Patent Document

TECHNICAL FIELD 
   The present invention is related generally to the field of semiconductor memory devices. More particularly, the present invention is related to content addressable memory systems and methods. 
   BACKGROUND OF THE INVENTION 
   Content Addressable Memory (CAM) devices are used in applications requiring matching operation on bit patterns, such as table lookup applications used by routing and switching systems in computer network applications. Typically, CAM devices provide for the direct comparison of stored data entries with a supplied value to be compared, called a comparand, in a single access. In contrast, when using conventional Random Access Memory (RAM) for the same search operation, stored data entries are compared by supplying the address of each of the stored data entries to the RAM device, retrieving each of the data entries stored at each of the addresses, and passing the data to an arithmetic logic unit (ALU), where it is then compared to the comparand. CAM devices, on the other hand, allow the comparand to be directly compared with all the stored data entries simultaneously, and any stored data entries matching the input entry generate a match signal. More specifically, each bit position of the comparand is compared with the corresponding bit positions of data entries stored in the CAM device. A priority encoder in the CAM device identifies which matching data entry is output first in the case of multiple matching data entries, with this data entry being termed the highest priority match, as will be explained later in this document. 
     FIG. 1  shows a conventional CAM device  100 . The CAM device  100  is directed by control logic  104 , which can conventionally write or retrieve data from the CAM array  108  by accessing an address decoder  112  through an address bus  116 . Data to be conventionally written to or retrieved from the CAM array  108  are provided to input/output (I/O) buffers  120 , which receive and supply data through a system bus  124 . The control logic  104  directs the I/O buffers  120  through a control bus  128 , and data is passed between the I/O buffers  120  and the CAM array  108  through a data bus  132 . The CAM device may be operated as a conventional random access memory device using just these functional elements to write and retrieve data from the CAM array  108  by the control logic  104  specifying address information to the CAM array  108  through the data bus  132 . 
   The primary differentiator between conventional random access memory (RAM) devices and CAM devices is the ability of CAM devices to perform search or matching operations in a deterministic time period as previously described, regardless of the number of data entries stored in the CAM array. Instead of the control logic  104  directing data access to the CAM array  108  through the address decoder  112 , a comparand can be moved to the Comparand Register  136 , and the control logic  104  then directs the CAM array  108  to compare all data entries to the data in the Comparand Register  136 . All matching data entries are prioritized by the Priority Encoder  140 , which determines which matching entry is the highest priority match. 
   An additional component of the CAM device  100  is the Mask Register  144 . This is a global mask register in that it applies equally to all data entries in the CAM array  108  per compare operation. The Mask Register  144  holds a data mask which is used to identify which bits in the data entries stored in the CAM array  108  are considered significant, and thus compared to the same bit locations in the Comparand Register  136 . For example, if the bit width of the data stored in the CAM array  108  is less than the native bit width of the CAM array  108 , the Mask Register  144  is invoked to include only those bits significant to the application in the compare operation. In the case of multiple data entries matching the data in the comparand register  136 , the Priority Encoder  140  determines which entry is output first. In a typical CAM device  108 , the Priority Encoder  140  selects the matching entry with the lowest physical address as the highest priority match. Note that typical CAM devices may have several global mask registers; each invoked for a different compare, write or read operation, and only invoked one at a time. 
   The CAM device  100  heretofore described is termed a binary CAM device because the matching operations for each bit across the CAM array  108 , aside from those bits indicated as not significant by the mask stored in the mask register  144 , will yields one of two states: match or no match. This is often called an exact match, as all non-globally masked bits of the stored entry must match the data in the comparand register  136  before the entry will indicate a match. 
   A ternary CAM device allows for matching operations which will yield, for each bit across its CAM array, one of three states: match, no match, or “don&#39;t care”. This third, don&#39;t care state is supported by each entry in the CAM array having its own individual, or local mask. This allows a third state to be specified individually for each bit of each data entry in the ternary CAM array  200 , as shown in  FIG. 2A . For each data entry  204  in the data portion  208  of the ternary CAM array  200 , shown arranged from a low address  0  to a high address N, there is an associated mask  212  in the mask portion  216  of the ternary CAM array  200 . 
     FIG. 2B  shows a four-bit example of a ternary CAM array  250 , and how the entries in the mask portion  254  affect the compare operation for each data entry in the data portion  258  for an example comparand of “1000”. Entries in the mask portion  254  specify a one for each bit that is significant in the compare operation, and a zero for each bit that is not significant, which forces a match for that bit location. In a compare operation with the comparand value of 1000, combining the data entries with their local masks, there are three matching entries and two no match entries in the ternary CAM array  250 . Starting at the low address data entry 1001 at address  0  with the mask value of 1111, there is a no match condition. Although the first three most significant bits match, the mask of 1111 makes all the bits significant in the comparison, and the least significant bit does not match. The next data entry at address  1  contains the value 1000, and is a perfect or exact match. The corresponding mask value is 1111, making all of the data bits significant for the compare operation, so the data value needs to match the comparand value exactly to indicate a match. 
   By contrast, the data entry at address  2 , with a value of 1011, when combined with the data mask value of 1100, will yield a match with the value of 1000 in the comparand because only the two most significant bits are compared, and the two least significant bits are a forced match. Similarly, the data value in address three of 1110, when combined with a mask value of 1100 will yield a no match with the value of 1000 in the comparand because the second most significant bit of the data does not match, and the mask value indicates this bit position as significant. Finally, at address N., the data value of 1111, when combined with the mask value of 1000 yields a match with the value of 1000 in the comparand because only the most significant bit is being compared, and they match. 
