Abstract:
Logic (also called “synchronizing logic”) in a co-processor (that provides an interface to memory) receives a signal (called a “declaration”) from each of a number of tasks, based on an initial determination of one or more paths (also called “code paths”) in an instruction stream (e.g. originating from a high-level software program or from low-level microcode) that a task is likely to follow. Once a task (also called “disabled” task) declares its lack of a future need to access a shared data, the synchronizing logic allows that shared data to be accessed by other tasks (also called “needy” tasks) that have indicated their need to access the same. Moreover, the synchronizing logic also allows the shared data to be accessed by the other needy tasks on completion of access of the shared data by a current task (assuming the current task was also a needy task).

Description:
PRIORITY CLAIM 
     This application is a continuation application of U.S. patent application Ser. No. 13/174,560, itself a continuation application of patent application Ser. No. 12/201,385, which in turn is a continuation application of U.S. patent application Ser. No. 10/117,781. U.S. patent application Ser. Nos. 13/174,560, 12/201,385 and 10/117,781 are each incorporated by reference herein in their entirety, including all Appendices. 
    
    
     CROSS-REFERENCE TO COMPUTER PROGRAM LISTING APPENDIX 
     Note that a computer program listing Appendix A originally filed in U.S. patent application Ser. No. 10/117,781 (see above) is hereby expressly incorporated by reference herein in its entirety. 
     Appendix A contains the following two files submitted herewith electronically, in IBM-PC format and compatible with MS-Windows. Appendix A is a part of the present disclosure and is incorporated by reference herein in its entirety, including: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
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                 Mar. 28, 2002 
                 02:31p 
                 365,686 
                 MEMCOP.TXT 
               
               
                   
                 Mar. 28, 2002 
                 02:33p 
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                 UCEXEC.TXT 
               
               
                   
                   
               
             
          
         
       
     
     The two files of Appendix A form source code of computer programs and related data of an illustrative embodiment of the present invention, as follows: 
     UCEXEC.TXT file describes the behavioral model of circuitry in a microcontroller&#39;s execution unit to decode and execute an instruction to provide a store-and-load command to a memory co-processor; and 
     MEMCOP.TXT file describes the behavioral model of circuitry of the memory co-processor, which is also known as a special processing unit (SPU). 
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and incorporates by reference herein in their entirety the following U.S. patent application(s): 
     (U.S. patent application Ser. No. 10/103,436) entitled “Dynamic Allocation of Packets to Tasks,” Nathan Elnathan et al., filed on Mar. 20, 2002, issued as U.S. Pat. No. 7,245,616; 
     (U.S. patent application Ser. No. 10/103,393) entitled “Reordering of Out-of-Order Packets,” Nathan Elnathan, filed on Mar. 20, 2002, issued as U.S. Pat. No. 7,072,342; 
     (U.S. patent application Ser. No. 10/103,415) entitled “Asymmetric Coherency Protection,” Ilan Pardo, filed on Mar. 20, 2002; 
     (U.S. patent application Ser. No. 10/117,394) entitled “Method and Apparatus to Suspend and Resume on Next Instruction for a Microcontroller,” Alexander Joffe, filed on Apr. 4, 2002, issued as U.S. Pat. No. 7,155,718; 
     (U.S. patent application Ser. No. 10/117,452) entitled “Method And Apparatus For Issuing A Command To Store An Instruction And Load Resultant Data In A Microcontroller,” Alexander Joffe et al., filed on Apr. 4, 2002; 
     (U.S. patent application Ser. No. 10/117,779) entitled “Memory Co-Processor for a Multi-Tasking System,” Alexander Joffe et al., filed on Apr. 4, 2002, issued as U.S. Pat. No. 6,938,132; and 
     (U.S. patent application Ser. No. 10/117,780) entitled “Sequencing Semaphore,” Alexander Joffe et al., filed on Apr. 4, 2002, issued as U.S. Pat. No. 6,978,330. 
     BACKGROUND OF THE INVENTION 
     A number of tasks executing in a microcontroller  20  (see  FIGS. 1A and 1B ) may take different paths (also called “code paths”) even if such tasks execute the same software program (also called “code”)  10 . For example, Task0 may make a jump in executing code  10  after performing a first policing function at a location S0, thereby to define a first code path  11 . Once Task0 makes the jump, Task0 does not need to perform the remaining policing functions S1 and S2 that are otherwise required during in-line (i.e. no jump) execution of software program  10 . 
