Patent Publication Number: US-9413665-B2

Title: CPP bus transaction value having a PAM/LAM selection code field

Description:
TECHNICAL FIELD 
     The described embodiments relate generally to the receiving of packet data from multiple sources, to the managing and storage of the packet data into a single memory, and to the forwarding of the packet data to a processing circuit. 
     BACKGROUND INFORMATION 
     A large integrated packet processing device such as a network flow processor integrated circuit may include multiple smaller specialized processors, where each such smaller processor is specially adapted and designed and/or specially programmed to do a particular type of packet processing. The large integrated device may receive packets on several different input ports, and may do initial processing on those packets in different initial processing circuits located in different places on the integrated circuit. For further processing, each packet then passes from one of the initial processing circuits to an appropriate one or more of the specialized processors. After processing by a specialized processor, a packet is forwarded to one of a plurality of output processing circuits, that in turn causes the packet to be output from the integrated circuit. If a packet received on a first input port and a first associated initial processing circuit is of a particular type, then it may be forwarded to one particular specially adapted processor. If another packet of a different type is received onto the same first input port, then that packet may be forwarded to another of the specialized processors. Similarly, a packet received on a second input port and a second associated initial processing circuit may be of the type that is to be further processed by same first specially adapted processor. In this way, some of the packets received on several different input ports and initial processing circuits may all be forwarded to the same one specialized processor. These packets may, for example, be written into different queues in a holding memory, or may be received by different ports of a multi-ported holding memory, or may be pushed into cooperating FIFO holding memories, or may be loaded into assigned buffers in a holding memory. Once a packet is in the holding memory, the specialized processor accesses the packet and performs the necessary further processing. The specialized processor is somehow made aware of the presence of the packet in the holding memory, or is otherwise provided with the packet. Throughput can be increased in a variety of ways, including by providing multiple specialized processors that perform the same specialized task. After being processed by a specialized processor, the packet is forwarded to an appropriate one of the output processing circuits, and is then output from the integrated circuit. 
     SUMMARY 
     Within a networking device such as an Island-Based Network Flow Processor (IB-NFP) integrated circuit, packet portions from multiple PDRSDs (Packet Data Receiving and Splitting Devices) are to be loaded into a single memory, so that the packet portions can later be processed by a processing device and then be output from the networking device. Rather than the PDRSDs managing and handling the storing of packet portions into the memory, a packet engine local to the memory is provided. 
     In a first novel aspect, the PDRSDs use a PPI (Packet Portion Identifier) Addressing Mode (PAM) in communicating across a Command/Push/Pull (CPP) bus with the packet engine and in instructing the packet engine to store packet portions. The packet engine uses linear memory addressing (in a Linear Address Mode (LAM)) to write the packet portions into the memory, and to read the packet portions from the memory. 
     In a second novel aspect, the packet engine allocates PPIs in response to PPI allocation requests. There are a fixed number of PPIs. Each PPI can be “in use” or “not in use” at given time. A PDRSD that has a packet portion, and that wants to have the packet portion loaded into the memory, sends a “PPI allocation request” across the CPP bus to the packet engine. The PPI allocation request includes an indication of the size of the packet portion to be stored. The packet engine uses this information to determine if there is adequate space in the memory and if a PPI is available. If the packet engine determines that a PPI is available and that there is adequate space in the memory for the packet portion, then the packet engine allocates an unused PPI and sends the newly allocated PPI to the requesting PDRSD across the CPP bus in a “PPI allocation response”. The PDRSD receives the allocated PPI, and sends the packet portion across the CPP bus to the packet engine tagged with the PPI. The PDRSD does not know where the packet portion will be stored in the memory, but rather supplies the PPI. The packet engine receives the PPI tagged to the packet portion, translates the PPI into a memory address or addresses, and then writes the packet portion into the memory using the memory address or addresses. 
     In a third novel aspect, a CPP bus transaction value has a PAM/LAM (PPI Addressing Mode/Linear Addressing Mode) mode selection bit. The CPP bus transaction value may, for example, be an “autopush” to push data to a target device across the CPP bus. If the PAM/LAM bit is set, then PAM addressing is indicated and a PPI value carried in the CPP bus transaction value is translated by the target device into a memory address, and the target then uses the memory address to store the data carried by the autopush into memory. If, on the other hand, the PAM/LAM bit is not set, then LAM addressing is indicated and an address value carried in the CPP bus transaction value is used to write the data carried by the CPP bus transaction value into the memory. The novel PAM/LAM mode selection bit is not limited to use with the CPP bus or in push bus transaction values, but rather sees general applicability and can be included in many different types of commands and instructions. A PAM/LAM mode selection bit can be part of a CPP read or write bus transaction value, such that the target of the receives a PPI and uses PPI addressing to perform the indicated read or write. A PAM/LAM mode selection bit can be part of an initial command of a bus transaction value exchange, or the PAM/LAM mode selection bit can be part of a later bus transaction value of the exchange. In one example, if PAM is selected then a first part of a memory is being accessed where a second part of the memory cannot be accessed, whereas if LAM is selected that the second part of the memory is being accessed but the first part of the memory cannot be accessed. Which part of the memory is being accessed is therefore determined by the addressing mode selected. CPP bus masters can transition between PPI addressing and linear addressing in a straightforward manner, with each part of the memory being addressed in the appropriate way. 
     In a fourth novel aspect, once processing has been completed on a packet portion and once the packet portion has been read out of the memory and is ready for outputting from the networking device, the egress device sends a “PPI de-allocate command” to the packet engine. The PPI de-allocate command includes an indication of the PPI to be de-allocated, but the de-allocate command does not include a number of credits to be returned, nor does it indicate the “owner” of the PPI being de-allocated. When the packet engine receives the PPI de-allocate command, the packet engine de-allocates the PPI by changing its used/not used status in a PAT (PPI Allocation Table) circuit from “used” to “not used”. In addition, the packet engine determines the original requestor of the PPI (“owner”) and determines the amount of buffer space in memory that was previously allocated (to accommodate storing the associated packet portion) but that now is available for other use. In response to receiving the PPI de-allocate command, the packet engine records the freed up buffer space as being available. Each PDRSD requestor is credit-aware in that it maintains a PPI “Credits Available” value and a Buffer “Credits Available” value. The PDRSD will only make a PPI allocation request if the PDRSD determines that is has adequate PPI credits available and has adequate buffer credits available. The PPI de-allocate operation results in one more PPIs being freed up (one PPI credit), and results in a certain amount of buffer credits of buffer space being free up. The packet engine sends a communication to the requesting PDRSD returning the freedup PPI credit and a buffer credit if appropriate. The requesting PDRSD adds the returned PPI credit to the PPI “Credit Available” value it maintains, and adds any returned buffer credit to the Buffer “Credit Available” value it maintains. 
     In a specific example, the memory is logically sectioned into 2K byte slices. Each 2K byte is one buffer credit, even though the packet portion size may be smaller (256 bytes, 212 bytes, 1K bytes, or 2K bytes). Each 2K byte slice is only permitted to be used by one “owner”. The 2K byte slice can, however, store more than one packet portion from the same requestor (same “owner”). If a requestor has adequate PPI and buffer credits as indicated by its stored PPI “Credits Available” value and its stored Buffer “Credits Available” value, and if the requestor has a packet portion to send to the packet engine, then the requestor subtracts one PPI credit and one buffer credit (the buffer credit is worth 2K bytes) from its stored credits available values, regardless of the size of the packet portion. The packet engine receives the PPI allocation request, and attempts to place the indicated size of the packet portion into unused space in a buffer that is already being used by the requestor but is only partly used. If the packet engine is successful, then the packet engine makes the PPI allocation and returns the one buffer credit to the requestor with the PPI allocation response. The requestor adds the returned one buffer credit back to its buffer “Credits Available” value. The allocation therefore did not cost the requestor any buffer credits because the packet portion will be stored into a buffer already assigned to the same requestor. If, however, the packet engine is not successful and the packet portion will have to be stored in a buffer that was not previously already assigned to the requestor, then the packet engine makes the PPI allocation and logs the requestor as being the “owner” of the newly assigned buffer and does not return the buffer credit to the requestor with the PPI allocation response. 
     Further details and embodiments and methods and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  is a diagram of a memory system having a packet engine, where the packet engine uses PPI addressing. 
         FIG. 2  is a flowchart that illustrates a method of allocating PPIs using PPI allocation requests and PPI allocation responses. 
         FIG. 3  is a diagram that illustrates a memory system that carries out the method of  FIG. 2 . 
         FIG. 4  is a diagram of a CPP bus transaction value that has a PPI Addressing Mode (PAM)/Linear Addressing Mode (LAM) selection code field. 
         FIG. 5  is a diagram that sets forth various fields of a PPI de-allocate command. 
         FIG. 6A  is a part of a larger  FIG. 6 , where  FIG. 6  is a flowchart of a method involving a PPI de-allocate request command. 
         FIG. 6B  is a part of the larger  FIG. 6 . 
         FIG. 6C  is a part of the larger  FIG. 6 . 
         FIG. 7  is a diagram of a credit-based PPI-addressed memory system that uses the PPI de-allocate command of  FIG. 7 . 
         FIG. 8  is a diagram of an Island-Based Network Flow Processor (IB-NFP) integrated circuit that has a packet engine in accordance with one novel aspect. 
         FIG. 9  is a diagram of a SerDes circuit in the IB-NFP integrated circuit of  FIG. 8 . 
         FIG. 10  is a diagram of an ingress MAC island in the IB-NFP integrated circuit of  FIG. 8 . 
         FIG. 11  is a diagram of an ingress NBI island in the IB-NFP integrated circuit of  FIG. 8 . 
         FIG. 12  is a table that sets forth the various components of the preclassification results generated by the picoengine pool in the ingress NBI island of  FIG. 11 . 
