Abstract:
In a networked system in which high speed busses interconnect sources and destinations of data, systems for and methods of data alignment, data re-timing, and circular buffer underflow/overflow detection, are described. The invention is directed to a system for detecting either or both underflow and overflow of a circular buffer capable of holding n entries. The invention is also directed to a method of detecting either or both underflow and overflow of a circular buffer capable of holding n entries.

Description:
This application is a divisional application of U.S. application Ser. No. 10/405,961, filed Apr. 1, 2003, now U.S. Pat. No. 7,366,935; which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates to the field of networked systems in which high speed busses interconnect sources and destinations of data, and, more specifically, to alignment, re-timing, and buffer underflow/overflow enhancements for such systems. 
   RELATED ART 
   In current network environments in which high speed busses interconnect senders and receivers of data, the senders and receivers may be embodied in various forms, including end user devices, such as PCs, intermediate devices such as switches, routers, and gateways, or even asynchronous integrated circuit chips (ASICs) situated within any of the foregoing. Such devices often operate substantially autonomously with respect to one another. Therefore, in the case in which the senders transmit data to various receivers around the networks, situations can arise in which the receivers are overwhelmed by the volume of data transmitted by the senders. For example, a sender can transmit a block of data to a receiver which exceeds the amount of free storage which is available in one or more first-in-first-out (FIFO) buffers maintained by the receiver for the purpose of buffering data received over the network. 
   To manage or avoid these situations, various mechanisms for flow control have emerged. According to a first mechanism, illustrated in  FIG. 1 , a sender  102  transmits data  106  to a receiver  104 . When the receiver  104  is unable to accommodate additional data, it transmits a transmission off (XOFF) signal  108  to the sender  102 , advising it to stop transmitting data to the receiver  104 . Responsive to the receipt of this signal, the sender  102  ceases transmitting data to the receiver  104 . When the receiver is able to accommodate additional data, it communicates a transmission on (XON) signal, also identified with numeral  108 , to the sender  102 . Responsive to the receipt of this signal, the sender  102  restarts transmitting data to the receiver  104 . 
   A problem with this mechanism is that it generates an excessive amount of overhead, embodied in the form of successive XON and XOFF signals, in the case in which the FIFO buffers used to buffer data at the receivers are relatively small. Therefore, other mechanisms for flow control have emerged. 
   One such mechanism is illustrated in  FIG. 2 . There, a sender  202  transmits a block of data  210  to the receiver  204 . The sender  202  maintains a counter  206  which is representative of the amount of free space available in one or more FIFO buffers  208  maintained at the receiver  204 . When a FIFO buffer is initially freed up at the receiver  204 , the receiver sends an increment command  212  to the sender, over one or more signal lines. Responsive thereto, the sender  202  increments the counter  206  with the (known) size of the buffer. When the sender  202  sends a block of data to the receiver  204 , it checks the contents of the counter  206  to determine if the one or more buffers  208  maintained at the receiver  204  can accommodate the block of data. If the block of data exceeds the contents of the counter  206 , it is determined that the one or more buffers maintained at the receiver  204  cannot accommodate the block of data, and the sender  202  defers sending the data until additional buffer space is available. If the block of data is less than or equal to the contents of the counter  206 , it is determined that the one or more buffers maintained at the receiver can accommodate the block of data. Hence, the sender  202  transmits the block of data to the receiver, and decrements the counter  206  by the size of the block of data. 
   This approach reduces the amount of overhead signals transmitted between the sender and receiver in the case in which small FIFO buffers are maintained at the receiver. It also avoids latency in the time required for the sender to determine the amount of free space at the receiver. A drawback is that the sender and receiver can easily get out of synchronization with respect to the perceived amount of free space at the receiver if, for example, an increment command sent to the sender is lost, or if a block of data transmitted to the receiver is lost. When this occurs, the error is catastrophic and cannot be recovered from. 
   Other problems occur when the data from the sender is source synchronous. Source synchronous data must often be synchronized to a clock which is used to synchronize events at the receiver. This is a difficult task to achieve, particularly when multiple source synchronous streams are concurrently being received at the receiver. Also, with source synchronous data, it is difficult to accurately detect buffer overflow/underflow conditions by comparing read and write pointers. 
   RELATED APPLICATION 
   This application is related to U.S. Pat. No. 7,272,672 entitled “High Speed Bus With Flow Control and Extended Burst Enhancements,” filed Apr. 1, 2003, and owned in common by the assignee hereof, which is fully incorporated by reference herein as though set forth in full. 
   SUMMARY 
   In a first aspect of this disclosure, a system is described for aligning a plurality of source synchronous streams of data. In this system, a buffer corresponding to each of the streams of data is provided. In addition, a status indicator corresponding to each of the streams of data is provided. This status indicator indicates whether or not data for the corresponding stream has begun to be clocked into the corresponding buffer. 
   First logic in the system clocks data from a stream into the corresponding buffer using the source synchronous clock for the steam, and also sets the status indicator for the stream to indicate that data for the stream has begun to be clocked into the corresponding buffer. 
   Meanwhile, second logic in the system checks the status indicators. Upon or after determining that data for all of the streams has begun to be clocked into the corresponding buffers, the second logic clocks data out of each of the buffers using a local clock common to all the buffers. In one embodiment, the local clock is a master clock for the system. 