     In the example of  FIG. 1A , another task, namely Task1 does not jump immediately after location S0 in software program  10 , and instead continues with in-line execution (e.g. executes a number of instructions immediately following location S0). However, Task1 may eventually make a jump after performing a second policing function at a location S1 in the software program  10 , thereby to define code path  12 . In a similar manner, Task2 may simply execute software program  10  without making any jumps immediately after locations S0 and S1, thereby to define code path  13 . 
     In the example being discussed, at various locations in the respective code paths, a decision to make a jump depends on the value of data that is shared among such tasks, Task0-Task2. For example, in a networking application, the policing functions performed at locations S0 and S1 may require that a packet that is being processed be dropped, for example if the rate exceeds a predefined bandwidth allocated to a channel to which the packet belongs. 
     Therefore, it is important for such tasks, Task0-Task2 to ensure that the data being used in the policing functions is accurate and updated properly. Access to such data may be arbitrated by use of a semaphore as a synchronization mechanism, to access a common location in memory, to ensure that data at that common location is not changed by one task while that data is being used by another task. Use of such a semaphore ensures, for example, that a packet counter is properly incremented (or a database entry is properly accessed) by each of a number of tasks that execute concurrently or simultaneously in the microcontroller. 
     In using a semaphore, when one task, e.g. Task0 in  FIG. 1B , is accessing a memory location at a location S0 in the code, other tasks, e.g. Task1, Task2, and Task3, that also need to access that same memory location are suspended (i.e. are made to wait). While such other tasks are waiting, Task0 may be activated from sleep, may issue a read request on being awakened, may again be put to sleep while waiting for a response from memory, may again be awakened on receiving the memory response, perform a read operation, and finally release the semaphore. Only at this point is the semaphore for code location S0 available for use by the next task, Task1. 
     Therefore, use of a semaphore effectively single threads the access to a common memory location in all of the tasks, even though a microcontroller (such as a network processor) may support multitasking. Such single threading causes the latency of each task to affect all subsequently-performed tasks. 
     U.S. Pat. No. 5,790,881 granted to Nguyen on Aug. 4, 1998 entitled “Computer system including coprocessor devices simulating memory interfaces” suggests (see abstract) “coupling a coprocessor to a master device, in which the coprocessor emulates an memory interface to the master device, like that of a memory device . . . . The coprocessor is disposed to receive data written from the master device, perform a coprocessing function on that data, and respond to a read data command from the master device with processing results.” 
     See also U.S. Pat. No. 6,338,108 granted to Motomura on Jan. 8, 2002 entitled “Coprocessor-integrated packet-type memory LSI, packet-type memory/coprocessor bus, and control method thereof” which states (see abstract) that “[a] memory section and coprocessor sections in a coprocessor-integrated packet-type DRAM are provided with unique memory device ID and coprocessor device IDs respectively . . . . ” 
     SUMMARY 
     The present invention relates to logic (also called “synchronizing logic”) that receives a signal (called a “declaration”) from each of a number of tasks, based on an initial determination of one or more paths (also called “code paths”) in an instruction stream (e.g. originating from a high-level software program or from low-level microcode) that a task is likely to follow. An initial determination by a task may be based on any information available prior to generation of the initial signal, including, for example, information in a header (of a packet or a cell) in the case of a networking application (wherein each task processes a packet/cell). 
     In some embodiments, each of the declarations identifies, for each of a number of predefined locations in a software program (e.g. one or more of locations S0-S2 in  FIGS. 1A and 1B ), whether or not that task expects to access data (also called “shared data”) that is also to be accessed by other tasks (e.g. on executing the same instructions in the instruction stream). 
     A task that indicates in the declaration a likely future need to access a shared data may decide at a later time that access to that shared data is no longer needed, and may indicate this to the synchronizing logic (via another declaration). However, in some embodiments, the opposite is not permitted, i.e. a task that has previously declared no need to access a shared data cannot change its no-need decision (e.g. due to changed circumstances), to declare that it now needs to access that shared data. 
     Once a task (also called “no-need” task) declares its lack of a future need to access a shared data (regardless of whether this happens in a first synchronization request or in a subsequent synchronization request), the synchronizing logic allows that shared data to be accessed by other tasks (also called “needy” tasks) that have indicated their need to access the same. Moreover, the synchronizing logic also allows the shared data to be accessed by the other needy tasks on completion of access of the shared data by a current task (assuming the current task was also a needy task). 
     In some embodiments, commands (such as a “store and load” command) to access the shared data from needy tasks are handled in order (so that allowing a no-need task to change its decision may result in out-of-order processing of access requests, which defeats the premise of in-order processing). In several such embodiments, each task is assigned a sequence number depending on the order in which the data being processed therein (e.g. a packet or a cell) is received, relative to the other tasks. Therefore, the synchronizing logic grants access to the shared data, based on the sequence number (and for this reason such a synchronizing logic is hereinafter called a “synchronizing sequencer”). 
