtual pages, from 256 bytes (enabled by the LSB bit) up to 64 K bytes (enabled by the MSB bit), with all power-of-two sizes in between.   
       
       FIG. 22  illustrates the VirtualPageEnable register.
     7. OverflowAddress (24)
       (a) Default Value: 0x40000 (the first atomic page in the EPM)   (b) Description: the 16 MSB bits correspond to the atomic page number in packet memory into which the packet that is overflowed will start to be stored. The 8 LSB are hardwired to ‘0’ (i.e. any value set by software to these bits will be disregarded). OverflowAddress is then the offset address within the 16 MB packet memory. The SIU will translate this offset into the corresponding physical address into the EPM. The first 1K atomic pages of the packet memory correspond to the LPM. If software sets the 16 MSB of OverflowAddress to 0 . . . 1023, results are undefined. When a packet is overflowed, the 16 MSB bits of OverflowAddress become the packetPage for that packet. The SPU allows the next packet overflow when it writes into this configuration register.   
       8. IntIfNoMoreXsizePages (4)
       (a) Default Value: 0xF (i.e. the interrupt will never be generated)   (b) Description: Specifies the index of a virtual page (0:256 bytes, 1:512 bytes, . . . , 8:64 K bytes, 9–15: no virtual page). Whenever the PMMU detects that there are no more virtual pages of that size in all the LPM, the NoMoreThanXSizePagesInt interrupt will be generated to the SPU.   
       9. IntIfLessThanXpacketIdEntries (9)
       (a) Default Value: 0   (b) Description: Minimum number of entries in the QS available for new packet identifiers. If the actual number of available entries is less than this number, an interrupt will be generated to the SPU. If this number is 0, the LessThanXPacketIdEntriesInt interrupt will not be generated.   
       10. DefaultPacketPriority (3)
       (a) Default Value: 0   (b) Description: Provides the priority level for the PacketAvailableButNoContextInt interrupt when PacketAvailableButNoContextMapping is 0.   
       11. ContextSpecificPatternMatchingMask: (8×(32×2))
       (a) Default Value:
           (1) SelectVector: select all bytes   (2) RegisterVector: store 4 bytes in each register (EndONask is hardwired to 1)   
           (b) Description: It specifies, for masked load/store operations, which bytes to load/store and in which (consecutive) registers. Software will guarantee that a stream only access its corresponding context-specific mask.   
       
       FIG. 23  illustrates the ContextSpecificPAtternMAtching mask configuration register.
     12. FirstInputQueue (5)
       (a) Default Value: 0   (b) Description: Specifies the smallest number of the queue into which packets from the PMMU will be inserted.   
       13. SoftwareOwned (4)
       (a) Default Value: 0 (not software owned)   (b) Description: one bit per LPM block. If ‘1’, the block is software owned, which implies that the memory of the block is managed by software, and that the VirtualPageEnable bits for that block are a don&#39;t care.   
       14. MaxActivePackets (32×9)
       (a) Default Value: 256 for each of the queues.   (b) Description: Specifies, for each queue q, a value between 0 and 256 that corresponds to the maximum number of packets within queue q that can be being processed by the SPU.   
       
       FIG. 24  illustrates the MaxActivePackets configuration register.
     15. CodeEntryPoint (32×30)
       (a) Default Value: 0 for each of the queues.   (b) Description: The contents of the CodeEntryPoint register associated to queue q are sent to the SPU when a context is activated which has been pre-loaded with a packet that resides in queue q.   
       16. CodeEntryPointSpecial (30)
       (a) Default Value: 0   (b) Description: The contents of this register are sent to the SPU when a context is activated due to the fact that all the contexts become PMU-owned.   
       17. Bypass Hooks (9)
       (a) Default Value: 0   (b) Description: See  FIG. 32 . Each bit activates one hardware bypass hook. The bypass hook is applied for as many cycles as the corresponding bit in this register is asserted.   
       18. InternalStateWrite (12)
       (a) Default Value: 0   (b) Description: See  FIG. 33 . Specifies one word of internal PMU state. The word of internal state will be available to software when reading the InternalStateRead configuration register. The InternalStateWrite configuration register is only used in one embodiment to debug the PMU.
 