   In a second aspect of this disclosure, a system for re-timing a source synchronous stream of data is described. In this system, first logic clocks data from the stream into a buffer using the source synchronous clock for the stream, and second logic clocks data out of the buffer using a local clock. In one embodiment, the local clock is a master clock for providing overall synchronization of the system or a networked system of which the re-timing system is a part. 
   In a third aspect of this disclosure, a system is described for detecting either or both underflow and overflow conditions of a circular buffer capable of holding n entries, wherein n is an integer of two or more. In this system, each entry in the buffer includes a tag portion. 
   A first counter is provided which circularly counts through m values, wherein m is an integer which is less than n and not evenly divisible into n. First logic performs a read operation by (1) reading an entry from the buffer, (2) comparing the tag portion of the entry with the contents of the counter, and (3) either or both (a) signaling an underflow condition if the tag portion of the entry is less than the contents of the counter, and (b) signaling an overflow condition if the contents of the tag portion of the entry exceed the contents of the counter. If the contents of the counter equal the tag portion of the entry, the contents of the first counter are updated to reflect the read operation. 
   Other systems, methods, features and advantages of the invention or combinations of the foregoing will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, advantages and combinations be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
       FIG. 1  is a block diagram illustrating a conventional transmit on/transmit off method of flow control. 
       FIG. 2  is a block diagram illustrating a conventional counter-based method of flow control. 
       FIG. 3  is a block diagram illustrating a counter-based method of flow control in which the counter may be updated based on incremental or absolute free storage available at the receiver. 
       FIG. 4  is a block diagram illustrating both source synchronous and master clock domains. 
       FIG. 5  is a timing diagram illustrating the clocking of commands over a plurality of signal lines synchronized to a double data recovery (DDR) source synchronous clock. 
       FIG. 6  is a block diagram illustrating the generation of multiple source synchronous data streams at a sender. 
       FIG. 7A  is a block diagram illustrating a multi-buffer arrangement for aligning multiple source synchronous data streams, and synchronizing the read out of the same to a master clock common to all the streams. 
       FIG. 7B  illustrates an example of a multi-stream status indicator for use in the system of  FIG. 7A . 
       FIG. 8  is a block diagram of a system for re-timing a source synchronous data stream from the source synchronous domain to the master clock domain. 
       FIG. 9  is a block diagram illustrating extended burst transfers. 
       FIG. 10A  is a block diagram illustrating an example of a write operation to a circular buffer which does not result in an overflow condition. 
       FIG. 10B  is a block diagram illustrating an example of a write operation to a circular buffer which results in an overflow condition. 
       FIG. 10C  is a block diagram illustrating the state of the circular buffer just prior to the read operation of  FIG. 10D . 
       FIG. 10D  is a block diagram illustrated an example of a read operation which results in an underflow condition. 
       FIG. 11  is a flowchart of a method of performing flow control between a sender and receiver in a networked system. 
       FIG. 12A  is a flowchart of a method of aligning multiple source synchronous data streams. 
       FIG. 12B  is a snapshot of the buffers in an example of the alignment system of  FIG. 7A  upon or after a determination that the status indicators for all the buffers are set. 
       FIG. 13  is a flowchart of a method of re-timing a source synchronous data stream. 
       FIG. 14  is a flowchart of a method extending a burst transfer. 
       FIG. 15  is a flowchart of a counter-based method of detecting either or both underflow and overflow conditions in a circular buffer. 
   

   DETAILED DESCRIPTION 
   As utilized herein, terms such as “about” and “substantially” and “near” are intended to allow some leeway in mathematical exactness to account for tolerances that are acceptable in the trade. Accordingly, any deviations upward or downward from the value modified by the terms “about” or “substantially” or “near” in the range of 1% to 20% should be considered to be explicitly within the scope of the stated value. 
   As used herein, the term “software” includes source code, assembly language code, binary code, firmware, macro-instructions, micro-instructions, or the like, or any combination of two or more of the foregoing. 
   The term “memory” refers to any processor-readable medium, including but not limited to RAM, ROM, EPROM, PROM, EEPROM, disk, floppy disk, hard disk, CD-ROM, DVD, or the like, or any combination of two or more of the foregoing, on which may be stored a series of software instructions executable by a processor. 
   The terms “processor” or “CPU” refer to any device capable of executing a series of instructions and includes, without limitation, a general- or special-purpose microprocessor, finite state machine, controller, computer, digital signal processor (DSP), or the like. 
   The term “logic” refers to implementations in hardware, software, or combinations of hardware and software. 
   The term “counter” means (1) a device such as a register or storage location used to represent the number of occurrences of an event, or (2) an instrument for storing integers, permitting these integers to be increased or decreased sequentially by unity or by an arbitrary integer, and capable of being reset to zero or to an arbitrary integer, or (3) a device with a finite number of states each of which represents a number which, upon receipt of an appropriate signal, can be incremented or decremented by a given constant, and which may be capable of being set to a particular state such as zero, or (4) a register or storage location used to accumulate the number of occurrences of some event. Examples include a modulo-n counter, or a reversible counter. 
   The term “receiver” means any recipient of data in a bussed or networked system. 
     FIG. 3  illustrates a system  300  for performing flow control between a sender  302  and receiver  304  of data interconnected by one or more busses  310 ,  312 ,  314  in a networked system. In this system, a counter  306  is maintained at the sender  302 , and the contents thereof, after initialization, are representative of free space available at the receiver  304  through one of more buffers  308 . 