     Such a synchronizing logic may be implemented in either hardware or in a microprocessor programmed with software in different embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate code paths of tasks in the prior art, wherein the tasks need to synchronize their access to shared data at various synchronization points. 
         FIG. 2A  illustrates each task issuing a “declaration” signal in accordance with the invention, to a synchronizing logic, followed by issuance of instructions to change shared data. 
         FIG. 2B  illustrates a table of states maintained in accordance with the invention for use in providing access to shared data. 
         FIG. 3A  illustrates, in a block diagram, use of a memory co-processor to interface a microcontroller to external memory, in accordance with the invention. 
         FIG. 3B  illustrates, in a flow chart, acts performed by a task in a microcontroller for accessing shared data in one embodiment. 
         FIG. 3C  illustrates, in a timing chart, a sequence of events when a task accesses shared data at a specific location in one embodiment. 
         FIG. 4A  illustrates, in a state diagram, transitions between various states of each task in accessing shared data in one embodiment. 
         FIGS. 4B and 4C  illustrate, in flow charts, acts performed by a synchronizing sequencer of one embodiment. 
         FIGS. 5A-5L  illustrate exemplary transitions that occur in a synchronization request array of the type illustrated in  FIG. 2B . 
         FIG. 6A  illustrates, in a block diagram, circuitry for a single synchronization point used to identify a sequence number of an instruction to be executed in one embodiment. 
         FIG. 6B  illustrates, in a block diagram, replication of the circuitry of  FIG. 6A  to implement a number of synchronization points in accordance with the invention. 
         FIG. 6C  illustrates, in a high-level block diagram, a memory co-processor of one embodiment, in which a sequencer of the type illustrated in  FIGS. 6A and 6B  is implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the invention, on startup or a short time thereafter prior to accessing any shared data, a number of tasks Task0-TaskM (wherein 0≦J≦M, M being the total number of tasks) issue signals (called “declarations”) to a logic (called “synchronizing logic”) as illustrated in  FIG. 2A , to identify whether or not the tasks expect to access shared resources at certain predefined synchronization points SA-SN (wherein A≦I≦N, N being the total number of synchronization points) in their instruction streams. If access to a shared resource is expected at a synchronization point SA, then this synchronization point SA is indicated as having a state “enabled” and otherwise indicated as having the state “disabled.” 
     The synchronizing logic receives such state signals from each task “TaskJ”, for each synchronization point SI, and stores the state locally in a buffer (e.g. in a two-dimensional table as illustrated in  FIG. 2B ). The synchronizing logic uses the states in deciding whether or not to allow an instruction from a task to access the shared data. When an instruction affecting a shared resource is received from a task for any specific synchronization point SI, the synchronizing logic changes a state associated with the task from “enabled” to “pending.” 
     If a task&#39;s state is “pending” and if it is now this task&#39;s turn, then the synchronizing logic issues any instructions from this task (such as a read-modify-write instruction) to the execution unit. Specifically, the synchronizing logic maintains a current pointer for each synchronization point SI, and this current pointer indicates which task&#39;s instruction (for accessing shared data) is to be now executed. 
     If the current pointer points to a task that has not yet provided an instruction and if the task is “enabled” then any instructions from other tasks that are received are made to wait until the current task&#39;s instruction is received and executed. If a task does not expect to issue such an instruction (and the state is “disabled”), then the current pointer is incremented to identify the next task that is current for this synchronization point SI. 
     Therefore, if a number of tasks do not expect to access a particular resource, then these tasks are assigned the state “disabled”. In providing access to that particular resource, all disabled tasks are skipped over, thereby to speed up the arbitration. The tasks just described as being “disabled” may expect to access one or more other shared resources, and indicate this need in their respective declarations. 
     If an instruction is received from a task “TaskJ” for a specific synchronization point SI, and if the current pointer for this synchronization point SI is pointing to TaskJ, then that instruction is immediately executed. Moreover, in some embodiments, the current pointer is incremented (or increased) to point to the next task from which an instruction is to be executed. 
     In certain embodiments, the increment (or increase) is unconditional (i.e. automatic), and alternatively in other embodiments the increment (or increase) depends on a specific signal from the task. The just-described signal (also called “lock current pointer” signal) may be activated by a task, to allow any number of instructions from the task to be executed for the current synchronization point SJ, prior to execution of an instruction from another task. 