         FIG. 13  is a table that sets forth the various components of the ingress packet descriptor as output by the ingress NBI island of  FIG. 11 . 
         FIG. 14  is a diagram of an ME island in the IB-NFP integrated circuit of  FIG. 8 . 
         FIG. 15  is a table that sets forth the various components of an egress packet descriptor. 
         FIG. 16  is a diagram of an MU island in the IB-NFP integrated circuit of  FIG. 8 . 
         FIG. 17  is a diagram of an egress NBI island in the IB-NFP integrated circuit of  FIG. 8   
         FIG. 18  is a diagram of an egress MAC island in the IB-NFP integrated circuit of  FIG. 8   
         FIG. 19  is a flowchart that illustrates steps involved in a CPP write operation. 
         FIG. 20  is a diagram of a CPP bus transaction value. 
         FIG. 21  is a table that sets forth the various fields in a command payload of a CPP bus command. 
         FIG. 22  is a table that sets forth the various fields in a pull-id payload of a CPP bus transaction. 
         FIG. 23  is a table that sets forth the various fields in a data payload of a CPP bus transaction. 
         FIG. 24  is a table that sets forth the various fields of a CPP data payload in the case of a pull. 
         FIG. 25  is a table that sets forth the various fields of a CPP data payload in the case of a push. 
         FIG. 26  is a flowchart that illustrates steps involved in a CPP read operation. 
         FIG. 27  is a diagram of the CTM (Cluster Target Memory) in the ME island of  FIG. 14 . 
         FIG. 28  is a diagram that illustrates an operation of the PPI Allocation Table circuit (PAT) in the packet engine of the CTM of  FIG. 27 . 
         FIG. 29  is a diagram that illustrates an operation of the Memory Allocation Table circuit (MAT) in the packet engine of the CTM of  FIG. 27 . 
         FIG. 30  is a diagram that sets forth various fields of a PPI allocation request command. 
         FIG. 31  is a table that sets forth the various fields of the PPI allocation request command of  FIG. 30 . 
         FIG. 32  is a diagram that sets forth various fields of a PPI allocation response bus transaction value. 
         FIG. 33  is a table that sets forth the various fields of the PPI allocation response of  FIG. 32 . 
         FIG. 34  is a diagram that sets forth various fields of a CPP bus transaction value that has a PAM/LAM selection bit. 
         FIG. 35  is a table that sets forth the various fields of the CPP bus transaction value of  FIG. 34 . 
         FIG. 36  is a diagram that sets forth various fields of a packet complete CPP command. 
         FIG. 37  is a table that sets forth the various fields of the packet complete CPP command of  FIG. 36 . 
         FIG. 38  is a diagram that sets forth various fields of a PPI de-allocate CPP command. 
         FIG. 39  is a table that sets forth the various fields of the PPI de-allocate CPP command of  FIG. 38 . 
         FIG. 40A  is a part of a larger  FIG. 40 , where  FIG. 40  is a block diagram of the packet engine in the CTM of  FIG. 27 . 
         FIG. 40B  is a part of the larger  FIG. 40 . 
         FIG. 41  is a diagram of one of the “find first in a slice” circuits of  FIG. 40 . 
         FIG. 42A  is a part of a larger  FIG. 42 , where  FIG. 42  sets forth CDL hardware description language for the state machine in the packet engine of  FIG. 40 . 
         FIG. 42B  is a part of the larger  FIG. 42 . 
         FIG. 42C  is a part of the larger  FIG. 42 . 
         FIG. 42D  is a part of the larger  FIG. 42 . 
         FIG. 42E  is a part of the larger  FIG. 42 . 
         FIG. 42F  is a part of the larger  FIG. 42 . 
         FIG. 42G  is a part of the larger  FIG. 42 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to background examples and some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  is a diagram of a packet engine that uses PPI (Packet Portion Identifier) addressing in accordance with a first novel aspect. An integrated circuit  1  includes a first Packet Data Receiving and Splitting Device (PDRSD)  2 , a second PDRSD  3 , a novel memory system  4 , and a processing device  5 , and an external memory unit interface circuit  6 . The external memory unit interface circuit is coupled to an external memory  7 . Both the external memory unit interface circuit  6  and the external memory  7  may be referred to together as a memory system  8 . The novel memory system  4  includes a memory  9  and a packet engine  10 . Packets or parts of packets are received onto the integrated circuit  1  and pass to the PDRSD  2 . Each such packet or part of a packet may be large, and memory resources on the integrated circuit may be scarce, so a first portion of each packet data is forwarded from the first PDRSD  2  to the memory system  4 , whereas a second portion of the packet data is stored in external memory  7 . Likewise, packets or parts of packets are received onto the integrated circuit  1  and pass to the second PDRSD  3 . A first portion of each such amount of packet data is forwarded from the second PDRSD  3  to the memory system  4 , whereas a second portion of the packet data is stored in the external memory  7 . In a specific example, first packet data  11  (PD#1) is received onto the first PDRSD  2  and is split. A first portion (PART#1 of PD#1)  12  of the first packet data  11  is sent to the memory system  4  along with a first PPI (9PPI#1). The first PPI is associated with and identifies the first packet data  11 . A second portion of the first packet data (PART#2 of PD#1)  13  is sent to the external memory unit interface circuit  6  and is stored in external memory  7 . In the specific example, second packet data PD#2  14  is received onto the second PDRSD  3  and is split. A first portion of the second packet data (PART#1 of PD#2)  15  is sent to the memory system  4  along with a second PPI. The second PPI is associated with and identifies the second packet data  14 . A second portion of the second packet data (PART#2 of PD#2)  16  is sent to the external memory unit interface circuit  6  and is stored in external memory  7 . Each first portion of each amount of packet data is to be stored into a different block of the memory  9 . The first portion of the first packet is to be stored into a first block in the memory  9 , and the first portion of the second packet is to be stored into a second block in the memory  9 . The memory system  4  includes circuitry, including a PPI Allocation Table circuit (PAT)  17 , that translates an incoming PPI into the starting memory address of the block associated with the packet data. The first PPI is translated into a first such memory address and the first memory address is used to write the first portion of the first packet data into the first block. In  FIG. 1 , the arrow  18  represents the first memory address and the first portion of the first packet data being supplied to the memory  9  so that the memory  9  can use the first memory address to store the first portion of the first packet data into the first block. Likewise, the second PPI is translated into a second such memory address and the second memory address is used to write the first portion of the second packet data into the second block. In  FIG. 1 , the arrow  19  represents the second memory address and the first portion of the second packet data being supplied to the memory  9  so that the memory  9  can use the second memory address to store the first portion of the second packet data into the second block. The processing circuit  5  obtains the first portion of the first packet data and the first portion of the second packet data, and performs processing on those first portions. In one example, the processing circuit  5  includes a holding memory and a specialized processor. In one example, the PDRSDs  2  and  3  are Network Bus Interface (NBI) circuits, each of which receives packet data from a different integrated circuit input port. The packet engine  10  is a small dedicated amount of digital logic circuitry that manages receiving PPI-tagged packet portions, that translates PPI values into memory addresses, and that handles writing packet portions into the memory  9 . The PDRSDs  2  and  3  can cause their respective packet portions to be loaded into the memory  9  without having to know the memory addresses of the blocks where the packet data is written, and without having to check with each other to make sure that there is not memory contention or conflicts. Due to the PPI addressing of the packet engine, the PDRSDs  2  and  3  need not intercommunicate or keep track of whether the other PDRSD has used a given block in the memory. The PDRSD simply sends the packet portion to be stored, tagged with a valid and assigned PPI (that was allocated to store the packet portion), to the packet engine  10  and the packet engine  10  then handles storing the packet portion in association with the PPI. In one example, the PPIs are PPIs are 9-bit numbers. There are 512 PPIs. Each such 9-bit PPI number, if it has been allocated for use by a PDRSD, is allocated for use by one and only one PDRSD. For each allocated and in-use PPI, the PAT  17  in the packet engine stores the starting address of the different block. The PDRSDs do not know the addresses of these blocks, but nevertheless still because packet data to be written into them by use of the allocated PPIs. In one example, a PPI is usable to read a packet portion out of the memory  9  and to forward the packet portion to the processing circuit  5 . 
       FIG. 2  is a flowchart that illustrates a method  20  of allocating PPIs in accordance with a second novel aspect.  FIG. 3  is a diagram of an integrated circuit  26  in which the method  20  is carried out, in one specific example. Integrated circuit  26  includes a first packet data source  27 , a second packet data source  28 , a bus  29 , and a memory system  30 . Memory system  30  includes a packet engine  31  and a memory  32 . In one specific example of the method  20 , a PPI is a 9-bit number and there are 512 PPIs. Each PPI may be either: 1) allocated and currently “in use”, or 2) not currently allocated and currently “not in use”. In a first step of the method  20  (step  21 ), the first packet data source device  27  has an amount of packet data. The amount of packet data may be a first portion of a packet. The first packet data source sends a PPI allocation request command  33  to the packet engine  30  so that the packet engine  30  receives the PPI allocation request  33 . The PPI allocation request  33  is received via bus  29 . The PPI allocation request  33  indicates the size of the packet data. The packet engine  30  maintains a PPI Allocation Table circuit (PAT)  34  and an associated Memory Allocation Table circuit (MAT)  35 . The packet engine uses these PAT and MAT circuits to identify a PPI (step  22 ) that is not currently being used, and to determine an associated block of memory that is large enough to hold the packet data. The packet engine stores an association between the identified PPI and the identified block of memory, and outputs (step  23 ) the PPI (as part of a PPI allocation response  36 ) from the packet engine  30  and onto the bus  29 , so that the PPI is received by the requesting packet data source device  27 . At this point the PPI is said to have been “allocated”. In response to receiving the PPI allocation response  36 , the requesting packet data source device  27  sends (step  24 ) the packet data tagged with the PPI to the packet engine. The packet data tagged with the PPI is identified in the diagram by reference numeral  37 . The packet engine receives the packet data along with the PPI, and translates the PPI into a memory address or addresses. The memory address or addresses is/are then used (step  25 ) to write the packet data into the block of memory (the block of memory associated with the PPI). Accordingly, the packet engine receives packet data from packet data source devices via bus  29  using PPI Addressing Mode (PAM) addressing, and writes the packet data into the memory  32  using Linear Address Mode (LAM) addressing. 