Read-Only Registers
   
       1. SizeOfOverflowedPacket (16)
       (a) Default Value: 0   (b) Description: Whenever the PMU has to overflow a packet, this register will contain the size in bytes of that packet.   
       2. TimeCounter (64)
       (a) Default Value: 0   (b) Description: Contains the number of core clock cycles since the last reset of the PMU.   
       
     The TimeCounter configuration register is illustrated in  FIG. 25 .
     3. StatusRegister (8)
       (a) Default Value: 1   (b) Description: Contains the state of the PMU. This register is polled by the SPU to figure out when the reset or freeze has completed (Freeze and Reset bits), or to figure out the source of packet error per inbound device identifier (Err: 1—error, 0—no error; EPM: 1—error has occurred while packet is overflowed to EPM, 0—error has occurred while packet is being stored in LPM; PSM: 1—error due to a packet size mismatch, 0—error due to a bus error).   
       

       FIG. 26  illustrates the StatusRegister configuration register 
     Interrupts 
     The PMU can interrupt the SPU when certain events happen. Software can disable all these interrupts using some of the configuration registers listed above. Moreover, each stream can individually mask these interrupts, which is the subject of a separate patent application. The list of interrupts that the PMU generate are as follows:
     1. OverflowStartedInt
       (a) Interrupt Condition: When the PMMU cannot store the incoming packet into the LocalPacketMemory, it will overflow the packet to the ExternalPacketMemory through the SIU.   (b) Disable Condition: OverflowEnable=‘0’   
       2. NoMorePagesOfXSizeInt
       (a) Interrupt Condition: When no more free virtual pages of the size indicated in IntIfNoMoreXSizePages are available.   (b) Disable Condition: IntIfNoMoreXSizePages ={10,11,12,13,14,15}.   
       3. LessThanXPacketIdEntriesInt
       (a) Interrupt Condition: When the actual number of available entries in the QS is less than IntIfLessThanXPacketIdEntries.   (b) Disable Condition: IntIfLessThanXPacketIdEntries=0   
       4. PacketAvailableButNoContextPriorityPint (P=0 . . . 7)
       (a) Interrupt Condition: When a packet identifier is received by the RTU from the QS but there is no available context.   (b) Disable Condition: PacketAvailableButNoContextPriorityPIntEnable=‘0’   
       5. AutomaticPacketDropInt
       (a) Interrupt Condition: When a packet cannot be stored in LPM and OverflowEnable=‘0’.   (b) Disable Condition: AutomaticPacketDroplntEnable=‘0’   
       6. PacketErrorInt
       (a) Interrupt Condition: When the actual size of the packet received from the ASIC does not match the value in the first two bytes of the ASIC-specific header, or when a bus error has occurred.   (b) Disable Condition: PacketErrorIntEnable=‘0’   
       

     Interrupts to the SPU in this embodiment are edge-triggered, which means that the condition that caused the interrupt is cleared in hardware when the interrupt is serviced. This also implies that the condition that causes the interrupt may happen several times before the interrupt is served by the SPU. Therefore, the corresponding interrupt service routine will be executed only once, even though the condition that causes the interrupt has happened more than once. 
     This behavior is not desirable for some of the interrupts. For these cases, a special interlock mechanism is implemented in hardware that guarantees that the condition will not happen again until the interrupt has been serviced. 
     An example of the special interlock mechanism is the case of the OverflowStartedInt and PacketAvailableButNoContextPriorityPInt interrupts. In the first case, when a packet is overflowed, no other packet are overflowed until the software writes a new address in the on-the-fly configuration register OverflowAddress. If a packet has been overflowed but the OverflowAddress register still has not been written by the software, any subsequent packet that would have otherwise been overflowed because it does not fit in the LPM must be dropped. 
     For the 8 PacketAvailableButNoContextPriorityPInt (P=0 . . . 7) interrupts, the PMU architecture implicitly guarantees that no multiple conditions (per each P) will occur. This is guaranteed by design since:
         (a) the PacketAvailableButNoContextPriorityPInt interrupt is only generated when a packet identifier of RTU priority P arrives to the RTU, and   (b) at most, only one packet identifier with RTU priority P resides in the RTU.       