     Use of such “lock current pointer” signal (and a version of this signal is called “update sequence number” signal) implements a critical section of any length while the signal is active, thereby causing the synchronizing logic to act as a semaphore of the type described in detail in a related U.S. patent application Ser. No. 10/117,780 that has been incorporated by reference above. 
     Increment of the current pointer described above may be optimized in some embodiments to be sufficiently large to skip over all tasks that do not expect to provide any instructions for accessing or changing the shared data for this synchronization point SI. In several embodiments, operation of a current pointer and allocation of a shared resource at one synchronization point SI is independent of the corresponding operation and allocation at another synchronization point SJ. Therefore, there may be no sequence relation between, for example, two policing operations (which may be used to control the flow of packets in a network, for example). For this reason the two policing operations may be identified as two synchronization points SI and SJ. 
     In one specific embodiment, the synchronizing logic selectively provides access to a number of shared data (e.g. eight shared data) among multiple tasks that execute in a microcontroller  301  ( FIG. 3A ), such as a network processor. Microcontroller  301  is coupled to and controls the operation of one or more channel processors that process packets and/or cells, as described in, for example, U.S. Pat. No. 6,330,584 granted to Joffe that is incorporated by reference herein in its entirety. Each of the channel processors assigns a sequence number to each packet according to their order of arrival, and the sequence number along with a pointer to the packet is supplied to microcontroller  301 . 
     The sequence number that is assigned to each packet may be any monotonically changing number (e.g. a number that is incremented or decremented by 1 for each packet arrival), and depending on the embodiment the sequence number is unique globally across all channel processors or alternatively the sequence number is unique only within a channel processor (or only in one direction of one channel processor). 
     On receipt of a packet pointer and its associated sequence number, microcontroller  301  assigns the packet to a task executing therein (an assignment may be performed by, for example, a request unit). The task generates a declaration followed by one or more instructions to the co-processor, to change shared data in memory. The just-described signals generated by a task are handled by a synchronizing logic which is hereinafter referred to as a synchronizing sequencer, by virtue of its use of sequence numbers in deciding the order of access. 
     A synchronizing sequencer of the type described herein can be implemented off-chip, i.e. outside of microcontroller  301  in which various tasks are executing. In one such embodiment illustrated in  FIG. 3A , a synchronizing sequencer  305 B is implemented in hardware in a separate co-processor  305  that is used in one embodiment to access external memory (as described in, for example, U.S. patent application Ser. No. 10/117,779 incorporated by reference above). 
     Memory co-processor  305  includes, in addition to synchronizing sequencer  305 B, a number of buffers  305 A in which are stored tables  305 A (of the type described above; also called “synchronization request arrays”). There is one table among tables  305 A for each synchronization point S0-SN. As noted above, multiple tables  305 A identify the state of access of shared data by various tasks (which in this example are identified by sequence numbers, because each task is assigned to a sequence number and works on a packet of that sequence number). 
     Each table  305 A holds the state of a group of (e.g. all) sequence numbers that identify which one of the tasks currently needs to supply a command to memory co-processor  305 . When a task has either indicated no need to access the shared data or the task&#39;s instruction (such as a read-modify-write instruction) to change the shared data has been executed, the sequence number is incremented (assuming the “update sequence number” signal is enabled with the read-modify-write instruction), after setting the synchronization point&#39;s state for the current sequence number (prior to incrementing) to “invalid”. 
     Soon after startup, each task compares one or more items of information in its packet header to predetermined values, thereby to classify the packet (see act  311  in  FIG. 3B ). Next, the task identifies (see act  312 ) synchronization points at which processing of the packet is likely to access data shared with other tasks, e.g. at points S0, S1 and S2 illustrated in  FIGS. 1A and 1B . For any given software program, synchronization points are known ahead of time, based on packet type. 
     Next, the task issues (see act  313  in  FIG. 3B ) a “declare” command to a synchronizing sequencer. In some embodiments, each “declare” command identifies, in addition to predefined synchronization points, an identifier of the task that processes the packet (as per  FIG. 3C ; also see bus  302  in  FIG. 3A ). 
     After issuing a declare command, each task simply continues, e.g. executes software to process the packet (see act  314  in  FIG. 3B ), until a synchronization point is reached. When a synchronization point is reached, each task goes to act  315  ( FIG. 3B ) to check if it is necessary to modify the shared data in memory. Under normal circumstances, this is true, and therefore the task goes to act  316  to issue an instruction to change the data (e.g. increment a packet counter). Alternatively, the task may go to act  317  (from act  315 ), e.g. if on performance of a policing function it is decided to drop the packet being processed. If so, the task indicates its change in expectation, about not needing to use shared data at one or more synchronization points that were previously marked “enabled”. 