       FIG. 4  is a diagram of a bus transaction value  38  that has a PAM/LAM selection code field  39  in accordance with a third novel aspect. In addition to the PAM/LAM selection code field  39 , the bus transaction value  38  also includes a PPI field  40 . The bus transaction value  38  is received by a device, such as a memory system having a packet engine and a memory. If the PAM/LAM selection code field  39  contains a value indicating that PAM is selected, then the value carried in the PPI field  40  is a PPI. This PPI is then translated by the receiving device into a memory address by the receiving device, and this memory address is then usable to identify the block of memory associated with the PPI. If, on the other hand, the PAM/LAM selection code field  39  contains a value indicating that LAM is selected, then a value carried by the bus transaction value is a linear address that is usable (either directly, or after the addition of an offset) as a memory address to access the memory. In some examples, the value of the same field  40  contains the address in the case of LAM being selected. In other examples, the value of another field  44  of the bus transaction value contains the address in the case of LAM being selected. In the particular example illustrated in  FIG. 4 , the bus transaction value  38  is an autopush bus transaction value. The autopush bus transaction value carries packet data in a packet data field  41 . The contents of the final destination field  42  and the data master field  43  together identify a receiving device (for example, the packet engine of a memory system) to which the autopush bus transaction value is directed. As a result of receiving the autopush bus transaction value, the receiving device writes the data carried by the packet data field  41  into the memory using either PAM addressing or LAM addressing, as determined by the value of the PAM/LAM selection code field  39 . In one specific example, the receiving memory system uses PAM addressing to access a first part of the memory, and uses LAM addressing to access a second part of the memory. If LAM is selected in a bus transaction value then the first part of the memory cannot be accessed using the bus transaction value, whereas if PAM is selected in a bus transaction value then the second part of the memory cannot be accessed using the bus transaction value. In one specific example, the bus transaction value is a Command/Push/Pull bus transaction value that is communicated across a CPP bus. An overall bus transaction includes the sending back and forth of a plurality of bus transaction values. If the bus transaction value is the last of such a plurality of bus transaction values, then the “last autopush” field  45  is set, otherwise the “last autopush” field  45  is cleared. 
       FIG. 5  is a diagram of a PPI de-allocate command  46  in accordance with a fourth novel aspect. In the specific example illustrated, the PPI de-allocate command  46  has a final destination field  47 , a target ID field  48 , an action field  49  and token  50 , a PPI field  51 , an island field  52 , a master ID field  53 , and a data reference field  54 . The PPI de-allocate command  46  is received onto a memory system having a packet engine and an associated memory as described above. Initially, the PPI is recorded in the PAT by the packet engine as being currently allocated (in use). The PPI de-allocate command  46  is sent via a bus to the memory system. The memory system to which the de-allocated PPI command is sent is identified by the values in the final designation field  47  and the target ID field  48 . The receiving memory system examines the content of the action field and token  49  and  50 . The content of the action field is a code (for example, “10010”) and the token is “00”. This indicates that the command  46  is a PPI de-allocate command. The receiving memory system then uses the value of the PPI field  51  to consult its PAT circuit, and to record in that PAT circuit that the PPI value carried by the de-allocate command is no longer in use (currently is not allocated). In addition, the amount of memory previously recorded by the MAT as being allocated to the PPI is also now recorded in the MAT as not being in use. At this point, the PPI is said to be “free” or “de-allocated”, and is available to be reallocated again. Similarly, the freed up memory is now available to be reallocated again. In one specific example, the PPI de-allocate command  46  is a Command/Push/Pull bus command that is communicated across a CPP bus. An overall bus transaction includes the sending back and forth of a plurality of bus transaction values, the first of which is the PPI de-allocate command  46 . In the response to receiving the PPI de-allocate command  46 , a receiving device may send back another bus transaction value. This other bus transaction value carries the value of the data reference field  54  of the original de-allocate command, and this value in the other bus transaction value is usable by device that sent the original de-allocate command to link or associate the other incoming bus transaction value with the original PPI de-allocate command. In the example of  FIG. 5 , the contents of the island field  52  and the master ID field  53  together identify the device that sent the PPI de-allocate command  46 . 
       FIG. 6  is a flowchart of a method  100  in accordance with another novel aspect. The method  100  is carried out in the integrated circuit  130  of  FIG. 7 . Integrated circuit  130  includes a first packet data source device  131 , a second packet data source device  132 , an embodiment of the novel memory system  133 , a processing device  134 , and a packet data destination device  135 . The first packet data source device  131  maintains a “credits available” value  136 . The second packet data source device  132  maintains a “credits available” value  137 . The packet engine  138  of the memory system  133  maintains a “Credits To Be Returned” (CTBR) value for each packet data source. CTBR value  139  is the CTBR value for the first packet data source device  131 . CTBR value  140  is the CTBR value for the second packet data source device  132 . In addition to the packet engine  138  and the memory  141 , the memory system  133  also includes a bulk engine  142 . The bulk engine  142  is a bulk data DMA (Direct Memory Access) data mover for moving data into and out of memory  141 . In one specific example, the first and second packet data source devices are ingress-NBI (Network Bus Interface) island circuits, the processing circuit  134  is a MicroEngine (ME) processor, the packet destination device  135  is an egress-NBI island circuit, and the memory system  133  is a CTM (Cluster Target Memory) located on the same ME island with the processing device  134 . 
     Initially in the method  100  of  FIG. 6 , if the packet data source device  131  (ingress-NBI) has received packet data and wants to send a “PPI allocation request” command, the process flow proceeds from decision diamond  101  to decision diamond  102 , otherwise process flow remains in decision diamond  101 . In this case, the first packet data source device  131  has received packet data, and wants to send a “PPI allocation request” command. In decision diamond  102 , the first packet data source device  131  consults its stored “credits available” value  136 . In this example, the “credits available” value is a number of buffer credits, where each buffer credit indicates a 2K byte amount of memory space in memory  141 . If the “credits available” value  136  is more than a predetermined configuration amount (that is set at configuration time for the packet data source device), then the packet data source device  131  is permitted to send a PPI allocation request command, otherwise the packet data source device  131  does not send an PPI allocation request command. If the packet data source device  131  has adequate buffer credits as indicated by the “credits available” value  136 , then the “credit available” value is decremented by one (assuming that a new buffer will be required to store the packet portion). In addition, a “PPI allocation request” command (indicated in  FIG. 7  by the circled “1”) is sent (step  103 ) from the packet data source device  131  to the memory system  133  across a CPP bus, where the “PPI allocation request” command includes an indication of the amount of packet data (the size of the packet portion). The packet engine  138  of the memory system  133  receives the “PPI allocation request” command via the CPP bus, and checks the PAT and MAT circuits of the packet engine to determine if there is an available PPI, and if there is adequate available buffer space in memory  141  to store the amount of packet data as indicated by the “PPI allocation request” command. If there is an available PPI and if there is adequate buffer space, then the packet engine  138  allocates a PPI and sends (step  104 ) a “PPI allocate response” back to the packet data source device  131 , where the “PPI allocate response” includes an indication of the allocated PPI. (The “PPI allocate response” is indicated in  FIG. 7  by the circled “2”). If a new buffer was required to store the packet portion then no buffer credit is returned to the requestor, but if the packet portion can be stored into an unused portion of a buffer already used by the requestor then one buffer credit is returned to the requestor in the PPI allocate response. Next, the packet data is transferred (step  105 ) from the first packet data source device  131  to the packet engine  138 . In actuality, the first part of the packet data is transferred to the packet engine, and the remainder of the packet data is stored in external memory external to the integrated circuit  130 . (The transfer of the packet data that is tagged with the PPI is indicated in  FIG. 7  by the circled “3”.) The packet engine  138  translates the PPI (that was tagged onto the packet data) into a memory address, and uses the memory address to write the packet data into memory  141 . The packet engine  138  maintains a “work queue”, where the entries on the work queue identify microengine processors (MEs) that are available to perform tasks on packet portions. The packet engine  138 , that has now written new packet data into memory  141 , pops this work queue and obtains (step  106 ) an indication of a processing device (processing device  134  in this example) that is the next available processing device. The packet engine  138  then performs a series CPP push bus transactions (step  107 ) to move the first 128 bytes of the packet data to the processing device  134 . Each autopush bus transaction communicates sixty-four bits (eight bytes) of the 128 bytes of packet data. If the packet data (stored in memory  141 ) is larger than 128 bytes, then the packet processing device  134  (an ME, in this case) does a PPI Addressing Mode (PAM) mode CPP read to the bulk engine  142 , thereby causing the bulk engine  142  to move the remaining part of the packet data (step  108 ) from memory  141  to the processing device  134 . In performing the bulk data move, the bulk engine uses (step  109 ) the PPI of the read command to obtain a linear address or addresses from the packet engine. The packet engine performs a PPI-to-address translation task for the bulk engine. There is a dedicated connection between the packet engine and bulk engine that is provided for this purpose. The bulk engine  12  uses the obtained linear address or addresses to read the remainder of the packet data out of the memory  141 , and then transfers that packet data back to the processing device  134  (an ME, in this case) to complete the CPP read operation. (The bulk data transfer of the packet data to the processing device  134  is indicated in  FIG. 7  by the circled “4”). At this point in this example, all the packet data is present in processing device  134 . Next, the processing device  134  