     The other interrupts can suffer from the multiple condition effect. Therefore, software should not rely on counting the number of times a given type of interrupt happens to figure out exactly how many times that condition has occurred. 
     Protection Issues 
     The architecture of the PMU in the instant embodiment creates the following protection issues:
     1. An stream could read/write data from a packet other than the one it is processing. An stream has access to all the packet memory, and there is no mechanism to prevent an stream from accessing data from a totally unrelated packet unless the packet memory is mapped as kernel space.   2. Since the configuration registers are memory mapped, any stream could update a configuration register, no matter whether the SPU is in single-stream mode or not. In particular, any stream could freeze and reset the PMU.   3. Whenever a packet is completed or moved with reactivation, nothing prevents software from continuing “processing” the packet.
 
Command Unit (CU)
   

     Software can update some information that the PMU maintains for a given packet and obtain this information. This is accomplished by software through some of the new XStream packet instructions referred to above. Some of these instructions are load-like in the sense that a response is required from the PMU. Others are store-like instructions, and no response is required from the PMU. 
       FIG. 27  is a diagram of Command Unit  213  of  FIG. 2 , in relation to other blocks of the XCaliber processor in this example, all of which bear the same element numbers in  FIG. 27  as in  FIG. 2 . The SPU dispatches, at most, two packet instructions per cycle across all contexts (one instruction per cluster of the SPU). The type of the packet instruction corresponds to the PMU block to which the instruction affects (PMMU, QS or RTU). When the SPU dispatches a packet instruction, a single command to the PMU is generated and inserted into one of three different queues in the CU block (one queue per PMU block to which the command goes). Commands to the PMU are issued to PMMU command queue  2703 , those to the QS go to QS command queue  2705 , and command to the RTU go to the RTU command queue  2707 . Each queue can hold up to 8 commands. The SPU only dispatches a command to the CU if there are enough free entries in the corresponding queue. 
     The CU is responsible for dispatching the commands to the respective blocks, and gathering the responses (if any) in an 8-entry ResponseQueue  2709 , which queues responses to be returned to the SPU. The CU can receive up to three responses in a given cycle (one from each of the three blocks). Since (a) only one outstanding packet instruction is allowed per stream, (b) the Response Queue has as many entries as streams, (c) only one command to the PMU is generated per packet instruction, and (d) only one response is generated per each load-like command, it is guaranteed that there will be enough space in the ResponseQueue to enqueue the responses generated by the PMU blocks. The ResponseQueue should be able to enqueue up to two commands at a time. 
     CU  213  also receives requests from SIU  107  to update the configuration registers. These commands are also sent to the PMMU, RTU and QS blocks as commands. The PMMU, QS, and RTU keep a local copy of the configuration registers that apply to them, The CU keeps a copy as well of all the configuration registers, and this copy is used to satisfy the configuration register reads from the SIU. 
     For read-only configuration registers, a special interface is provided between the CU and the particular unit that owns the read-only configuration register. In XCaliber&#39;s PMU, there exists two read-only configuration registers: one in the PMMU block (SizeOfOverflowedPacket) and the other one in the CU block (StatusRegister). Whenever the PMMU writes into the SizeOfOverflowedPacket register, it notifies the CU and the CU updates its local copy. 
     Commands in different queues are independent and can be executed out of order by the PMU. Within a queue, however, commands are executed in order, and one at a time. The PMU can initiate the execution of up to 3 commands per cycle. The PMMU and QS blocks give more priority to other events (like the creation of a new packetPage when a new packet arrives -PMMU-, or the extraction of a packet identifier because it needs to be sent out -QS-) than to the commands from the SPU. This means that a command that requests some data to be sent back to the SPU may take several cycles to execute because either the PMMU or QS might be busy executing other operations. 
     RTU  227  has two sources of commands: from the QS (to pre-load packet information into an available context) and from the SPU (software command). The RTU always gives more priority to SPU commands. However, the RTU finishes the on-going context pre-load operation before executing the pending SPU command. 
     Command/Response Formats 
     A command received by the CMU has three fields in the current embodiment:
     1. Context number, which is the context associated to the stream that generated the command.   2. Command opcode, which is a number that specifies the type of command to be executed by the PMU.   3. Command data, which is the different information needed by the PMU to execute the command specified in the command opcode field.   