     A synchronizing sequencer receives each instruction issued by each task, and uses a buffer (of the type described above in reference to  FIG. 2B ) to determine the action to be taken. For example, synchronizing sequencer temporarily stores each instruction in a buffer (also referred to as an “out of sequence” buffer), unless the following two conditions are met: (1) the task is currently enabled to access the shared data for this synchronization point and (2) it is now the current task&#39;s turn (as noted above in reference to  FIG. 2B ) to access the shared data. 
     In one example, the signals exchanged between a microcontroller-based task and coprocessor-based synchronizing sequencer are illustrated in  FIG. 3C . Specifically, in this example, a task first assembles and issues a declare command for each of a number of synchronization points that it expects to encounter when processing the just-received packet. Thereafter, the task assembles and issues a command to change shared data at a first synchronization point SA. 
     The issued command may be, for example, a “store-and-load” command (of the type discussed in the related U.S. patent application Ser. No. 117,452 incorporated by reference above), and if so, the memory co-processor returns a result to the microcontroller, indicating, for example, the task that issued the command and optionally a register in which the result is to be stored. Any number of such store-and-load commands or alternatively store commands may be issued by the task, based on synchronization points SA-SN identified in the declaration. 
     On receipt of a declare command identifying a particular synchronization point SI as being “enabled”, the synchronizing sequencer changes a value in the above-described table from “invalid” (which is a default value) to “enabled”, as illustrated by branch  401  ( FIG. 4A ). Thereafter, when the synchronizing sequencer receives a store-and-load (or store) command, the synchronizing sequencer changes an “enabled” value in the table to “pending” (as illustrated by branch  402 ). When a task&#39;s command has been executed, the synchronizing sequencer changes a “pending” value in the table to “invalid” (as illustrated by branch  403 ). 
     When a task indicates its change in expectation, about not needing to use shared data at one or more synchronization points that were previously at the state “enabled”, the synchronizing sequencer changes an “enabled” state in the table to “disabled” (as illustrated by branch  404 ). Branch  404  is only a unidirectional branch, meaning that a state “disabled” is not to be changed to “enabled”. A task may also indicate at the inception that it does not expect to participate in changing data at a specific synchronization point, and in this case, the synchronizing sequencer changes an “invalid” state in the table to “disabled” (as illustrated by branch  405 ). 
     The synchronizing sequencer changes a “disabled” state in the table to “invalid” (as illustrated by branch  406 ) when incrementing a current pointer (beyond the sequence number for the current task). The synchronizing sequencer never changes an “enabled” state in the table to “invalid”. 
     In one embodiment, the synchronizing sequencer is implemented by a number of processes that execute in parallel with one another. Specifically, one process  410  (illustrated in  FIG. 4B ) receives and handles all commands from the various tasks, while other processes  420  (illustrated in  FIG. 4C ) issue to an execution unit, instructions received from these tasks if so indicated by the state of each task. Therefore, in this embodiment, a single process  410  ( FIG. 4B ) handles all commands, for all synchronization points (i.e. for all request arrays). 
     Initially, process  410  simply waits for a command (as per act  411 ), and on receipt of a command from a task in a microcontroller, goes to act  412  to check if the command was a “declare” command. If so, process  410  goes to act  413  to copy enable/disable states for this task from the command into all request arrays (for all synchronization points), and thereafter returns to act  411 . 
     If in act  412  the command received is not a declare command, then the command contains an instruction for changing the shared data (such as a store-and-load instruction or a store instruction). If so, process  410  goes to act  414  and changes a state value of a location in the request array corresponding to the task that generated the command (e.g. for a specific sequence number), for the current synchronization point from “enable” to “pending”, and thereafter returns to act  411 . 
     As noted above, a number of processes  420  ( FIG. 4C ) execute in the synchronizing sequencer, one for each request array. Specifically, the sequencer retrieves in act  421  the state of a location (in the request array) that is identified by the current pointer. If the retrieved state is “invalid” (see act  422 ), then the sequencer simply returns to act  421  (after the current clock cycle has completed). If the state is not “enabled” (see act  424 ), the sequencer simply resets the state to “invalid” (see act  425 ), and then increments the current pointer (see act  426 ), and returns to act  421 . 