processes (step  110 ) the packet data. When the processing is done, the processing device  134  sends a “packet complete” command (step  111 ) back to the packet engine  138 . (This transfer of the “packet complete” command is indicated in  FIG. 7  by the circled “5”). The packet engine  138  forwards (step  112 ) the “packet complete” command to the packet data destination device  135 . (This forwarding of the “packet complete” command is indicated in  FIG. 7  by the circled “6”). In response, the packet data destination device  135  schedules (step  113 ) the packet data to be output from the integrated circuit  130 . When the packet data is to be transmitted from the integrated circuit  130 , the packet data destination device  135  pulls (step  114 ) the packet data from the memory system  133 . In addition, as explained above, there is packet payload data stored in memory external to the integrated circuit  130 . The packet data destination device  135  causes the bulk engine  142  to read this externally-stored packet payload data (step  115 ) out of external memory (the external memory is not shown in  FIG. 7 ) and to send it to the packet data destination device  135 . (The transfer of the packet data and the PPI to the packet data destination device  135  is indicated in  FIG. 7  by the circled “7”). When the packet data destination device  135  has all the packet data (both the first part of the packet data stored in memory  141  as well as the remainder of the packet data that was stored in external memory), then the packet data destination device  135  sends a “PPI de-allocate” command (step  116 ) to the packet engine  138 . (The sending of the “PPI de-allocate” command is indicated in  FIG. 7  by the circled “8”). The “PPI de-allocate” command indicates the PPI, but does not include a number of credits to be de-allocated, nor does it include the size of the buffer space in memory  141  that is to be freed. The packet engine  113  uses the PPI (step  117 ) from the PPI de-allocate command to consult its MAT and PAT circuits. From its MAT and PAT circuits, the packet engine  138  determines: 1) the registered owner of the PPI, and 2) whether a 2K byte buffer has been entirely freed up due to the memory no longer having to store the packet portion associated with the de-allocated PPI. If only part of a 2K byte buffer is freed up, then the buffer is still in use, and no buffer credit is returned. Only if the freeing up of buffer space results in an entire 2K byte buffer being free is buffer credit be returned. The determined number of buffer credits to be returned (one or zero) is then added (step  118 ) to the buffer CTBR value for the owner of the PPI. In the example of  FIG. 7 , the owner of the PPI is the first packet data source device  131 . The buffer CTBR value  139  is therefore increased by the number of de-allocated buffer credits. In the system of  FIG. 7 , credits can be returned to a packet data source device by including the number of “credits to be returned” in a field of a “PPI allocate response” that is being sent to the packet data source device. Accordingly, when the next “PPI allocate response” is to be sent from the packet engine  138  to the first packet data source device  131 , the updated number of “credits to be returned” as recorded in the CTBR register  139  is included (step  119 ) in the PPI allocate response. (The sending of the number of credits to be returned is indicated in  FIG. 7  by the circled “9”). The packet data source device  131  receives the “PPI allocate response” (step  120 ), and obtains the “credits to be returned” value, and adds the “credits to be returned” value to the previously stored “credit available” value  136 , thereby generating an updated “credits to be returned” value  136  (step  121 ). 
     Operational Example 
       FIG. 8  is a diagram that illustrates one example of packet traffic passing through an Island-Based Network Flow Processor (IB-NFP) integrated circuit  150 , where the IB-NFP  150  includes a packet engine in accordance with one novel aspect. 
     The NFP integrated circuit  150  includes a peripheral first area of input/output circuit blocks  151 - 179 . Each of the SerDes I/O circuit blocks  156 - 167  and  168 - 179  is duplex in that it has four 10 Gbps lanes for receiving SerDes information and it also has four 10 Gbps lanes for transmitting SerDes information. A SerDes circuit can communicate information in both directions simultaneously. The three SerDes circuits  162 - 164  can therefore communicate information at 120 gigabits per second in both directions. Respective ones of the DDR physical interfaces  151 ,  152 ,  154 ,  155 ,  166  and  167  are used to communicate with corresponding external memory integrated circuits  180 ,  181 ,  183 ,  184 ,  185  and  186 , respectively. GPIO interface block  153  is used to receive configuration information from external PROM  182 . 
     In addition to the first peripheral area of I/O blocks, the NFP integrated circuit  150  also includes a second tiling area of islands  187 - 211 . Each of these islands is either a full rectangular shape, or is half the size of the full rectangular shape. For example, the island  192  is a full island. The island  197  is a half island. The functional circuits in the various islands of this second tiling area are interconnected by: 1) a configurable mesh Command/Push/Pull (CPP) data bus, 2) a configurable mesh control bus, and 3) a configurable mesh event bus. Each such mesh bus extends over the two-dimensional space of islands with a regular grid or “mesh” pattern. In the case of the CPP data bus, as described in further detail below, functional circuitry in one island can use the CPP data bus to send a command to functional circuitry in another island, to read data from functional circuitry in another island, or a write data to functional circuitry in another island. 
     In addition to the second tiling area, there is a third area of larger sized blocks  212 - 216 . The mesh bus structures do not extend into or over any of these larger blocks. The functional circuitry of a larger sized block may connect by direct dedicated connections to an interface island within the tiling area and through this interface island achieve connectivity to the mesh buses and other islands. 
     In the operational example of  FIG. 8 , packet traffic is received into three SerDes input/output circuit blocks  162 - 164 . The packet data in this particular example passes through dedicated connections from three SerDes circuit blocks  162 - 164  to the ingress MAC island  208 . Ingress MAC island  208  converts successive symbols delivered by the physical coding layer into packets by mapping symbols to octets, by performing packet framing, and then by buffering the resulting packets in an SRAM memory for subsequent communication to other processing circuitry. After buffering in the SRAM, the resulting packets are communicated from ingress MAC island  208  across a single private inter-island minipacket bus, to ingress NBI (Network Bus Interface) island  209 . Prepended to the beginning of each packet is a MAC prepend value that contains information about the packet and results of analyses (parse results PR) performed by the ingress MAC island. For each packet, the functional circuitry of ingress NBI island  209  examines fields in the header portion to determine what storage strategy to use to place the packet into memory. In one example, the ingress NBI island  209  examines the header portion and from that determines whether the packet is an exception packet or whether the packet is a fast-path packet. If the packet is an exception packet, then the ingress NBI island  209  determines a first storage strategy to be used to store the packet so that relatively involved exception processing can be performed efficiently, whereas if the packet is a fast-path packet then the ingress NBI island determines a second storage strategy to be used to store the packet for more efficient transmission of the packet from the NFP integrated circuit  150 . The ingress NBI island  209  examines a packet header, performs packet preclassification, determines that the packet is a fast-path packet, and determines that the header portion of the packet should pass to ME (Microengine) island  203 . The header portion of the packet is therefore communicated across the configurable mesh CPP data bus from ingress NBI island  209  to ME island  203 . The ME island  203  determines header modification and queuing strategy for the packet based on the packet flow (derived from packet header and contents) and the ME island  203  informs egress NBI island  200  of these. In this simplified example being described, the payload portions of fast-path packets are placed into internal SRAM (Static Random Access Memory) MU block  215  and the payload portions of exception packets are placed into external DRAM  185  and  186 . Half island  205  is an interface island through which all information passing into, and out of, SRAM MU block  215  passes. The functional circuitry within half island  205  serves as the interface and control circuitry for the SRAM within block  215 . Accordingly, the payload portion of the incoming fast-path packet is communicated from ingress NBI island  209 , across the configurable mesh CPP data bus to SRAM control island  205 , and from control island  205 , to the interface circuitry in block  215 , and to the internal SRAM circuitry of block  215 . The internal SRAM of block  215  stores the payloads so that they can be accessed for flow determination by the ME island  203 . 
     In addition, a preclassifier in the ingress NBI island  209  determines that the payload portions for others of the packets should be stored in external DRAM  185  and  186 . For example, the payload portions for exception packets are stored in external DRAM  185  and  186 . Interface island  206 , IP block  216 , and DDR PHY I/O blocks  166  and  167  serve as the interface and control for external DRAM integrated circuits  185  and  186 . The payload portions of the exception packets are therefore communicated across the configurable mesh CPP data bus from ingress NBI island  209 , to interface and control island  206 , to external MU SRAM block  216 , to 32-bit DDR PHY I/O blocks  166  and  167 , and to the external DRAM integrated circuits  185  and  186 . At this point in the operational example, the packet header portions and their associated payload portions are stored in different places. The payload portions of fast-path packets are stored in internal SRAM in MU block  215 , whereas the payload portions of exception packets are stored in external memories  185  and  186 . 
     ME island  203  informs egress NBI island  200  where the packet headers and the packet payloads can be found and provides the egress NBI island  200  with an egress packet descriptor for each packet. Egress NBI island  200  places packet descriptors for packets to be output into the correct order. The egress packet descriptor indicates a queuing strategy to be used on the packet. For each packet that is then scheduled to be transmitted, the egress NBI island  200  uses the egress packet descriptor to read the header portion and any header modification, and to read the payload portion, and to assemble the packet to be transmitted. The egress NBI island  200  then performs packet modification on the packet, and the resulting modified packet then passes from egress NBI island  200  and to egress MAC island  207 . Egress MAC island  207  buffers the packets, and converts them into symbols. The symbols are then delivered by dedicated conductors from the egress MAC island  207  to three SerDes circuits  171 - 173  and out of the IB-NFP integrated circuit  150 . The SerDes circuits  171 - 173  together can provide 120 gigabits per second of communication throughput out of the integrated circuit. 
       FIG. 9  is a more detailed diagram of one of the SerDes I/O blocks  162 . 