     The PMU, upon receiving a command, determines to which of the command queues the command needs to be inserted. A command inserted in any of the queues has a similar structure as the command received, but the bit width of the opcode and the data will vary depending on the queue. The table of  FIG. 28  shows the format of the command inserted in each of the queues. Not included are the Read Configuration Register and Write Configuration Register commands that the CU sends to the PMMU, QS and RTU blocks. 
     Each command that requires a response is tagged with a number that corresponds to the context associated to the stream that generated the command. The response that is generated is also tagged with the same context number so that the SPU knows to which of the commands issued it belongs. 
     As described above, there is only one ResponseQueue  2709  ( FIG. 27 ) that buffers responses from the three PMU blocks. Note that there is no need to indicate from which block the response comes since, at most, one packet instruction that requires a response will be outstanding per stream. Therefore, the context number associated to a response is enough information to associate a response to a stream. 
       FIG. 29  is a table showing the format for the responses that the different blocks generate back to the CU. Not included in the table are the configuration register values provided by each of the blocks to the CU when CU performs a configuration register read. 
     The RTU notifies the SPU, through a dedicated interface that bypasses the CU (path  2711  in  FIG. 27 ), of the following events:
     1. A masked load/store operation has finished. The interface provides the context number.   2. A GetContext has completed. The context number associated to the stream that dispatched the GetContext operation, and the context number selected by the RTU is provided by the interface. A success bit is asserted when the GetContext succeeded; otherwise it is de-asserted.   3. A pre-load either starts or ends. The context number and the priority associated to the packet is provided to the SPU.
 
Reset and Freeze Modes
   

     The PMU can enter the reset mode in two cases:
     1. SPU sets the Reset configuration flag.   2. XCaliber is booted.   

     The PMU can also enter the freeze mode in two cases:
     1. SPU sets the Freeze configuration flag.   2. PMU finishes the reset sequence.   

     The reset sequence of the PMU takes several cycles. During this sequence, the Reset bit in the StatusRegister configuration register is set. After the reset sequence, all the configuration registers are set to their default values, and the PMU enters the freeze mode (the Reset bit in the StatusRegister is reset and the Freeze bit is set). When this is done, the SPU resets the Freeze configuration flag and, from that time on, the PMU runs in the normal mode. 
     When the SPU sets the Freeze configuration flag, the PMU terminates the current transaction or transactions before setting the Freeze bit in the StatusRegister. Once in the freeze mode, the PMU will not accept any data from the network input interface, send any data out through the network output interface, or pre-load any packet 
     The PMU continues executing all the SPU commands while in freeze mode. 
     The SPU needs to poll the StatusRegister configuration register to determine in which mode the PMU happened to be (reset or freeze) and to detect when the PMU changes modes. 
     The CU block instructs the rest of the blocks to perform the reset and the freeze. The following is the protocol between the CU and any other block when the CU receives a write into the reset and/or freeze configuration bit:
     1. The CU notifies to some of the blocks that either a freeze or a reset needs to be performed.   2. Every block performs the freeze or the reset. After completion, the block signals back to the CU that it has completed the freeze or reset.   3. The CU updates the StatusRegister bits as soon as the reset or freeze has been completed. Software polls the StatusRegister to determine when the PMU has completely frozen.
 
The different blocks in the PMU end the freeze when:
   1. IB, LPM, CU and QS do not need to freeze.   2. As soon as the PMMU finishes uploading inbound packets, if any, and downloading outbound packets, if any.   3. As soon as the RTU has finished the current pre-load operation, if any.   4. As soon as the OB is empty.
 