     If the state is “enabled” then the sequencer waits for the state to change (see act  427 ), and this may take several clock cycles, depending on the code path of a task (associated with the current location in the request array). When the state changes, the sequencer checks (in act  428 ) if the state is “pending” and if not, goes to act  425  (discussed above). If the state is “pending” then the sequencer supplies the instruction to the execution unit (see act  429 ), thereby to allow access to shared data. 
       FIG. 5A  illustrates two arrays: (1) a two-dimensional array  501  (which is 4×4 in size in this example) that functions as a synchronization request array for a specific synchronization point (e.g. the first synchronization point S0) and (2) a one-dimensional array  502  (which is 1×4 in size in this example) that is used to navigate to the next location in array  501 , as discussed below. 
     In  FIG. 5A , array  501  has a row pointer Rptr pointing to the bottom-most row and a column pointer Cprt pointing to the left-most column, so that the two pointers together point to a first location in array  501 , in column 0, row 0, which is labeled as “00”. For example, based on certain number of high order bits of the sequence number, an entire row may be read in hardware, and the remaining low order bits of the sequence number are used to select one value from the row. 
     In one example illustrated in  FIGS. 5A-5L , all possible sequence number values are mapped to one of the locations in array  501 . For example, the sequence numbers may be mapped sequentially from left to right in the bottom row, followed by left to right in the row adjacent to the bottom row and so on (with the top right corner location of array  501  representing the largest possible sequence number). 
     In one example, a task that processes a packet with sequence number 0 does not expect to change the data (at the first synchronization point S0), and for this reason the “00” location in array  501  is initially set to value “0” which indicates “disabled” state (e.g. as a result of a “declare” command). The synchronizing sequencer resets the value in the first location “00” of array  501  to the value “x” which indicates “invalid” state, and moves the column pointer to point to the next column on the right (see  FIG. 5B ). Note that the row pointer remains the same until the column pointer reaches the right-most location of array  501 . 
     The synchronizing sequencer notes that the second location “01” of array  501  is currently set to value “x” and therefore, a declare command has not yet been received (see  FIG. 5C ) from a task with sequence number 1, for the first synchronization point S0. At some time, a declare command from a task with sequence number 2 is received, and the value “1” received therefrom is loaded into array  501 , in the location “02”. Next, a declare command from a task with sequence number 5 is received (for the first synchronization point S0), and the value “1” received therefrom is loaded into array  501 , in the location “11”. Then, at a later time, a declare command from a task with sequence number 1 is received (for the first synchronization point S0), and the value “1” received therefrom is loaded into array  501 , in the location “01”, as shown in  FIG. 5D . 
     Next, a “store-and-load” command is received for the first synchronization point S0, from a task with sequence number 2. So the synchronizing sequencer loads the value “2” into array  501  (see  FIG. 5E ), in the third location “02”. Since it is not yet the turn of sequence number 2 (because the current pointer is still pointing to the second location “01” of array  501 ), the synchronizing sequencer simply places the received command in a buffer (called “out-of-sequence” buffer). 
     An out-of-sequence buffer which is used may be, for example, dedicated to holding instructions only for the first synchronization point S0 (so that there are multiple such buffers, one for each synchronization point), although a larger buffer to hold instructions for all synchronization points, may be used in other embodiments. 
     In view of this disclosure, the to-be-executed instructions being held in out-of-sequence buffers may be handled in a manner similar or identical to the prior art handling of “flows” of packets, with one flow for each synchronization point. Hence, each flow will have a task (and sequence number) that is identified by a current pointer. 
     Thereafter, at some point, a task with sequence number 1 issues a “store-and-load” command for the first synchronization point S0. The synchronization point for which a command has been issued is identified in the command, e.g. as the value of an operation number OPN (e.g. one of OP0-OP7 as per  FIG. 6B ). As a result of receiving a store-and-load command, the synchronizing sequencer loads the value “2” into array  501  (see  FIG. 5F ), in the second location “01”. 
     Since the current pointer is also pointing to the second location “01”, it is now the turn of the task with sequence number 1. Therefore, the synchronizing sequencer immediately passes an instruction in the just-received command for execution by an execution unit in the memory co-processor. For details on the memory co-processor, see the related U.S. patent application Ser. No. 10/117,779 that was concurrently filed with U.S. application Ser. No. 117,781 and incorporated by reference above. 
     After supplying the instruction to the execution unit, the synchronizing sequencer resets the value in the second location “01” of array  501  (see  FIG. 5F ) to the value “x” which indicates “invalid” state, and moves the column pointer to point to the next column on the right (see  FIG. 5G ), while the row pointer remains stationary. 