   The sender  302  is normally in a first default state. While in this state, the sender, when it is desired to transmit a block of data to the receiver  304 , first compares the size of the block with the contents of the counter  306 . If the size of the block exceeds the available free space at the receiver  304 , the sender  302  defers or avoids transmitting the block. If the size of the block equals or is less than the available free space at the receiver  304 , the sender  302  transmits the block over one or more signal lines  310 , and the receiver  304 , upon receiving the data, stores it in the one or more buffers  308 . Upon or after transmission of a block to the receiver  304 , first logic at the sender  302  decrements the counter  306  by the size of the block. 
   When free space in one or more buffers  308  becomes available, the receiver  304  sends a first command, INCREMENT n, to the sender advising it of the same. In  FIG. 3 , this command is transmitted over one or more signal lines  312 . The argument n of the command is an integer representative of the incremental amount of free space which has become available at the receiver  304 . In response to the sender&#39;s receipt of this command, second logic at the sender  302  increments the counter  306  by the argument n, i.e., the amount of incremental free space which has become available in the one or more buffers  308  maintained at the receiver  304 . 
   From time to time, the receiver  304  may also send a second command, ABSOLUTE m, to the sender, advising it of the amount of absolute free space which is available in one or more buffers maintained at the receiver  304 . In  FIG. 3 , this command is sent over or more signal lines  314  (which may be the same as or different from signal lines  312 ). In response, third logic at the sender stores a value representative of this absolute free space in the counter  306 . Through this third command, synchronization between the contents of the counter  306  and the absolute amount of free space at the receiver  304  can be easily maintained. Moreover, unlike the conventional flow control system illustrated in  FIG. 2 , in the system illustrated in  FIG. 3 , error recovery is now possible if it is ever determined that the contents of the counter  306  are not representative of the amount of free space actually available at the receiver  304 . 
   In one implementation, upon reset or initialization, the sender  302  enters a third reset state, and the contents of the counter  306  are initialized to zero. An ABSOLUTE m command, upon receipt thereof by the sender  302 , transitions the sender  302  to the first state, and also initializes the counter  306  with the amount of absolute free space available at the receiver  304 . 
   In one implementation example, a lock out counter is maintained at the sender  302  to avoid race conditions where data is transmitted from the sender  302 , and an ABSOLUTE m command is sent from the receiver  304  prior to its receipt of the data due to the latency of the bus. Since the ABSOLUTE m command is sent prior to receipt of the data at the receiver  304 , it overstates the absolute free space available at the receiver  304 . 
   Upon being reset, the lockout counter is configured to automatically count up to a predetermined number of clock pulses, e.g.,  255 , and then stop. The counter is reset when the sender  302  transmits data to the receiver  304 . Any ABSOLUTE m command received while the counter is counting up to its final state is ignored. When the counter reaches its final state, it stops counting. Any ABSOLUTE m command received while the counter is in this state is executed. Other approaches for implementing this lock-out feature are possible so nothing in the foregoing should be taken as limiting. 
   In one embodiment, the system  300  further comprises fifth logic in the sender  302  for transitioning the sender  302  to a second state upon or after receipt of a third command from the receiver. In one example, this third command may be referred to as the BLACK HOLE command. This second state is characterized by the sender  302  transmitting data to the receiver  304  without regard to the contents of the counter  306 . 
   In a second embodiment, the system  300  further comprises sixth logic in the sender  302  for transitioning the sender  302  from the second state to the first state upon or after receipt of the second command, i.e., the ABSOLUTE m command, from the receiver  304 . 
   In one example, the sender  302  and receiver  304  are ASICs, and the one or more buffers  308  are first-in-first-out (FIFO) buffers. In one implementation, the system  300  is embodied in the sender  302 . 
     FIG. 4  illustrates a networked system  400  in which source synchronous data is transmitted from a sender  402  to a receiver  404 . The source synchronous data comprises a data portion, which is transmitted over one or more signal lines  410 , and a clock portion, which is transmitted over one or more signal lines  412 . At the receiver  404 , the data portion is sampled using the clock portion. Since the data and clock portions are equally affected by latency in the system, there is little risk of misalignment between the data and clock portions. In one implementation, the clock portion comprises a double data rate (DDR) clock in which the data portion is sampled on both the rising and falling edges of the clock. 
   Turning back to  FIG. 3 , the commands INCREMENT n, ABSOLUTE m, or BLACK HOLE, as previously discussed, may be transmitted from receiver  304  to sender  302  in the form of source synchronous signals. In one example, each of these commands may be communicated to the sender  302  over three signal lines. In  FIG. 5 , the first signal line, identified with numeral  502 , carries the source clock. In the particular example illustrated, the source clock is a DDR clock, which means that data is carried on the other two signals at both positive-going and negative-going transitions of the clock. The second signal line, identified with numeral  504 , carries four bits, CMD 3 , CMD 2 , CMD 1 , and CMD 0 , specifying the command type, and four bits, PORT 3 , PORT 2 , PORT 1 , and PORT 0 , specifying one of sixteen ports at the receiver that the command relates to. It is assumed in this example that the senders and receivers each have 16 ports, and a communication between the two can take place using any of the 16 ports. It is also assumed in this example that a value of 0x8 for the four command bits, CMD[3:0] specifies the INCREMENT command, a value of 0xC (hex) for these bits specifies an ABSOLUTE command, and a value of 0xE (hex) for these bits specifies a BLACK HOLE command. The third signal line, identified with numeral  506 , carries eight bits, DATA 7 , DATA 6 , DATA 5 , DATA 4 , DATA 3 , DATA 2 , DATA 1 , and DATA 0 . These eight bits specify the argument, i.e., n or m, of the command. With these 8 bits, an INCREMENT or ABSOLUTE command specifying up to 256 bytes of incremental or absolute free space may be communicated to the sender. 