       FIG. 10  is a more detailed diagram of the ingress MAC island  208 . The symbols pass from the three SerDes I/O blocks  162 - 164  and to the ingress MAC island  208  across dedicated conductors  217 . The symbols are converted into packets by a 100 Gbps ethernet block  218 . The packets are parsed and analyzed, and a “MAC prepend value”  220  that contains information about the packet is placed at the beginning of the packet  221 . The resulting packets and associated MAC prepend values are then buffered in SRAM  219 . The MAC prepend value  220  includes: 1) an indication of the length of the packet, 2) an indication whether the packet is an IP packet, 3) and indication of whether the checksums are correct, and 4) a time stamp indicating when the packet was received. Packets that are buffered in SRAM  219  are then output from the ingress MAC island  208  to the ingress NBI island  209  in the form of one or more 256-byte minipackets  222  that are communicated across dedicated connections  223  of a minipacket bus to the ingress NBI island  209 . The event bus mesh, the control bus mesh, and the CPP data bus mesh mentioned above are represented in  FIG. 10  by reference numerals  224 - 226 , respectively. For additional detailed information on the structure and operation of the ingress MAC island  208 , see: U.S. patent application Ser. No. 14/321,732, entitled “Merging PCP Flows As They Are Assigned To A single Virtual Channel”, filed on Jul. 1, 2014, by Joseph M. Lamb (the entire contents of which is incorporated herein by reference). 
       FIG. 11  is a more detailed diagram of the ingress NBI island  209 . Ingress NBI island  209  receives the MAC prepend and the minipacket information via dedicated minipacket bus connections  223  from the ingress MAC island  208 . The first 256 bytes of the frame and the MAC prepend pass through multiplexing circuitry and are analyzed by a pool  227  of forty-eight picoengines. Pool  227  generates preclassification results  228 .  FIG. 12  is a diagram that describes various parts of the preclassification results  228 . The preclassification results  228  include: 1) a determination of which one of multiple buffer pools to use to store the frame, 2) a sequence number for the frame in a particular flow of frames through the NFP integrated circuit, and 3) user metadata. The user metadata is typically a code generated by the picoengine pool  227 , where the code communicates certain information about the packet. In one example, the user metadata includes a bit that indicates whether the frame was determined by the picoengine pool  227  to be an exception frame or packet, or whether the frame was determined to be a fast-path frame or packet. The frame is buffered in SRAM  229 . A buffer pool is a set of targets in ME islands where header portions can be placed. A buffer list is a list of memory addresses where payload portions can be placed. DMA engine  230  can read the frame out of SRAM  229  via conductors  231 , then use the buffer pools to determine a destination to which the frame header is to be DMA transferred, and use the buffer lists to determine a destination to which the frame payload is to be DMA transferred. The DMA transfers occur across the configurable mesh CPP data bus. In the case of an exception packet, the preclassification user metadata and buffer pool number indicate to the DMA engine  230  that the frame is an exception frame and this causes a first buffer pool and a first different buffer list to be used, whereas in the case of a fast-path frame the preclassification user metadata and buffer pool number indicate to the DMA engine that the frame is a fast-path frame and this causes a second buffer pool and a second buffer list to be used. CPP bus interface  232  is a CPP bus target. CPP bus interface  232  is a CPP bus interface through which the configurable mesh CPP data bus  226  is accessed. Arrow  233  represents frames (packets) that are DMA transferred out of the ingress NBI island  209  by DMA engine  230  and through CCP bus interface  232 . Each frame (packet) is output with a corresponding ingress packet descriptor.  FIG. 13  sets forth the parts of an ingress packet descriptor. An ingress packet descriptor includes: 1) an address indicating where the header portion is stored (in which ME island), 2) an address indicating where the payload portion is stored (which MU island, either for internal SRAM or for external DRAM), 3) how long the frame (packet) is, 4) a sequence number for the flow to which the frame (packet) belongs, 5) user metadata. 
     After the picoengine pool  227  in the ingress NBI island  209  has done its analysis and generated its preclassification results for the packet, the ingress NBI island  209  then DMA transfers the frame headers (packet headers) and associated preclassification results across the CPP configurable mesh data bus  226  and into the ME island  203 . Within the ME island  203 , one or more microengines (MEs) then perform further processing on the header and preclassification results as explained in further detail in U.S. patent application Ser. No. 13/399,888, entitled “Island-Based Network Flow Processor Integrated Circuit”, filed Feb. 17, 2012, by Stark et al. (the entire subject matter of which is hereby incorporated by reference). 
       FIG. 14  is a more detailed diagram of ME island  203 . In the operational flow of  FIG. 8 , packet headers and the associated preclassification results are DMA transferred from the ingress NBI island  209  across the configurable mesh CCP data bus and into the Cluster Target Memory (CTM)  234  of ME island  203 . A DMA engine  230  in the ingress NBI island  209  is the master and CTM  234  in ME island  203  is the target for this transfer. The packet header portions and the associated ingress packet descriptors pass into the ME island  203  via CPP data bus island bridge  235  and data bus interface circuitry  236 . Once in the CTM  234 , the header portions are analyzed by one or more of twelve microengines (MEs)  237 - 248 . The MEs have, through the DB island bridge  235 , a command out interface, a pull-id in interface, a pull-data out interface, and a push data in interface. There are six pairs of MEs, with each pair sharing a memory containing program code for the MEs. Reference numerals  237  and  238  identify the first pair of MEs and reference numeral  249  identifies the shared memory. As a result of analysis and processing, the MEs modify each ingress packet descriptor to be an egress packet descriptor.  FIG. 15  is a diagram that describes the parts of an egress packet descriptor. Each egress packet descriptor includes: 1) an address indicating where and in which ME island the header portion is found, 2) an address indicating where and in which MU island the payload portion is found, 3) how long the packet is, 4) a sequence number of the packet in the flow, 5) an indication of which queue the packet belongs to (result of the packet policy), 6) an indication of where the packet is to be sent (a result of the packet policy), 7) user metadata indicating what kind of packet it is. Memory errors and other events detected in the ME island are reported via a local event ring and the global event chain back to the ARM island  188 . A local event ring is made to snake through the ME island  203  for this purpose. Event packets from the local event chain are received via connections  250  and event packets are supplied out to the local event chain via connections  251 . The CB island bridge  252 , the cluster local scratch  253 , and CTM  234  can be configured and are therefore coupled to the control bus CB via connections  254  so that they can receive configuration information from the control bus CB  255 . The event bus and the control bus are shown in simplified form in these diagrams. 
       FIG. 16  is a diagram of MU half island  205  and SRAM block  215 . MU half island  205  includes several hardware engines  255 - 259 . In the operational example, fast path packet payloads are DMA transferred directly from ingress NBI island  209  and across the configurable mesh data bus, through data bus interface  260  of half island  205 , and into the data cache SRAM  261  of block  215 . The ingress NBI DMA engine  230  issues a bulk write command across the configurable mesh data bus to the bulk transfer engine  255 . The destination is the MU half island  205 . The action is bulk write. The address where the data is to be written into the MU half island is the address taken out of the appropriate buffer list. The bulk write command received at the MU is a bulk write, so the data bus interface  260  presents the command to the bulk engine  255 . The bulk engine  255  examines the command which is a write. In order to perform a write, the bulk engine needs data. The bulk engine therefore issues a pull-id through the pull portion of interface  260 , which in turn issues a pull-id back onto the configurable mesh CPP data bus. The DMA engine  230  in NBI island  209  receives the pull-id. Part of the pull-id is a data reference which indicates to the DMA engine which part of the packet is being requested as data. The DMA engine uses the data reference to read the requested part of the packet, and presents that across the data part of the CPP data bus back to bulk engine  255  in MU island  205 . The bulk engine  255  then has the write command and the packet data. The bulk engine  255  ties the two together, and it then writes the packet data into SRAM  261  at the address given in the write command. In this way, fast path packet payload portions pass from DMA engine  230  in the ingress NBI island  209 , across the configurable mesh CPP data bus, through the data bus interface  260  of the MU half island  205 , through a bulk transfer engine  255 , and into data cache SRAM  261  of block  215 . In a similar fashion, exception packet payload portions pass from the DMA engine  230  in ingress NBI island  209 , across the configurable mesh CPP data bus, through the data bus interface of half island  206 , through the bulk transfer engine of half island  206 , and through DDR PHYs  166  and  167 , and into external memories  185  and  186 . 
       FIG. 17  is a diagram of egress NBI island  64 . In the operational example, ME island  203  instructs the egress NBI island  209  to transmit a packet by supplying the egress NBI island with an egress packet descriptor of the packet to be transmitted. The ME island  203  supplies the egress packet descriptor to the egress NBI island  200  by issuing a transmit packet command across the configurable mesh CPP data bus and to the packet reorder block  262 . The packet reorder block  262  responds by pulling the egress packet descriptor from the ME island across the CPP data bus. In this way, multiple egress packet descriptors enter packet reorder block  262 . These egress packet descriptors are reordered so that the descriptors for the packets of a flow are in proper sequence. The scheduler  263  receives the properly ordered egress packet descriptors and pushes them onto appropriate queues in queue SRAM  264 . Each such queue of egress packet descriptors is per port, per data type, per group of connections. Reference numeral  265  identifies one such queue. Packets of a connection in this case share the same set of source and destination IP addresses and TCP ports. Scheduler  263  schedules packets to be transmitted by popping egress packet descriptors off the queues in appropriate orders and at appropriate times, and by supplying the popped egress packet descriptors via conductors  266  to the DMA engine  267 . DMA engine  267  receives such an egress packet descriptor, and based on the information in the egress packet descriptor, transfers the payload portion and the header portion of the packet across CPP data bus and DB interface  268  and into FIFO  269 . As a result, each entry in FIFO  269  includes a complete packet having a script code portion  270 , the header portion  271 , and the payload portion  272 . Information can be written into FIFO  269  as larger values, but information passes out of FIFO  269  and into the packet modifier  273  in ordered 32-byte chunks. The script code  270  at the beginning of the packet was added by the microengine in the ME island. As a result of the lookup performed at the direction of the microengine, a packet policy was determined, and part of this packet policy is an indication of what of the packet header to change and how to change it before the packet is transmitted. The packet modifier  273  receives a packet in 32-byte chunks from FIFO  269 . As each 32-byte chunk passes through the packet modifier  273 , it can increase in size due to the insertion of bits, or it can decrease in size due to the deleting of bits. The chunks pass through the pipeline in sequence, one after the other. The resulting modified chunks as they come out of the pipeline are aggregated at the end of the packet modifier  273  into larger 256-byte portions of a packet, referred to here as minipackets. A minipacket includes a number of chunks, along with associated out-of-band control information. The out-of-band control information indicates how the data of the minipacket can be assembled with the data of other minipackets to reform the overall modified packet. In this way, the resulting modified packet is output from the egress NBI island  200  as a sequence of 256-byte minipackets across dedicated connections  274  to egress MAC island  207 . Reference numeral  275  identifies one such minipacket. For additional detailed information on the structure and operation of the egress NBI  200 , see: U.S. patent application Ser. No. 13/941,494, entitled “Script-Controlled Egress Packet Modifier”, filed on Jul. 14, 2013, by Chirac P. Patel et al. (the entire contents of which is incorporated herein by reference). 