     At this point, the current pointer is pointing to a location “02” of array  501 , with value “2” which means an instruction is pending (e.g. in the out-of-sequence buffer). The synchronizing sequencer sends this instruction for execution, and changes the value to “x” to indicate that this sequence number has been now processed for the first synchronization point S0. 
     At this point ( FIG. 5H ), the synchronizing sequencer notes that the next location has value “0” (meaning “disabled”), and changes this to value “x”. Then synchronizing sequencer moves the current pointer up to the next row, to location “10” (which represents sequence no. 4). Since the values for location “10” is “0” (meaning “disabled”), the synchronizing sequencer changes this to value “x” and moves the pointer by one location to the right (not shown). 
     The location “11” (which represents sequence no. 5) has a value “2” which means an instruction is pending (e.g. in the out-of-sequence buffer), and the synchronizing sequencer supplies this instruction to the execution unit, changes this location to value “x” and moves the pointer by one location to the right (see  FIG. 5I ). 
     At this stage, the pointer is pointing to the location “12”, which has a value “2” indicating that an instruction is pending (e.g. in the out-of-sequence buffer). Again, the synchronizing sequencer supplies this instruction to the execution unit, changes this location to value “x” and moves the pointer by one location to the right (not shown). The location “13” has value “0” thereby to indicate this sequence number is “disabled” and therefore the value is changed to “x” and the pointer is moved up the next row (see  FIG. 5J ). 
     In a similar manner, the upper-most row is eventually reached as illustrated in  FIG. 5K , and the first two locations “30” and “31” are skipped because of the value “0” indicating that these sequence numbers are “disabled” and these values are changed to “x”. The pointer now points to the location “32” which has a “pending” instruction which is thereafter executed, thereby to yield the array illustrated in  FIG. 5L . 
     In the above-described manner, whenever the synchronizing sequencer receives a command from a task, it identifies and updates an appropriate array  501  associated with the specified synchronization point and sequence number. The synchronizing sequencer also (independently in one embodiment) evaluates the value at a location identified by the current pointer, and moves the current pointer if appropriate. 
     Some embodiments sequentially increment a row pointer until the right-most location of array  501  is reached. However, in other embodiments combinational logic is used to determine the next state to be processed, hence skipping a number of disabled states. Also, in some embodiments, another array  502  (also called “navigation array”) is used to identify the next row that has a location to be evaluated (e.g. all intermediate rows in which all locations are disabled are skipped). 
     Specifically, in one embodiment, an entire row of array  501  is read simultaneously (in hardware). Values in the row are simultaneously evaluated by combinational logic, and as a result of evaluation, the row may be updated (e.g. by changing the state to “invalid”) if execution is to proceed to another row. Also as a result of evaluation navigation array  502  may be updated. 
     The evaluation result can have one of the following three values: value 0 if all locations in a row are “disabled” (in which case this row in array  501  will be completely skipped by the synchronizing sequencer when issuing instructions to the execution unit), value 1 if at least one location in the row is “pending” (in which case the synchronizing sequencer will read the row and possibly issue an instruction; issuance of an instruction can happen under the following two conditions (1) if no intermediate sequence number is “enabled” i.e. waiting to become “pending” or “invalid” and (2) if no intermediate sequence number is “invalid” i.e. waiting for state to become “enabled” or “disabled”), and value x if all locations in a row are not disabled and there is no instruction pending (in which case the synchronizing sequencer may not even read the row). 
     Therefore, in a single clock cycle, the synchronizing sequencer skips one or more rows that do not have relevant values. The synchronizing sequencer also avoids processing a row that doesn&#39;t have any pending instruction. 
     The number of locations in a row of array  501  is limited only by the number of values that can be evaluated in a single clock cycle, e.g. evaluating 32 values needs more time than evaluating 16 values (e.g. 8 nanoseconds may be required for 16 values to be evaluated). In one specific implementation, array  501  is 16×16 in size, with 2 bits of value per location. 
       FIG. 6A  illustrates, in a block diagram, circuitry (also called “synchronizer element”) for a single synchronization point used to extract a sequence number indicative of a pending instruction to be executed, from a request array  501 , using a navigation array  502  (both arrays are illustrated in  FIGS. 5A-5L ). The synchronizer element of  FIG. 6A  also includes combinational circuitry  503  to implement state transitions, as described above in reference to, for example,  FIG. 4A . The synchronizer element of  FIG. 6A  also includes circuitry  504  to perform the process described above in reference to, for example,  FIG. 4C . Also illustrated in  FIG. 6A  are various combinational logic elements and storage elements to perform various acts of the type described in general herein and in detail in the attached Appendix A (filed electronically, but incorporated by reference herein in its entirety). 