     FIG. 6  illustrates a particular configuration of a sender in which a single stream of data  606  emanating from a media access controller (MAC)  602  is split up (for pin conservation purposes) into a plurality of separate source synchronous streams  616   a ,  616   b ,  616   c  and  616   d . The process of splitting up the stream  606  into separate streams, and associating each of the streams with its own separate clock, is performed by corresponding data path elements  614   a ,  614   b ,  614   c  and  614   d  under the control of controller  610 , which in turn is directed by the MAC  602  through control line  608 . These separate source synchronous streams are then transmitted to a receiver. 
   The situation is depicted in  FIG. 4 , which shows a source synchronous stream  410 ,  412  transmitted from the sender  402  to the receiver  404 . (For ease of illustration only, one source synchronous stream is illustrated in  FIG. 4 , it being understood that for purposes of this discussion multiple such streams may be present). A master clock  414  is common to both the sender  402  and receiver  404 . When the multiple streams are received at the receiver  404 , there is a need to combine and align the data so it is synchronous with the master clock. 
     FIG. 7A  illustrates a system for performing this function which may be embodied at the receiver. The data portions of the incoming streams are identified with numerals  708   a ,  708   b ,  708   c ,  708   d , and  708   e , and the clock portions of the incoming streams are identified with numerals  710   a ,  710   b ,  710   c ,  710   d , and  710   e . A buffer  702   a ,  702   b ,  702   c ,  702   d , and  702   e  for each of the incoming streams is provided. In addition, a status indicator (not shown in  FIG. 7A ) corresponding to each of the streams of data is provided. This status indicator indicates whether or not data for the corresponding stream has begun to be clocked into the corresponding buffer. An example of such a status indicator, implemented as a multi-bit storage location, with a bit allocated for each of the streams, is illustrated in  FIG. 7B . The individual bits in the example illustrated are identified with numerals  716   a ,  716   b ,  716   c ,  716   d , and  716   e . The default state of each these bits may be ‘0’, and when data for a stream has begun to be clocked into a buffer, the corresponding bit is set to a ‘1’. 
   First logic in the system clocks data from a stream into the corresponding buffer using the source synchronous clock for the steam, and also sets the status indicator for the stream to indicate that data for the stream has begun to be clocked into the corresponding buffer. 
   Meanwhile, second logic in the system, embodied as state machine (SM)  704  and alignment logic  706  in the particular implementation illustrated in  FIG. 7B , checks the status indicators. Upon or after determining that data for all of the streams has begun to be clocked into the corresponding buffers, the second logic clocks this data out of each of the buffers using a master clock  714  common to all the buffers to form a combined stream  712  which is synchronized to the master clock  714 . 
   In one embodiment of this system, the buffers  702   a ,  702   b ,  702   c ,  702   d , and  702   e  are FIFO buffers, and the source synchronous clocks  710   a ,  710   b ,  710   c ,  710   d  and  710   e  for each of the streams are double data rate (DDR) source synchronous clocks. 
   In one implementation, each of the FIFO buffers  702   a ,  702   b ,  702   c ,  702   d , and  702   e  has a write pointer. Moreover, a single read pointer is provided which is common to all the buffers. In this implementation, the first logic is configured to update the write pointer for a buffer as data for a stream is clocked into the buffer using the source synchronous clock for the stream, and the second logic is configured to update the read pointer common to all the buffers as data is clocked out of the buffers using the master clock. In one example, the first logic updates the write pointer for a buffer during a write operation by incrementing it, and the second logic updates the read pointer during a read operation as data is clocked out the buffers by incrementing it. Alternatively, the first logic may update the write pointer for a buffer during a write operation by decrementing it, and the second logic may update the read pointer during a read operation by decrementing it. 
   Turning back to  FIG. 4 , as stated, a master clock  404  is common to both the sender  402  and receiver  404 . When even a single stream of source synchronous data is received at the receiver  404 , there is often a need to re-time the data so it is synchronous with the master clock (or other local clock) rather than the source synchronous clock. Accordingly, the system  800  illustrated in  FIG. 8 , which may be embodied in the receiver  404 , is provided for performing this function. 
   In this system, first logic clocks the data portion  804  of the stream into buffer  802  using the clock portion  806  of the source synchronous stream. After a time, second logic clocks the data out of the buffer  802  using the master clock  808  (or other local clock). The data as it is clocked out of the buffer is identified in  FIG. 8  with numeral  810 . 
   In one embodiment, the clock portion of the stream is a DDR source synchronous clock, and the buffer  802  is a FIFO buffer. In one implementation, the FIFO buffer has a read pointer and a write pointer, and the first logic is configured to update the write pointer during a write operation using the source synchronous clock  806  for the stream, and the second logic is configured to update the read pointer during a read operation using the master clock  808 . In one example, the write pointer is updated during a write operation by incrementing it, and the read pointer is updated during a read operation by incrementing it. Alternatively, the write and read pointers may both be decremented during their respective operations. 