       FIG. 18  is a diagram of egress MAC island  65 . In the presently described example, the packet traffic discussed in connection with  FIG. 3  flows out of the egress MAC island  207  and through three SerDes I/O circuits  171 - 173  and out of the IB-NFP integrated circuit  150 . 
     CCP Data Bus Operation: Operation of the Command/Push/Pull data bus is described below in connection with  FIGS. 19-26 . The CPP data bus includes four “meshes”: a command mesh, a pull-id mesh, and two data meshes data 0  and data 1 .  FIG. 19  is a flowchart of a write operation method  1000  that might occur across the configurable mesh CPP data bus. In a first step (step  1001 ), certain functional circuitry in one of the islands uses its data bus interface to output a bus transaction value onto the configurable mesh CPP data bus. This functional circuitry is referred to as the “master” of the write operation. The format of the bus transaction value is as set forth in  FIG. 20 . A bus transaction value  1006  includes a metadata portion  1007  and a payload portion  1008  as shown. The metadata portion  1007  includes a final destination value  1009  and a valid bit  1010 . 
     The bus transaction value in this case is a write command to write data into functional circuitry in another island. The functional circuitry that receives the bus transaction value and the data to be written is referred to as the “target” of the write operation. The write command is said to be “posted” by the master circuit onto the command mesh. As indicated in  FIG. 20 , the write command includes a metadata portion and a payload portion. The metadata portion includes the 6-bit final destination value. This final destination value identifies an island by number, where the island identified is the final destination of the bus transaction value. The final destination value is used by the various crossbar switches of the command mesh to route the bus transaction value (i.e., the command) from the master circuit to the appropriate target circuit. All bus transaction values on the data bus that originate from the same island that have the same final destination value will traverse through the configurable mesh data bus along the same one path all the way to the indicated final destination island. 
     A final destination island may have more than one potential target circuit. The 4-bit target field of payload portion indicates which one of these targets in the destination island it is that is the target of the command. The 5-bit action field of the payload portion indicates that the command is a write. The 14-bit data reference field is a reference usable by the master circuit to determine where in the master the data is to be found. The address field indicates an address in the target where the data is to be written. The length field indicates the amount of data. 
     In a next step (step  1002 ) in the method  1000  of  FIG. 19 , the target circuit receives the write command from the command mesh and examines the payload portion of the write command. From the action field the target circuit determines that it is to perform a write action. To carry out this action, the target circuit writes (i.e., posts) a bus transaction value (step  1003 ) called a pull-id onto the pull-id mesh. The pull-id is also of the format indicated in  FIG. 20 . The payload portion of the pull-id is of the format set forth in  FIG. 22 . The final destination field of the metadata portion of the pull-id indicates the island where the master circuit is located. The target port field identifies which sub-circuit target it is within the target&#39;s island that is the target circuit of the command. The pull-id is communicated through the pull-id mesh back to the master circuit. 
     The master circuit receives the pull-id from the pull-id mesh and uses the content of the data reference field of the pull-id to find the data. In the overall write operation, the master circuit knows the data it is trying to write into the target circuit. The data reference value that is returned with the pull-id is used by the master circuit as a flag to match the returning pull-id with the write operation the master circuit had previously initiated. 
     The master circuit responds by sending (step  1004 ) the identified data to the target across one of the data meshes data 0  or data 1  as a “pull” data bus transaction value. The term “pull” means that the data of the operation passes from the master to the target. The term “push” means that the data of the operation passes from the target to the master. The format of the “pull” data bus transaction value sent in this sending of data is also as indicated in  FIG. 20 . The format of the payload portion in the case of the payload being pull data is as set forth in  FIG. 24 . The first bit of the payload portion is asserted. This bit being a digital high indicates that the transaction is a data pull as opposed to a data push. The target circuit then receives (step  1005 ) the data pull bus transaction value across the data 1  or data 0  mesh. The target circuit writes the content of the data field (the data field of  FIG. 24 ) of the pull data payload portion into target memory at the appropriate location indicated by the address field of the original write command. 
       FIG. 26  is a flowchart of a read operation method  2000  that might occur across the configurable mesh CPP data bus. In a first step (step  2001 ), a master circuit in one of the islands uses its data bus interface to output (to “post”) a bus transaction value onto the command mesh bus of the configurable mesh CPP data bus. In this case, the bus transaction value is a read command to read data from a target circuit. The format of the read command is as set forth in  FIGS. 20 and 21 . The read command includes a metadata portion and a payload portion. The metadata portion includes the 6-bit final destination value that indicates the island where the target is located. The action field of the payload portion of the read command indicates that the command is a read. The 14-bit data reference field is usable by the master circuit as a flag to associated returned data with the original read operation the master circuit previously initiated. The address field in the payload portion indicates an address in the target where the data is to be obtained. The length field indicates the amount of data. 
     The target receives the read command (step  2002 ) and examines the payload portion of the command. From the action field of the command payload portion the target circuit determines that it is to perform a read action. To carry out this action, the target circuit uses the address field and the length field to obtain the data requested. The target then pushes (step  2003 ) the obtained data back to the master circuit across data mesh data 1  or data 0 . To push the data, the target circuit outputs a push bus transaction value onto the data 1  or data 0  mesh.  FIG. 25  sets forth the format of the payload portion of this push bus transaction value. The first bit of the payload portion indicates that the bus transaction value is for a data push, as opposed to a data pull. The master circuit receives the bus transaction value of the data push (step  2004 ) from the data mesh bus. The master circuit then uses the data reference field of the push bus transaction value to associate the incoming data with the original read command, and from the original read command determines where the pushed data (data in the date field of the push bus transaction value) should be written into the master circuit. The master circuit then writes the content of the data field of the data field into the master&#39;s memory at the appropriate location. 
       FIG. 27  is a more detailed diagram of the Cluster Target Memory (CTM)  234  in the ME island  203  of  FIG. 14 . CTM  234  includes a data cache SRAM  276 , an engine  277  called the miscellaneous engine (MISC), an atomic engine  278 , a bulk mover engine  279 , and a novel packet engine  280 . The packet engine  280  includes, among other parts not illustrated, a master CPP bus interface circuit  281 , a target CPP bus interface circuit  282 , a data cache interface circuit  283 , a bulk engine interface circuitry  284 , a state machine  285 , a DMA resource handler  286 , a PPI Allocation Table circuit (PAT)  287 , and a Memory Allocation Table circuit (MAT)  288 . The packet engine  280  can operate as a CPP bus master in a CPP bus transaction by using its master CPP bus interface circuit  281  and the master portion of the data bus interface circuit  236  to access the CPP data bus  226 . Another CPP bus master located elsewhere on the integrated circuit can access the packet engine via the CPP data bus  226 , with the packet engine acting as a CPP bus target. The target portion of the data bus interface circuit  236  and the target interface circuit  282  together function as a CPP data bus target. The packet engine  280  can write to and read from the data cache SRAM  276  via the data cache interface circuit  283 . The bulk engine mover  279  can use the packet engine  280  as a PPI-to-address translation resource. The bulk engine uses the packet engine  280  an a PPI-to-memory address translation resource by presenting a PPI to be translated to the packet engine on interface  284  and by receiving in response a memory address from the packet engine  280  on interface  284 . 