While in freeze mode, the blocks will not:
   1. start uploading a new packet; start downloading a completed packet; or generate interrupts to the SPU (PMMU)   2. pre-load a context or generate interrupts to the SPU (RTU).   

     If software writes a ‘1’ in the Freeze/Reset configuration register and then writes a ‘0’ before the PMU froze or reset, results are undefined. Once the PMU starts the freeze/reset sequence, it completes it. 
     Performance Counters Interface 
     The PMU probes some events in the different units. These probes are sent to the SIU and used by software as performance probes. The SIU has a set of counters used to count some of the events that the PMU sends to the SIU. Software decides which events throughout the XCaliber chip it wants to monitor. Refer to the SIU Architecture Spec document for more information on how software can configure the performance counters. 
       FIG. 30  shows a performance counter interface between the PMU and the SIU. Up to 64 events can be probed within the PMU. All 64 events are sent every cycle to the SIU (EvenVector) through a 64-bit bus. 
     Each of the 64 events may have associated a value (0 to 64K−1). Software selects two of the events (EventA and EventB). For each of these two, the PMU provides the associated 16-bit value (EventDataA and EventDataB, respectively) at the same time the event is provided in the EventVector bus. 
     Events are level-triggered. Therefore, if the PMU asserts the event for two consecutive cycles, the event will be counted twice. The corresponding signal in the EventVector will be asserted only if the event occurs, and for as many cycles as the event condition holds. 
     The SIU selects which events are actually counted (based on how software has programmed the SIU). If the SIU decides to count an event number different from EventA or EventB, a counter within the SIU counts the event for as many cycles the corresponding bit in the EventVector is asserted. If the events monitored are EventA and/or EventB, the SIU, in addition to counting the event/s, increments another counter by EventDataA and/or EventDataB every time the event occurs. 
       FIG. 31  shows a possible implementation of the internal interfaces among the different blocks in PMU  103 . CU acts as the interface between the PMU and SIU for the performance counters. CU  213  distributes the information in EventA and EventB to the different units and gathers the individual EventVector, EventDataA and EventDataB of each of the units. 
     The CU block collects all the events from the different blocks and send them to the SIU. The CU interfaces to the different blocks to notify which of the events within each block need to provide the EventDataA and/or EventDataB values. 
     Performance events are not time critical, i.e. they do not need to be reported to the SIU in the same cycle they occur. 
       FIGS. 34 through 39  comprise a table that lists all events related to performance counters. These events are grouped by block in the PMU. The event number is shown in the second column. This number corresponds to the bit in the EventVector that is asserted when the event occurs. The third column is the event name. The fourth column shows the data value associated to the event and its bit width in parentheses. The last column provides a description of the event. 
     The CU block collects all of the events from the different blocks and sends them to the SIU. The CU interfaces to the different blocks to notify which of the events within each block need to provide the EventDataA and the EventDataB values. 
     Performance events are not time critical, i.e. they do not need to be reported to the SIU in the same cycle that they occur. 
     Debug Bypasses and Trigger Events 
     Hardware debug hooks are implemented in the PMU to help debugging of the silicon. The debug hooks are divided into two categories:
     1. Bypass hooks: will bypass potentially faulty functions. Instead of the faulty results generated by these functions (or, in some cases, no result at all), the bypass hook will provide at least some functionality that will allow other neighboring blocks to be tested.   2. Trigger events: when a particular condition occurs in the PMU (trigger event), the PMU will enter automatically in single-step mode until, through the OCI Interface (Section), the SIU sends a command to the PMU to exit the single-step mode.   