   Turning back to  FIG. 4 , it is often convenient to transfer data from the sender  402  to the receiver  404  in the form of bursts of predetermined size. In one example, in which commands, such as the INCREMENT n, ABSOLUTE m, or BLACKHOLE, may also be transmitted from the sender  402  to the receiver  404  in the form of four clock cycle/eight clock transition bursts as illustrated in  FIG. 5 , it is convenient to set the possible predetermined sizes of the data bursts to integer or fractional multiples of the burst size needed to transmit the commands. In one example, the possible predetermined sizes of the data bursts are 32, 48, 64, and 80 bytes. In this particular example, the initiation of a command burst can be determined by examining the STAT C signal line  504  illustrated in  FIG. 5 . A 0 to 1 transition on that signal line, consistent with 0x8, 0xC (hex), or 0xE (hex) values for CMD[3:0], indicates the initiation of a command burst. 
   When the data is transmitted in the form of bursts, however, the issue of fractional bursts, a burst less than one or more of the possible, normally-used burst sizes, must be dealt with. This problem is particularly acute in the case in which the possible burst sizes are expressed in the form of a plurality of discrete, predetermined burst sizes. In many cases, the size of the fractional burst may not be sufficient to justify the overhead needed to transfer the burst. 
   A system  900  which addresses this problem is illustrated in  FIG. 9 . In this system, data is transmitted over bus  906  in the form of bursts. The system  900  selectively varies the number of bursts used to transfer a block of data in order to overcome the problem of excessive overhead associated with small sized fractional bursts. 
   In this system, first logic transmits a block of data as n bursts (wherein n is an integer of two or more) provided the size of any fractional burst in the n bursts exceeds a threshold level. 
   However, second logic transmits the block as less than n burstsif the size of any fractional burst is less than or equal to the threshold level. 
   In one embodiment, a block of data which would otherwise be represented as n bursts of one or more predetermined sizes, where the nth burst is a fractional burst having a size less than or equal to the threshold level, is in fact transmitted as n−1 bursts, where the first n−2 bursts are of a first predetermined size, and one of the bursts, normally but not necessarily the last, (n−1)st burst, is of a second predetermined extended size, equal to the sum of the first predetermined size and an amount sufficient to accommodate the size of the fractional burst. Since the block is transmitted as n−1 bursts, the overhead associated with the nth burst is avoided. 
   In one implementation, as illustrated in  FIG. 9 , the sender  902  indicates to the receiver  904  through a signal line  910  that an extended burst is being is being transmitted, and also indicates to the receiver  904  the size of the current burst being sent over one or more signal lines  908 . Upon receipt of this information, the receiver  904  is able to take appropriate action to buffer or otherwise handle the burst. 
   In one example, the first predetermined burst size is 64 bytes, the second predetermined extended burst size is 80 bytes, and the threshold level is 16 bytes. If a fractional burst is less than or equal to 16 bytes in size, the block is transmitted as n−1 bursts, with n−2 bursts being of the first predetermined size of 64 bytes, and the (n−1)th burst having the second predetermined extended size of 80 bytes. If, on the other hand, the fractional burst is greater than 16 bytes in size, the block is transmitted as n bursts, with n−1 bursts being of the first predetermined size of 64 bytes in size, and the nth burst having a predetermined size equal to 32, 48 or 64 bytes, whichever is needed to accommodate the size of the fractional burst as a standalone entity. In this example, the sender  902  communicates to the receiver  904  the size of a burst using a 6 bit value, which is capable of indicating absolute sizes of up to 64 bytes. When the extended burst indicator is asserted, the 6 bit value which is transmitted is the absolute size of the burst minus 64. Through this approach, the 6 bits can be used to communicate burst sizes of up to 80 bytes. Thus, for a burst size of 80 bytes, the sender  902  in this example communicates a value of 16 to the receiver  904  using these 6 bits. 
   Turning back to  FIG. 8 , the traditional method of detecting underflow or overflow for the buffer  802  is based on a comparison of read and write pointers. However, in the system  800  of  FIG. 800 , since the write pointer for the buffer  802  is typically updated through the source synchronous clock  806 , and the read pointer for this buffer is typically updated through a different clock, the master clock  808 , the traditional method will not always be accurate in detecting underflow or overflow conditions. Instead, race conditions can cause the traditional method to detect underflow or overflow conditions when none exist, or fail to detect underflow or overflow conditions when such are in fact present. 
   To avoid this problem, a counter-based system for detecting either or both underflow and overflow conditions of a circular buffer such as a FIFO buffer is provided. In this system, which is illustrated through the examples of  FIGS. 10A and 10B , the buffer  1000  is assumed to be capable of holding n entries, wherein n is an integer of two or more. Moreover, each entry in the buffer includes a tag portion  1002 . 
   A first counter  1008  is provided which is capable of circularly counting through m values, wherein m is an integer which is less than n and not evenly divisible into n. 
   First logic performs a read operation from the buffer  1000  by (1) reading an entry from the buffer, (2) comparing the tag portion  1002  of the entry with the contents of the counter  1008 , and (3) either or both (a) signaling an underflow condition if the tag portion  1002  of the entry is less than the contents of the counter  1008 , and (b) signaling an overflow condition if the tag portion  1002  of the entry exceeds the contents of the counter  1008 . If the contents of the counter  1008  equal the tag portion  1002  of the entry, the contents of the first counter  1008  are updated. 