     Packet engine  280  of  FIG. 27  operates in the same way that the packet engine  138  of  FIG. 7  operates. It receives “PPI allocation request” CPP commands from credit-aware requestors (for example, from the DMA controller in an ingress NBI island) via the CPP data bus. If the packet engine  280  receives such a PPI allocation request command, then it consults it PAT and MAT circuits. If the PAT and MAT circuits indicate that there is an available PPI and that there is adequate memory space to store the packet data associated with the PPI allocation request, then the packet engine allocates an unused PPI to the packet data. The packet engine updates its PAT and MAT circuits to reflect that the newly-allocated PPI is now being used and that the appropriate amount of buffer space is allocated to the PPI. The packet engine also sends the credit-aware requestor back a “PPI allocation response” where the PPI allocation response includes the PPI. Another function of the packet engine is to receive amounts of packet data that are tagged with PPIs. If the packet engine receives such an amount of packet data that is tagged with a PPI, then the packet engine its PAT and MAT circuits to translate the PPI into a memory address or addresses and uses the memory address or addresses to write the packet data into the appropriate buffer or buffers in data cache memory  276 . In addition, the packet engine maintains the work queue. Each entry in the work queue indicates a microengine (ME) that is available to process a packet portion. The packet engine uses information stored in its PAT and MAT circuits to read packet data associated with a PPI, and to send that packet data and the PPI to the next available microengine as indicated by the work queue. Another function of the packet engine is to receive “packet complete” CPP commands from microengines. A “packet complete” CPP command serves to tell the packet engine that the micoengine has completed its processing of the packet data associated with a particular PPI. If the packet engine receives such a “packet complete” CPP command, it logs in its PAT and MAT circuits the updated status of the processing associated with the PPI, and it forwards the “packet complete” CPP command to the appropriate egress NBI island. Another function of the packet engine is to receive “de-allocate PPI” CPP commands from egress NBI islands. If the packet engine receives such a “de-allocate PPI” CPP command, then the packet engine de-allocates the indicated PPI. The PPI changes the information stored in the PAT circuit to reflect that the PPI is now not being used. The PPI also changes the information stored in the MAT circuit to reflect that the buffer space previously used to store the packet data associated with the PPI is now available for use in storing other packet data. The packet engine also sends the original PPI requestor a PPI “Credits To Be Returned” (CTBR) value and a Buffer CTBR value, so that the credit-aware requestor can add the credits back to its PPI “Credits Available” value and to its buffer “Credits Available” value. See  FIG. 7 , the flowchart of  FIG. 6 , and the associated textual description above for further details on the operation of the packet engine. 
       FIG. 28  is a diagram that illustrates operation of the PPI Allocation Table circuit (PAT)  287  in the packet engine  280  of the CTM  234  of  FIG. 27 . The circuit is not a table, but rather is circuitry that implements the table and the described table operations. There are five hundred and twelve 9-bit PPIs, each of which is either being used or is unused at a given time. If a PPI is being used (i.e., has been allocated), then the valid bit in the row of the PPI is set, otherwise the valid bit in that row is not set. In addition, for each used PPI, the PAT circuit stores an indication of the “owner” of the PPI. The owner is the device (for example, the DMA engine in ingress NBI-0, the DMA engine in ingress NBI-1, or an ME) that originally submitted an allocation request for the PPI and to which the PPI is currently allocated. In addition, for each PPI, the PAT circuit stores an indication of the size of the “packet portion” identified by the PPI (Packet Portion Identfier). The “packet portion” can be either 256 B, 512 B, 1 KB and 2 KB in size. In addition, for each PPI, the PAT circuit stores the starting address in dcache SRAM that is reserved for storing the “packet portion” associated with the PPI. In addition, the PAT circuit stores an indication of whether the first part of the packet portion has been received (by the packet engine) from the requestor, an indication of whether the last part of the packet portion has been received (by the packet engine) from the requestor, and an indication of whether the entire packet portion has been communicated from the packet engine to a processing ME. When a PPI is de-allocated, the valid bit in the row for the PPI is cleared. 
       FIG. 29  is a diagram that illustrates operation of the Memory Allocation Table circuit (MAT)  288  in the packet engine  280  of the CTM  234  of  FIG. 27 . The circuit is not a table, but rather is circuitry that implements the table and the described table operations. As illustrated in  FIG. 29 , each row of the MAT circuit  288  has a field for indicating the “owner” of a “slice” of buffer space represented by the remaining eight bits, as well as the eight bits. The 2K byte slice is also referred to as a “buffer”. Each of the eight bits represents one 256 byte portion of the 2K byte “buffer” in data cache SRAM  276 . If a bit is set, then the corresponding 256 byte portion is allocated for use in storing the “packet portion” associated with a PPI allocated to the owner (the “owner” indicated by the first entry in the row). Each “buffer” can only be assigned to one “owner”, but a buffer can store more than one packet portion of the same “owner”. As illustrated in the diagram, there are 128 such rows in the MAT circuit  288 . When a requestor (for example, the DMA engine in ingress NBI-0) sends an “PPI allocation request” CPP command to the packet engine, the packet engine consults the MAT circuit, and more particularly examines any row whose indicated “owner” is the requestor that sent the PPI allocation request. If such a row is found, and if there are enough (adjacent) cleared bits in the row to indicate that the entire packet portion (the amount indicated in the “PPI allocation request”) can be stored in contiguous available buffer space corresponding to the row, then the appropriate number of cleared bits are set to reflect that the packet portion will be stored in buffer space corresponding to these bits. If a row is not found that is “owned” by the requestor and that has adequate available storage space to accommodate the packet portion, then a row is chosen that is not get assigned to any owner. The owner of that row is set to be the requestor, and the appropriate number of adjacent bits are set according to the size of the packet portion. Regardless of whether a new row in the MAT is used, the row in the PAT circuit for the newly allocated PPI is updated so that the starting address logged in the PAT circuit for the PPI is the starting address of the first of the adjacent newly allocated 256 byte portion. When a PPI is de-allocated, the bits in the MAT circuit that were set (to reserve buffer space for use in storing the packet data associated with the PPI) are cleared. 
       FIG. 30  is a diagram of a “PPI allocation request” command  289  that the DMA engine in an ingress NBI island can send to the packet engine  280  in the CTM  234  in the ME island  203 .  FIG. 31  is a diagram that sets forth the contents of the various fields of the PPI allocation request command of  FIG. 30 . The “PPI allocation request” command  289  is a CPP bus command whose action field  293  and token field  294  contain particular codes that identify the bus transaction value as being a “PPI allocation request” command as opposed to another type of command. The values in the final destination field  290  and in the target ID field  292  identify the packet engine to which the PPI allocation request command is directed. A 2-bit value in the length field  295  indicates the size of the packet portion for which a PPI is being requested. The values of two bits of the address field  296 , the value of the data master island field  297 , and the value of the master ID field  298  identify the requestor device. The value of the data reference field  299  is supplied, as in other CPP commands, so that a response to the command can include the data reference value so that the response can be associated with the original command. In the case of the “PPI allocation request” command, the data reference value is included as part of the associated “PPI allocation response”. In the table of  FIG. 31 , the notation [X;Y] indicates a field that is X bits long, starting at bit number Y. So, for example, the [2;0] notation in the table for the 2-bit “length of packet portion” code indicates that the code is two bits long, and that these two bits start at bit  0  of the 5-bit LENGTH field. 
       FIG. 32  is a diagram of a “PPI allocation response” bus transaction value  300  that the packet engine in the CTM in the ME island can back in response to a “PPI allocation request” command.  FIG. 33  is a diagram that illustrates various fields  301 - 307  of the PPI allocation response bus transaction value  300  of  FIG. 32 . The value of the data reference field  304  associates this response with a prior “PPI allocation request” command. The PPI value that is being communicated as having been allocated is indicated by the value in the PPI field  305 . The PPI field is a 9-bit part of the 64-bit DATA field as set forth in  FIG. 33 . The values of the final destination field  301  and the master ID field  303  identify the original requestor to which the “PPI allocation response” is being sent. As explained above, a “PPI allocation response” can, in addition to setting forth a PPI that is being allocated, also set forth PPI credits to be returned to the requestor (due to completion of one or more prior allocate operations that were then de-allocated) and buffer credits to be returned to the requestor (due to completion of the prior allocate operations that were then de-allocated). The value of the PPI credit field  306  indicates a number of PPI credits being returned to the requestor. The value of the buffer credit field  307  indicates a number of buffer credits being returned to the requestor. When the requestor receives these credit values, the requestor adds the PPI credits being returned value to the PPI “credits available” register value maintained in the requestor, and the requestor adds the buffer credits being returned to the buffer “credits available” register value maintained in the requestor. 
       FIG. 34  is a diagram of a bus transaction value  308  that is usable to transfer data, where the bus transaction value  308  has a PAM/LAM mode selection field  313 .  FIG. 35  is a diagram that illustrates the fields of the bus transaction value of  FIG. 34 . The bus transaction value  308  is a CPP “autopush”. The values of the final destination field  309  and the data master field  311  indicate the destination device to which the data is being sent. If the bit of PAM/LAM mode selection field  313  is set, then PPI addressing is employed and the PPI is carried in nine bits of the bus transaction value, where the first eight bits of the PPI are carried in the SIGNAL MASTER field as indicated in  FIG. 35  and where the ninth bit of PPI is bit eleven of the 14-bit DATA REF field as indicated in  FIG. 35 . If, on the other hand, the bit of the PAM/LAM mode selection field  313  is cleared, then LAM addressing is employed and the address is carried in eight bits of the 14-bit DATA REF field, starting at bit three, as indicated in  FIG. 35 . In the example of the method set forth in  FIG. 6 , multiple such “autopush” bus transaction values may be sent from the ingress NBI  209  to the packet engine  280  in order to communicate 2 k bytes the first 128 bytes of the packet portion as set forth in step  105  of the flowchart of  FIG. 6 . Each such “autopush” only transfers sixty-four bits (eight bytes) of data, so multiple such autopush bus transaction values are required to transfer the data. 
     An autopush bus transaction value  308  can also be directed to the packet engine  280 . If the bit of the PAM/LAM mode selection field  313  is set, then the packet engine converts the PPI carried by the autopush into a memory address, and the data carried by the autopush is written into the data cache SRAM starting at this memory address. If the bit of the PAM/LAM mode selection field  313  is not set, then the address carried by the autopush is used to write the data into the data cache SRAM. In one example of the packet engine, PAM addressing can write into a first part of the data cache SRAM but not into a second part, whereas LAM addressing can write into the second part of the data cache SRAM but not into the first part. How the data cache SRAM is partitioned into these two parts is configurable via the control bus (CB). 