     Moreover, the PMU has the capability of being single-stepped. A signal (SingleStep) will come from the OCI Interface. On a cycle-by-cycle basis, the different blocks of the PMU will monitor this signal. When this signal is de-asserted, the PMU will function normally. When SingleStep is asserted, the PMU will not perform any work: any operation on progress will be held until the signal is de-asserted. In other words, the PMU will not do anything when the signal is asserted. The only exception to this is when a block can lose data (an example could be in the interface between two block: a block A sends data to a block B and assumes that block B will get the data in the next cycle; if SingleStep is asserted in this cycle, block B has to guarantee that the data from A is not lost). 
     Bypass Hooks 
     The different bypass hooks in the PMU are activated through the on-the-fly BypassHooks configuration register.  FIG. 40  is a table illustrating the different bypass hooks implemented in the PMU. The number of each hook corresponds to the bit number in the BypassHooks register The bypass hook is applied for as many cycles as the corresponding bit in this register is asserted. 
     Trigger Events 
     The following is a list of trigger events implemented in the PMU.
     1. A new packet of size s bytes is at the head of the IBU.
       (a) s=0: any packet.   
       2. A packetId from source s with packetPage pp is inserted in queue q in the QS.
       (a) s=0: PMM, s=1: QS, s=2: CMU; s=3: any   (b) pp=0x10000: any   (c) q=33: any   
       3. A packetId from queue q with packetPage pp and packetNumbet pn is sent to RTU.
       (a) pp 0x10000: any   (b) q=33: any   (c) pn=256: any   
       4. A packetId with packetPage pp and packetNumber pn reaches the head of queue q in the QS.
       (a) pp=0x10000: any   (b) q=33: any   (c) pn=256: any   
       5. A packet with RTU priority p and packetPage pp and packetNumber pn is pre-loaded in context c.
       (a) pp=0x10000: any   (b) q=33: any   (c) pn=256: any   (d) c=8:any   
       6. A packetId from queue q with packetPage pp and packetNumber pn is sent for downloading to PMM.
       (a) pp 0x10000: any   (b) q=33: any   (c) pn=256: any   
       7. A packetId with packetPage pp and packetNumber pn reaches the head of queue q in the QS.
       (a) pp=0x10000: any   (b) q=33: any   (c) pn=256: any   
       8. Packet command pc is executed by block b.
       (a) pc=0: GetSpace; pc=1: FreeSpace; pc=2: InsertPacket; pc=3: ProbePacket; pc=4: ExtractPacket; pc=5: CompletePacket; pc=6: UpdatePacket; pc=7: MovePacket; pc=8: ProbeQueue; pc=9: GetContext; pc=10: ReleaseContext; pc=11: MaskedLoad; pc=12: MaskedStore; pc=13: any   b) b=0: RTU; b=1: PMM; b=2: QS; b=3: any
 
Detailed Interfaces with the SPU and SIU
   
       