   In one embodiment, the system further comprises a second counter  1010  for circularly counting through m values. In this second embodiment, second logic performs a write operation by (1) forming an entry to be written to the buffer, the tag portion  1002  of the entry representative of the contents of the second counter  1010 , (2) writing the entry to the buffer, and (3) updating the contents of the second counter. 
   In one implementation, the first logic is configured to update the contents of the first counter  1008  during a read operation by incrementing it, and the second logic is configured to update the contents of the second counter  1010  during a write operation by incrementing it. 
   In an alternate implementation, the first logic is configured to update the contents of the first counter  1008  during a read operation by decrementing it, and the second logic is configured to update the contents of the second counter  1010  during a write operation by decrementing it. 
   To further explain the operation of this system, consider the example illustrated in  FIG. 10A . In this example, circular buffer  1000  is assumed to be a FIFO buffer with five possible entries, and read counter  1008  and write counter  1010  are configured to circularly count between 0 and 2 in increments of 1. In other words, the pattern of values achieved by the counters follow a repeating pattern as follows: 0, 1, 2, 0, 1, 2, 0, 1, 2, etc. Three entries are assumed to have been written into the buffer  1000 , bearing respectively tag values of 0, 1, and 2. The write pointer  1006  points to the next available entry in the buffer  1000  for writing to, and the read pointer  1010  points to the next entry in the buffer  1000  to be read from. 
   When a read operation is performed, the tag value of the entry pointed to by the read pointer is compared with the contents of the read counter  1008 . Since the two agree, an overflow condition is not detected. Instead, the value is read, and the read pointer and read counter are both incremented. 
   When a write operation is performed, the contents of the write counter  1010  becomes the tag value for the entry written into the buffer  1000  at the location pointed to by the write pointer  1006 . Then, both the write pointer and write counter are both incremented. 
   Now, consider the example illustrated in  FIG. 10B . This example represents that illustrated in  FIG. 10A  after four additional successive write operations to the buffer  1000 . At the conclusion of these write operations, the tag portions of the buffer entries, the contents of the read and write counters  1008 ,  1010 , and the locations pointer to by the read and write pointers  1002 ,  1004  are as illustrated in  FIG. 10B . 
   When the next read operation is performed, the tag value for the location pointed to by the read pointer  1002 , 2, is compared with the contents, 0, of the read counter  1008 , an overflow condition is detected since the tag value for the entry exceeds the contents of the read pointer  1002 . That is an accurate assessment of the situation since, as can be seen from  FIG. 10B , two valid entries in the buffer have been overwritten: that pointed to by the read pointer, and the next successive location. 
     FIG. 10C  illustrates the state of buffer  1000  of  FIG. 10A  after two more write operations, followed by two additional read operations.  FIG. 10D  illustrates the buffer of  FIG. 10C  after three more read operations. During the third of these operations, the tag portion of the entry  1014 , 0, is compared with the contents of the read counter  1008 , 2. Since the tag portion of the entry  1014  is less than the contents of the read counter  1008 , an underflow condition is detected. That is an accurate assessment of the situation since, as illustrated in  FIG. 10D , the entry  1014  has been re-read even though this value has already been read and a new data value has not yet been written into this entry. 
     FIG. 11  is a flowchart of one embodiment of a method  1100  of performing flow control between a sender and receiver of data in a networked system. In this method, the sender is configured to maintain a counter whose contents, after initialization, are representative of available free space in one or more buffers maintained by the receiver. As illustrated, in one embodiment, upon the inception of the method, program control or method flow may pass down one of three branches, identified respectively with numerals  1102 ,  1114 , and  1118 . Each one of these three branches will be explained in turn. 
   Branch  1102  is followed if it is desired to transmit a block of data from the sender to the receiver. Inquiry step  1104  is performed, to inquire whether the sender is in a first (default) state indicating that the contents of the counter should first be checked before performing the transfer. If so, inquiry step  1106  is performed. In inquiry step  1106 , it is determined whether or not the size of the block to be transmitted is less than or equal to the contents of the counter. If so, step  1108  is performed. In step  1108 , the block of data is transmitted to the receiver. Then, step  1110  is performed. In step  1110 , the contents of the counter are decremented by the size of the block. Program control or method flow is then exited or terminated. 
   If, in inquiry step  1106 , it is determined that the size of the block to be transmitted exceeds the contents of the counter, step  1107  is performed. In step  1107 , transmission of the block is either deferred or avoided. Program control or method flow is then exited or terminated. 
   Turning back to inquiry step  1104 , if it is determined that the sender is not in the first state, program control or method flow is exited or terminated. Alternatively, in one embodiment, it is assumed that the sender is in a second state, characterized by the fact that data is transmitted by the sender to the receiver without regard to the contents of the counter. (Instead of assuming that the sender is in the second state, an inquiry step could be performed here to ensure that the sender is in the second state. If not, an error condition could be flagged.). Step  1112  is then performed. In step  1112 , the block of data is transmitted to the receiver, while avoiding the inquiry step  1106 . Program control or method flow is then exited or terminated. 
   Branch  1114  is followed if a first command is received advising the sender of the amount of incremental free space which has become available at the receiver. In one embodiment, as discussed, this first command is of the form INCREMENT n, where n indicates the incremental free space which has become available at the receiver. In step  1116 , upon or after receipt of such a command at the sender, the counter is incremented by n, the incremental free space which has become available at the receiver. Program control or method flow is then exited or terminated. 