       FIG. 36  is a diagram of a “packet complete” command  316  that the processing ME in the ME island  203  can send to the packet engine  280  in the CTM  234  in ME island  203 .  FIG. 37  is a diagram that sets forth the contents of various fields  315 - 330  of the packet complete command  316  of  FIG. 36 . The action field  317  carries a particular 5-bit code that identifies the CPP command as a “packet complete” command. The values of the final destination field  315  and the target ID field  317  identify the target device to which the “packet complete” command is directed. In the present example, the target device is the packet engine  280 . The value of the PPI field  320  indicates the PPI, the processing of whose corresponding packet portion is now indicated to have been completed. The value in the NBI NUM field  321  indicates the egress NBI to which the “packet complete” command should be forwarded by the packet engine. 
       FIG. 38  is a diagram of a PPI de-allocate command  322  that an egress NBI island can send back to the packet engine  280  in the ME island  203 .  FIG. 39  is a diagram that sets forth the contents of various fields of the PPI de-allocate command  322  of  FIG. 38 . The action field  326  and token field  327  carry a particular code that identifies the CPP command as a “de-allocate PPI” command. The PPI to be de-allocated is indicated by the value in the PPI field  328 . As indicated in  FIG. 39 , the PPI field is the first nine bits of the 40-bit ADDRESS field. The particular CTM target to which the PPI de-allocate command is directed is indicated by the values in the final destination field  323  and in the target ID field  325 . The device that sent the PPI de-allocate command is set forth by the values in the data master island field  329  and in the master ID field  330 . 
       FIG. 40  is a conceptual block diagram of the circuitry of the packet engine  280  of the CTM  234  of  FIG. 27 .  FIG. 41  is a more detailed diagram of one of the “find first in the slice” circuits in the MAT circuit  288  of  FIG. 40 . The circuitry of the “master interface circuit” block  281  in the diagram of  FIG. 27  is the circuitry  332  and  333  in  FIG. 40 . The circuitry of the “target interface circuit” block  282  in the diagram of  FIG. 27  is the circuitry  334  and  335  in  FIG. 40 . A “PPI allocation request” command is received by the T CMD portion of the data bus interface  236 . The command is decoded by the command decoder  336 , thereby causing the signals on conductors  337  to load a PPI allocation request into FIFO ALLOC  338 . FIFO ALLOC  338  stores PPI allocation requests, whereas FIFO DE-ALLOC  339  stores PPI de-allocation requests. The round robin arbiter  341  arbitrates between the requests output from the FIFOs  338 ,  339  and  340 , and through the WINNER signal on conductors  342  causes the state machine  285  to service one of the requests. In the case of the request being a PPI allocation request, the state machine  285  sends an allocation request signal ALLOC_REQ to the PAT circuit  287 , and a PPI is received back from the PAT circuit  287  in the form of PAT_NUM (PPI). The state machine  285  also sends an allocation request signal ALLOC_REQ to the MAT circuit  288 , and receives back a memory address CTM_ADDRESS. If the PAT and MAT circuits indicate that the PPI can be allocated, then the state machine  285  generates a PPI allocation response  343  and sends it to the CPP bus interface circuitry. The CPP bus interface circuitry handles transmitting a proper “PPI allocation response” onto the CPP data bus. In the event that a “PPI de-allocate” request command is received by the T CMD portion of the data bus interface  236 , then this command is decoded by decoder  336  and a PPI de-allocate request is loaded into FIFO DE-ALLOC  339 . After arbitration, the request is passed to the state machine  285 , which in turn sends de-allocation requests to the PAT and MAT circuits. Reference numeral  344  identifies the “Credits To Be Returned” (CTBR) registers that are maintained in the state machine  285 . For each potential “owner” of a PPI, there is a PPI CTBR and a buffer CTBR. The PPI CTBR stores the number of PPI credits to be returned to the owner on the next PPI allocation response, and the buffer CTBR stores the number of buffer credits to be returned to the owner on the next PPI allocation response. In the case of a de-allocate request command coming through the state machine, one PPI is being de-allocated so the PPI CTBR value for the “owner” indicated by the PAT is incremented by one. Similarly, the MAT indicates whether a buffer has been made available (due to memory space no longer being necessary to store the packet portion associated with the de-allocated PPI), and this number of buffer credits (one or zero) is added to the buffer CTBR value for the “owner”. The next time a PPI allocation response is sent to that owner, the CTBR values are included in the PPI allocation response. The CTBR values stored in the state machine for that owner are then zeroed. If a PPI allocation request passes out of arbiter  341  to the state machine  285 , but if the PAT or MAT circuits indicate that a PPI allocation cannot be made, then either: 1) the PPI allocation request is recirculated for a later attempt by loading it into FIFO OOC  340 , or 2) the failed PPI allocation request is signaled back to the requestor by setting an error code in the next PPI allocation response sent back to the requestor. 
     If the packet engine receives an “autopush” of the type set forth in  FIG. 34  and  FIG. 35 , then the address and data to be pushed pass through multiplexing circuit  345 . If PAM is selected as indicated by the PAM/LAM selection bit in the autopush bus transaction value, then the PPI  346  is supplied onto one of the translate inputs of the PAT circuit  287 . The PAT circuit  287  translates the PPI into a memory address ADDRESS  347 , which is supplied to the data cache interface  283 . Because time is required to perform the PPI-to-address translation, the data to be written in the autopush is pipelined in FIFO  348  so that the data and address (for the autopush write to memory) is supplied to the data cache interface circuit  283  at the same time. The data cache interface circuit  283  uses the address and data to write the data into the data cache SRAM  276 . The PAT circuit  287  performs PPI-to-address translation functions for the bulk engine  279  through bulk interface  284 . The PAT circuit  287  also performs PPI-to-address translation functions for the DMA handler  286 . Reference numeral  348  identifies a PPI value coming from the DMA handler, and reference numeral  349  identifies the returned address. 
     Reference numerals  390 - 392  identify first find and forwarding circuits. First find and forwarding circuit  390  identifies the first one of thirty-two requests from the ENTRY_ 0  to ENTRY_ 31  circuits and forwards the request to FIFO  393 . Second find and forwarding circuit  391  identifies the first one of thirty-two requests from the ENTRY_ 0  to ENTRY_ 31  circuits and extracts a PPI from the request, and forwards the PPI to the PAT  287  for translation into a memory address, and receives a memory address in response, and forwards the memory address to the particular one of the ENTRY_ 0  to ENTRY_ 31  circuits. Third find and forwarding circuit  392  identifies the first one of thirty-two requests from the ENTRY_ 0  to ENTRY_ 31  circuits and forwards the request to DMA master command FIFO  394 . 
       FIGS. 42A-42G  together form a larger  FIG. 42 . How  FIGS. 42-42G  fit together to form the larger  FIG. 42  is set forth in the key at the bottom of  FIG. 42G .  FIG. 42  is an amount of CDL hardware language description code that describes and specifies the state machine  285  of the packet engine  280  of  FIG. 40 . The state machine services requests from the allocation request FIFO  338 , the out of credits request FIFO  340 , and the de-allocate request FIFO  339 . The state machine has three states: an IDLE state, a PAT_STATE_ALLOC state, and a PAT_STATE_FREE state. In the CDL code, the PPI allocation table may be referred to in short as the “packet allocation table”. 
     In the case of allocating a PPI, the state machine is initially in the IDLE state. Upon receiving a PPI allocation request from the allocation request FIFO (FIFO ALLOC) or the out of credits FIFO (FIFO  00 C), the state machine: 1) sends an allocation request (ALLOC_REQ) to the PPI allocation table circuit, and 2) sends an allocation request (ALLOC_REQ) to the memory allocation table circuit. The memory allocation request has a size and an owner field. The state machine then moves to the PAT_STATE_ALLOC state. In the PAT_STATE_ALLOC state, the state machine waits for three cycles to receive allocation responses from the PPI allocation table circuit and the memory allocation table circuit. If in this period it has received both a memory allocation response (CTM_ADDRESS) from the memory allocation table circuit and a packet allocation response (PPI_NUM) from the PPI allocation table circuit, then the allocation is successful. In response, the state machine sends a “taken” signal to each of the PPI allocation table circuit and the memory allocation table circuit indicating that the responses are being used, and that the table circuits should mark their appropriate table entries for the PPI and CTM address as now being “in use”. The state machine then returns to the IDLE state. If, on the other hand, the state machine does not receive both a successful memory allocation response (because a memory slice was not available) and a successful PPI allocation response (because a PPI was not available for allocation) in the period, then one of the following actions is taken: 1) the allocation request is added to the FIFO OOC to retry at a later time, or 2) a PPI allocation response, whose value is all ones to indicate that the allocation has failed, is sent out of the packet engine. For a successful allocation, the PPI number is sent out of the packet engine in the allocation response, and in addition any packet and/or buffer credits to be returned (due to another earlier PPI de-allocation) are also indicated in the allocation response. The CPP fields of the allocation response come from the PALLOC_RE_FIFO (FIFO ALLOC) if the PALLOC_REQ_FIFO had won arbitration. Otherwise, they come out of the FIFO OOC. 
     In the case of de-allocating (also referred to as “freeing”) a PPI: the state machine is initially in the IDLE state. If FIFO DE-ALLOC won arbitration, then the state machine sends a DE-ALLOC_REQ signal to the PPI allocation table circuit and moves to the PAT_STATE_FREE state. In the PAT_STATE_FREE state, the state machine receives a “DE-ALLOC RESPONSE” from the PPI allocation table circuit. This “DE-ALLOC RESPONSE” contains the memory address (of the start of where the packet portion is stored) of the packet portion to be freed, and the size of the packet portion, and the owner of the packet portion. The state machine sends a memory de-allocation request to the memory allocation table circuit, which will then cause the associated buffer space in the memory to be indicated in the memory allocation table as being unused (free). Any credits (packet credits and buffer credits) that are now freed up due to the de-allocating operation are added back into the “Credits To Be Returned” (CTBR) registers for that owner. The state machine then moves back to the IDLE state. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.