     In one embodiment (see  FIG. 6A ), the sequencer maintains a table with a 2 bits of state per sequence number. These 2 bits indicate if the entry has a valid pending request to be issued to the execution unit or has an entry which could be skipped. The arrangement of this table is not in an array of 256*2 bits but in an array of 16 entries*16 consecutive states thus covering the 256 sequence numbers. 
       FIG. 6B  illustrates, in a block diagram, replication of the synchronizer element of  FIG. 6A  in blocks op0-op7, to implement a number of synchronization points, and using arbitration (e.g. round-robin) to select one of the sequence numbers across all blocks, and use of that sequence number to pass to a decoder an instruction to be executed. In the implementation illustrated in  FIG. 6B , all parameters for an instruction are stored in a parameters table that is indexed by a task number. The task number is supplied from a task number table that in turn is indexed by a sequence number. As noted above, each synchronizer element of  FIG. 6A  provides a sequence number, and one of these numbers is selected by arbitration. Although there may be a large number of values for a sequence number, the number of tasks is limited, and for this reason one level of indirection saves storage space in the parameters table (which holds, e.g. 32 entries, one entry for each task number). 
       FIG. 6C  illustrates, in a high-level block diagram, a memory co-processor in which a sequencer of the type illustrated in  FIGS. 6A and 6B  is implemented. A memory co-processor illustrated in  FIG. 6C  also contains an input control block  601  that directs commands received on a bus  602  either to a sequencer  603  or to a command FIFO  604  that holds various instructions (along with a memory address and a task identifier) for decoding by decode unit  606  and execution by execution unit  608 . 
     Such a decision is made based on the type of command received, e.g. if the command type is not “SPU” (which is an name for the memory co-processor of one embodiment), then the command is written directly to the FIFO. If the command type is “SPU” and if the “enable sequencing” signal (which is in a field in the command that has been received) is inactive then the command is written directly to the FIFO. In all other cases, the command is passed to the sequencer. As noted elsewhere herein, the sequencer buffers commands that are received out of sequence, and after the next command (based on a previously processed command&#39;s sequence number) is received, then as many commands as can be issued in sequence are issued to the command FIFO. 
       FIG. 6C  also contains a decode unit  606  that decodes each instruction in the FIFO, and passes the decoded instruction and parameters to the execution unit  608 . The execution unit in turn executes the instruction, e.g. performs a read-modify-write operation to read data from memory, change the data based on the specified modify function (such as policing) and writes the data back to memory. 
     Note that in some embodiments, there is one sequencer for eight ingress ports and another sequencer for eight egress ports. Moreover, in some embodiments, a cross-connect (called “XMAU”) connects an execution unit of a microcontroller to a sequencer in a memory co-processor. 
     In one particular embodiment, there are two command FIFOs in the memory co-processor, for holding (1) commands transferred directly to the execution unit (e.g. simple load, store and crc store commands), and (2) commands that go through the sequencer first (so that the sequencer reorders these commands according to their sequence number). 
     In such an embodiment, a memory co-processor distinguishes between the two types of commands, because the commands that go through the sequencer usually need to use the alu stage of the execution unit pipe, and the alu stage is not available every cycle. The simple load and store commands can be inserted then, in order to avoid loss of cycles due to stalls. 
     The memory co-processor of one embodiment implements the Virtual Scheduling Algorithm of the ITU I.371 to perform policing of ATM cells. The memory co-processor uses an extension to this algorithm for variable length frames. Furthermore, in some embodiments, for ATM cells the memory co-processor performs the Virtual Scheduling Algorithm GCRA(I, L). For variable size frames the memory co-processor performs extended GCRA(I, L, S) algorithm (S stands for frame size in pre-defined units, e.g., 64 bytes). The memory co-processor returns a single bit to indicate conforming/non-conforming. 
     Numerous modifications and adaptations of the embodiments, examples, and implementations described herein will be apparent to the skilled artisan in view of the disclosure. 
     For example, under certain circumstances, no-need tasks may be allowed to change their decision, e.g. if a needy task that is to be granted access has not yet issued an access request. Moreover, instead of granting access to the shared data to tasks as per sequence number, a synchronizing logic of the type described herein may grant access in any order among the tasks, depending on the embodiment. For example, such a synchronizing logic may operate in a first-in-first-out manner by granting access first to a task that sends in its synchronization request first. Alternatively, the synchronizing logic may grant access to the shared data based on a priority associated with each task, or simply in a round-robin fashion. 
     Numerous such modifications and adaptations of the embodiments described herein are encompassed by the attached claims.