   Branch  1118  is followed if a second command is received advising the sender of the absolute amount of free space which is available at the receiver. In one embodiment, as discussed, this command is of the form ABSOLUTE m, where m is the absolute amount of free space available at the sender. Upon or after receipt of such a command at the sender, step  1120  is performed. In step  1120 , the value m, the absolute amount of free space available at the sender, is stored in the counter. 
   In one embodiment of branch  1118 , a check is made whether the sender is in the third reset state. If so, in addition to storing the value m in the counter in step  1120 , the sender is transitioned into the first state. 
   In one implementation of branch  1118 , a check is also made whether the lock-out timer (previously discussed) is still counting up to its final state. If so, the ABSOLUTE m command is ignored. If not, the ABSOLUTE m command is implemented. 
   Program control or method flow is then exited or terminated. Alternatively, in one embodiment, inquiry step  1122  is performed. In inquiry step  1122 , it is determined whether or not the sender is in the second state which, as discussed earlier, is characterized by the fact that transmission occurs without regard to the contents of the counter. If the sender is in the second state, step  1124  is performed. In step  1124 , the sender is transitioned from the second state to the first state which, as discussed previously, is characterized by the fact that the contents of the counter are compared with the size of a block before that block is transmitted to the receiver. Program control or method flow then exits or terminates. 
   In inquiry step  1122 , if it is determined that the sender is not in the second state, it is assumed that the sender is already in the first state, and program control or method flow exits or terminates. Alternatively, an inquiry step could be performed here to explicitly determine whether or not the sender is in the first state. If not, an error condition could be flagged. 
   In one embodiment, another branch is followed upon or after receipt of a third command directing the sender to transmit data without regard to the contents of the counter. As discussed previously, this command may take the form of BLACK HOLE. In this embodiment, upon or after receipt of such a command, the sender is transitioned to the second state. 
     FIG. 12  is a flowchart of a method  1200  of aligning a plurality of source synchronous streams of data. In one embodiment, the method begins with step  1202 , which comprises having the sender enter an alignment mode, and transmit a predetermined pattern of data through the multiple streams at the same time. In the configuration illustrated in  FIG. 6 , this step is performed by controller  610  arranging to have each of the DPs  614   a ,  614   b ,  614   c ,  614   d , and  614   d  transmit the predetermined pattern, in source synchronous format, at the same time. 
   Turning back to  FIG. 12 , step  1204  is then performed. In step  1204 , the data from each of the streams is clocked into a corresponding buffer using the source synchronous clock for the stream. Step  1206  is then performed. In step  1206 , a status indicator for the stream is set to indicate that data for the stream has begun to be clocked into the corresponding buffer. 
   Inquiry step  1208  is also performed. Although this step is shown in  FIG. 12  as being performed after steps  1202 - 1206 , it may be performed concurrently with these steps. In inquiry step  1208 , the status indicators are checked. If it is determined that the status indicators for all the buffers indicate that data for all of the streams has begun to be clocked into the corresponding buffers, step  1210  is performed. In step  1210 , data from each of the buffers is clocked out using a master clock common to all the buffers. At this point, since alignment has been achieved, in one embodiment, the sender exits the alignment mode of operation, and enters a normal mode of operation. 
   In one implementation of this method, the source synchronous clocks are double data rate (DDR) source synchronous clocks, and the buffers are each FIFO buffers. 
   In one implementation example, each of the buffers are FIFO buffers having a separate write pointer, and there is also a read pointer is common to all the buffers. In this implementation example, the write pointer for a buffer is updated as data for the stream is clocked into the buffer using the source synchronous clock for the stream, and the read pointer common to all the buffers is updated as data is clocked out of the buffers using the master clock. 
   The step of updating the write pointer for a buffer may comprise incrementing the pointer for each entry written into the buffer, and the step of updating the read pointer may comprise decrementing the pointer for each group of entries read out in common from the buffers. Alternatively, the step of updating the write pointer may comprise decrementing it for each entry written into the buffer, and the step of updating the read pointer may comprise incrementing it for each group of entries read out in common from the buffers. 
   An example of the process of updating the write pointers and the common read pointer is illustrated in  FIG. 12B . In this example, it is assumed that there are three streams of incoming data, and hence three separate buffers, identified respectively with numerals  1212   a ,  1212   b , and  1212   c . It is also assumed that the write pointers for each of the buffers, and the common read pointer, were each initialized to point to the first entry in each of the buffers, identified respectively with numerals  1216   a ,  1216   b , and  1216   c . It is further assumed that  FIG. 12B  provides a snapshot of the state of affairs in this arrangement upon determining that the status indicators for each of the buffers have all been set. 
   At this point, it is assumed that four entries have been written into buffer  1212   a , one entry has been written into buffers  1212   b , and three entries have been written into buffer  1212   c . Hence, the write pointer for buffer  1212   a , identified with numeral  1214   a , points to the fifth entry from the top in buffer  1212   a ; the write pointer for buffer  1212   b , identified with numeral  1214   b , points to the second entry from the top in buffer  1212   b ; and the write pointer for buffer  1212   c , identified with numeral  1214   c , points to the fourth entry from the top in buffer  1212   c.    