     The architecture explained in the previous sections is implemented in the hardware blocks shown in  FIG. 41 : 
     SPU-PMU Interface 
       FIGS. 42–45  describe the SPU-PMU Interface. 
     SPU-PMU Interface 
       FIGS. 46–49  describe the SIU-PMU Interface. 
     The specification above describes in enabling detail a Packet Memory Unit (PMU) for a Multi-Streaming processor adapted for packet handling and processing. Details of architecture, hardware, software, and operation are provided in exemplary embodiments. It will be apparent to the skilled artisan that the embodiments described may vary considerably in detail without departing from the spirit and scope of the invention. It is well-known, for example, that IC hardware, firmware and software may be accomplished in a variety of ways while still adhering to the novel architecture and functionality taught. 
     Insulative Packet Storage 
     In one aspect of the present invention, the inventor provides a method for insuring that consecutively stored data packets are positioned in memory in such a way that they may be allowed to grow in size to a certain amount during processing without requiring packet move operations. 
     As previously described the hardware-controlled Local Packet Memory is partitioned into sectors of 256 bytes. Therefore, under normal circumstances, if a packet of 257 bytes arrives, that packet will be assigned to two separate available 256 byte regions, one of which will be occupied by a single byte. If the software is aware of the 256 byte partition, then there is a built-in tail growth offset for this packet, and if the packet grows the new data can be stored in the second of the two assigned regions. It is, however, more desirable that the software not be aware of the partition size of the hardware-controlled memory, because, in new and later implementations of the system the partition size might change, then the software would not work. 
     A software-configurable header growth and tail growth offset is then desirable because it hides the hardware implementation from the software, and allows software to be safely ported to a next generation. Such a configurable header and tail growth offset system is described below. 
     Referring now to  FIG. 8  of Ser. No. 09/737,375, a portion of packet memory is illustrated containing a data packet for processing. It is described above with regard to  FIG. 8  that a data packet can grow or shrink as a result of SPU processing. Moreover, a data packet may also retain the same size in bytes of data.  FIG. 8  illustrates both head growth and shrink as well as tail growth and shrink possibilities. There may also be a case where the header of a data packet grows during processing while the tail portion shrinks, and so on. It is described in Ser. No. 09/737,375 that the PMU can be configured to allocate extra memory space acting as insulation against a packet overwriting a next packet stored in the memory. It has also been described that virtual and atomic pages are allocated by the PMMU for the purpose of noting where in memory packets will be stored. An offset from a beginning address denotes the beginning of the data packet and the size of the data packet determines how many pages of memory it will occupy. Therefore the PMMU can select a space in memory beginning with an offset that is large enough to hold the data given the known size of a data packet. 
       FIG. 50  is a block diagram illustrating a portion of memory space  5000  for holding data packets according to prior art. Packet memory  5000  is illustrated herein and is analogous to the packet memory described with respect to  FIG. 8  above. This example is intended to illustrate possible overwriting of data resulting in corrupt data packets. Packet memory  5000  can be assumed in this example to be local packet memory (LPM) described in Ser. No. 09/737,375 as hardware-controlled memory. Memory  5000  has three exemplary data packets stored therein. These are data packets  5001  (Packet B),  5002  (Packet A), and 5003 (packet C). Data packets  5001 – 5003  will hereinafter be referred to by their alphabetical labels A–C. 
     Data packets B and C are illustrated as crosshatched diagonally in the same direction. Data packet A is illustrated as crosshatched diagonally in the reverse direction from the crosshatching of packets B and C. In this example, it is packet A that is currently being processed. During processing, packet A has a Packet A head growth denoted by X and a Packet A tail growth denoted herein by Y. It can be seen in this example that the growth of packet A has forced an overwrite of data into the portion of memory  5000  occupied by packet B and into the portion of memory  5000  occupied by packet C. Had packet A stayed the same size or shrunk during processing, there would be no conflict. However, the illustrated overwrites denoted by X and Y cause corruption of both packets B and C. 
     In actual practice, the overwrite would not take place, because software running in the SPU would have moved packet A into another location of memory large enough to accept the packet growth that occurred during processing, before actually overwriting data into allotted memory space. It is described above with reference to  FIG. 8  that a packet move operation contains the following steps:
     1. The software running on the SPU requests from the PMU for a chunk of contiguous space of the new packet size. The PMU will return a new packetPage that identifies (points to) this new space.   2. The SPU software writes the data into the new memory space.   3. The SPU software renames the old packetPage with the new packetPage.   4. The SPU software requests the PMU to de-allocate the space associated to the old packetPage.   5.   