   Since the status indicators for each of the buffers has been set, the process of reading out groups of entries common to all the buffers may commence, starting with the first entry in each buffer, i.e., entries  1216   a ,  1216   b , and  1216   c , since that is the location pointed to by the read pointer. 
     FIG. 13  is a flowchart of a method  1300  of re-timing a source synchronous stream of data. The method commences with step  1302 , which comprises clocking data from the stream into a buffer using the source synchronous clock for the stream. Step  1302  is followed by step  1304 , which comprises clocking data out of the buffer using a local clock. From step  1304 , the method may then jump back to step  1302  for additional iterations. 
   In one embodiment, the method occurs in a networked system. In one implementation, the local clock is a master clock for the networked system, such as clock  414  in  FIG. 4 . In one example, the source synchronous clock is a DDR source synchronous clock, and the buffer is a FIFO buffer. 
   The FIFO buffer may have a read pointer and a write pointer. If so, the write pointer is updated as data is clocked into the buffer using the source synchronous clock, and the read pointer is also updated as data is clocked out of the buffer using the local clock. 
   The step of updating the write pointer may comprise incrementing the write pointer for each entry written into the buffer, and the step of updating the read pointer may comprise incrementing the read pointer for each entry read from the buffer. 
   Alternatively, the step of updating the write pointer may comprise decrementing the write pointer for each entry written into the buffer, and the step of updating the read pointer may comprise decrementing the read pointer for each entry read from the buffer. 
     FIG. 14  is a flowchart of a method  1400  of selectively varying the number of burst transfers used to transmit a block of data. It is assumed that a block of data is desired to be transmitted from a sender to a receiver in a networked system, and it has been determined that the block is such as to require n bursts, where n is an integer of two or more, and that at least one of the bursts is a fractional burst, i.e., a burst in which the amount of data to be transmitted by the burst is less than one or more of the possible, normally-used sizes (or capacities) of a burst. 
   The method begins with inquiry step  1402 . In inquiry step  1402 , it is determined whether the size of the factional burst exceeds a threshold level. If so, step  1404  is performed. If not, step  1406  is performed. 
   In step  1404 , the block of data is transmitted as n bursts. In step  1406 , the block of data is transmitted as less than n bursts. Program control or method flow then terminates or exits. 
   In one embodiment, step  1406  comprises transmitting the block of data as n−1 bursts comprising n−2 bursts of a first predetermined size, and one burst having a second predetermined extended size equal to the first predetermined size plus an amount sufficient to accommodate the size of the fractional burst. In one implementation, data representative of the size of each of the bursts is also transmitted, as well as an indicator of whether the burst is of the second extended size. 
   In one example, the first predetermined burst size is 64 bytes, the second predetermined extended burst size is 80 bytes, and the threshold level is 16 bytes. In this example, step  1404  comprises transmitting the block as n bursts comprising (n−1) 64 byte bursts, and one burst having a predetermined size of either 32, 48, or 64 bytes, whichever is needed to accommodate the fractional burst as a standalone burst. Step  1406  comprises transmitting the block as (n−1) bursts comprising (n−2) 64 byte bursts, and one burst having a predetermined extended burst size of 80 bytes. 
     FIG. 15  is a flowchart of a method  1500  of detecting either or both underflow and overflow conditions of a circular buffer capable of holding n entries, wherein n is an integer of two or more, in which each entry of the buffer includes a tag portion. 
   In one embodiment, the method is incidental to the performance of a read operation from the buffer. In this embodiment, the method begins with step  1502 , which comprises reading an entry from the buffer. 
   From step  1502 , the method proceeds to step  1504 , which comprises comparing the tag portion of the entry with the contents of a first counter configured to circularly count up to m values, where m is an integer less than n and not evenly divisible into n. 
   Inquiry step  1506  follows step  1504 . In inquiry step  1506 , it is determined whether the tag portion of the entry agrees with the contents of the first counter. If so, step  1508  is performed. If not, step  1510  is performed followed by step  1508 . 
   Step  1508  comprises updating the contents of the first counter to reflect the read operation. Step  1510  comprises performing either or both of the following steps: (a) signaling an underflow condition if the tag value of the entry is less than the contents of the first counter, and (b) signaling an overflow condition if the tag portion of the entry is greater than the contents of the first counter. 
   From step  1508 , program control or method flow exits or terminates. 
   In one embodiment, the method  1500  further comprises performing a write operation. In one embodiment, the process of performing a write operation comprises forming an entry to be written to the buffer, the tag portion of the entry representative of the contents of a second counter configured to circularly count up to m entries; writing the entry to the buffer; and updating the contents of the second counter. 
   In one implementation, the step of updating the contents of the first counter during a read operation comprises incrementing it, and the step of updating the contents of the second counter during a write operation comprises incrementing it. 
   In another implementation, the step of updating the contents of the first counter during a read operation comprises decrementing it, and the step of updating the contents of the second counter during a write operation comprises decrementing it. 
   In one implementation example, the circular buffer is a FIFO buffer. 
   The methods of  FIGS. 11-15 , and any of the embodiments, implementations and examples which have been discussed, may be implemented through software, hardware, or any combination of hardware and software. In relation to the software implementation, the methods may be embodied in the form of software instructions stored in a memory. Furthermore, this memory may be accessible by a processor in a system, wherein the processor is configured to successively retrieve and execute the software instructions. 
   While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.