     It can be readily seen by one with skill in the art that many packet move operations would certainly require considerable processing resources. 
     To enable allocation of extra memory space to insure space for a packet to grow during processing, two pre-boot configuration registers (not shown) are provided. These are termed a TailGrowthOffset register and a HeadGrowthOffset register. These configuration registers in a preferred embodiment have a digital value indicating the number of 16 byte-wide lanes assigned to head and tail offset. For example, in one embodiment the HeadGrowthOffset is 4 bits, to allow for 16 16-byte lines, and the TailGrowthOffset is 6 bits to account for 63 16-byte lines. The goal of the present invention is to provide suitable memory space for a packet to grow during processing such that consecutively stored data packets do not overwrite each other, or that packet move operations can be sharply reduced or avoided altogether. 
       FIG. 51  is a block diagram illustrating a portion of memory space  5100  for holding consecutively stored data packets according to an embodiment of the present invention. Packet memory  5100  is analogous to packet memory  5000  described with regards to  FIG. 50  above with the exception of how data packets are stored. 
     In this example data packet B ( 5001 ) has a portion of memory allocated thereto in addition to the portion allocated for the actual size of packet B. A portion represented herein by the letter h is allocated for possible head growth of packet B during processing. A portion represented herein by the letter t is allocated for possible tail growth of packet B. The actual data size of packet be can be represented by the letter S (not shown). Therefore, the amount of memory  5100  allocated for data packet B can be expressed as M=(s+h+t) where M is the total memory allocated. The same description above applies equally to packets A and B. 
     Data packets A and C exhibit the same buffer zones h and t bounding s (original packet size). It is noted herein that in actual practice, the values of h and t are pre-configured at boot time and remain constant values for all incoming data packets whether stored in hardware or software-controlled memory. For example, the value of h is the same for packets A, B and C. Similarly, the value of t remains constant for all packets as well. The only variable is s, which is the starting size of any packet before processing. However, in another embodiment higher levels of granularity may be introduced. For example, all VoIP packets may have specific h and t values while all other packet types have constant h and t values different from the VoIP values. In this case, additional configuration registers would be provided for isolating specific types of data packets. This embodiment, of course, assumes that the packet type is known at the time of memory allocation for storage. 
     It is described above with reference to Ser. No. 09/737,375, that the PMMU is responsible for memory allocation and for determining if and where in local packet memory a packet will be stored. The PMMU handles memory allocation in hardware-controlled LPM. Software in the SPU handles memory allocation in the event that it is determined that a packet does not fit in LPM. Software can access the offset registers containing h and t values, so that the allocation of extra memory space for packet growth is practiced for all of memory  5100 . 
     In actual practice, the values of h and t are configurable as follows for application to all preprocessed data packets. A configuration register HeadGrowthOffset (h) has a minimum value of 0 and a maximum value of 240 bytes of memory in a preferred embodiment. Another configuration register TailGrowthOffset (t) has a minimum value of 0 and a maximum value of 1008 bytes of memory in the same embodiment. These values are stored in the previously mentioned registers associated with h and t during boot. When a data packet arrives, the PMMU consults the registers to read out the values of h and t and compute the values along with the value of s (original packet size) to produce M. The PMMU will look for a space in memory  5100  that can accommodate the total computed size M. It is noted herein that the PMMU still only needs to generate a single offset to store the data packet because h, t, and s are all known at the time of page allocation. The generated offset is from the initial address where the packet is to be stored. 
     Packets in many cases will not grow to fill the allocated memory space h and t. In some cases, a packet may actually shrink after processing. Moreover, consecutively stored packets may exhibit differing growth amounts. The only time a packet move operation is required is if the head grows more than h or the tail grows more than t. If SPU software is charged with storing a data packet, the PMU can provide the required values of h and t to the SPU because it has already consulted the registers when determining that a particular packet did not fit memory  5100 . In a case where SPU software is solely responsible for storage of a packet, then it may access and read the registers independently from PMU hardware. 
     In one aspect of the present invention, there may be a plurality of different registers holding h and t values for a plurality of differing types of data packets. Also in one embodiment, changed values of h and t may be introduced into the registers while the system is running and processing data. In this case new values may be determined dynamically, based on such as historical performance. Keeping historical performance records can be accomplished in order to refine predicted packet growth for differing types of data packets. 
     It has been previously described that at some point processing is complete, and the packet may be downloaded from the LPM to be sent out of the system. This downloading is a function of the PMMU. Clearly, when download commences, the PMMU has to read the header growth offset for a packet, to determine where the packet really starts in LPM. For example, if a packet, ready for download, was stored with a header growth offset, but the packet header has not grown or shrunk in processing, then the first data for the packet will be at the beginning of the first page allocated, pls the header growth offset. 
     If the header for a packet has grown or shrunk, the software needs to notify the PMMU of the magnitude of the header change so the PMMU can find the beginning of the packet in LPM. For example, if the header grew by X, the first packet data will be at the beginning of the first page allocated, plus the offset, less the amount of the growth, and so on. 
     Additionally, in those embodiments wherein offsets may change dynamically, that is, on-the-fly, there are mechanisms to ensure that offset changes are made only at times that the changes will not create confusion regarding the actual starting point of a packet in memory at download time. 
     Accordingly the claims that follow should be accorded the broadest interpretation.