   The priority encoders employed in typical ternary CAM devices will resolve multiple matches by returning the matching location with the lowest physical address first. In the previous example, where addresses  1 ,  2 , and N matched, the ternary CAM will respond with the location address  1  as the highest priority match. In applications, this requires that the data stored in the ternary CAM array be ordered by the value of the mask, with higher priority values having lower physical addresses within the ternary CAM array. In applications where the mask may have interleaved ones and zeros, the designer must decide what determines priority, and sort the data appropriately before storing in the ternary CAM array. It is important to note that all data entries with a common mask value will comprise a block of data values within the ternary CAM array when sorted by mask value. It is not necessary to further sort the data entries within each block 
   For most computer networking applications, the networking protocols specify the priority of multiple matching entries by the value of the mask. These protocols simplify the issue of determining priority and sorting by defining valid mask values that do not allow interleaved ones and zeros in the mask, and always include some minimum number of bits starting with the most significant data bit. This masking technique is often referred to as the network mask, or net mask. 
   In the previous example of  FIG. 2B , the best match is the location with the longest string of ones in the mask, and having matching data in the significant bit positions. The ultimate match is an exact match with a mask value of all ones, as in address  1  of  FIG. 2B . If address  1  was empty, so there would not be an exact match, the best match would be at address  2  because two most significant bits are being compared as opposed to one significant bit in address N, even though both locations would register a match. Note that in  FIG. 2B , the mask value in address  3  would be an illegal net mask because it contains interleaved ones and zeros. 
   A typical networking application is a router. A network message received by a router might be targeted to all devices serviced by a second router connected to the first router. The devices connected to the second router have thirty-two bit addresses with the first twenty-four bits being identical. Because the message is directed to all the devices serviced by the second router, the first router is only concerned with identifying packets destined for the second router, regardless of where the second router sends the packets. The first router could keep a copy of all the network addresses of all of the devices connected to the second router, but that is an inefficient use of memory space within the first router. What the routing protocols allow is for the first router to represent all of the devices serviced by the second router with a single entry in its routing table. The entry would be stored with the value of the twenty-four identical network address bits in the most significant bits of the thirty-two bit address space, and zeros in the least significant eight bits. The corresponding mask value would be all ones in the first twenty-four bit positions, and zeros in the remaining eight bit positions. Any incoming packet to the first router with a destination address having the most significant twenty-four bits matching this entry will be forwarded to the second router, regardless of the value in the least significant eight bits. 
   In reality, a network router is connected to several other routers, which in turn, are connected to yet more routers, as well as end devices such as personal computers and servers. Each router connected to the first router may have one or more blocks of addresses that can be represented with one entry in the routing table of the first router, but with differing numbers of common significant bits. This means that the first router will have a variety of different net masks within its routing table; from a mask value of all ones for exact matches for end devices connected directly to this router, to a mask with few ones. These entries need to be sorted by net mask length, with the longest net mask entries at the lowest physical addresses within the CAM device, which takes processing power and time. The network space tends to be very dynamic, so this table may need to be resorted frequently, again taking processing power and time, potentially causing the ternary CAM device to not be available for a search operation when it is needed. 
   In conventional ternary CAM devices, gaps or empty entries are left in each net mask block of entries. This allows routers to update the routing table by inserting a new entry in an available empty location within the correct block without having to re-sort the entire table each time a new entry is inserted. Although this improves entry insertion time, it clearly wastes available resources by reserving ternary CAM array space for possible future use. 
   Another concern with conventional ternary CAM design is that the presentation of a comparand for a priority matching operation necessitates a search of the entire ternary CAM array. The switching of many logic gates necessary for this operation is appreciable. Some comparands might be seeking to retrieve only data entries which refer to a common table to which multiple devices might be assigned. Typically, this table information is included in each data entry assigned to the common table. Incorporating the table identifier within the address represented in the data entry consumes additional bits within the data space. Moreover, because the data entries themselves must be searched to identify data entries included in the table, a full search of the entire CAM array must be made even if only addresses are assigned to a single table are needed. Finally, some conventional CAM arrays provide for address spaces much wider than conventional IPv4 32-bit addresses, and allow for data words in excess of 144 bits in width. These devices allow for the segmentation of the native data word width of the CAM device into half-words and quarter-words when addresses short of the maximum word width are being used. Even when these words are empty or partially empty, a priority matching operation still will necessarily involve a search of the entire CAM array. 
   What is desired is a way to avoid having to use and occupy system resources in sorting and resorting data entries to support conventional priority matching protocols. What also is desired is a way to eliminate searching an entire ternary CAM array when the comparand seeks information, such as addresses assigned to a single, common table, or when only some of the word width in an extended width ternary CAM is used. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved ternary CAM device having, in addition to a data entry and a ternary mask entry, one or more additional control words which can specify a net mask length and status, a table identifier, and/or a validity word to specify a detailed status of segmented data words stored. This allows for the priority matching of the ternary CAM device entries with a comparand without sorting the data entries stored in the ternary CAM device. In addition, the additional status words can be used for table identification to not only save space in the actual device entries, but also allows for matching of data entries with selected table identifiers, precluding the need to search the entire ternary CAM array. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a conventional content addressable memory (CAM) device. 
       FIG. 2A  is a table depicting a conventional ternary CAM array storing data words and associated mask words. 
       FIG. 2B  is a table depicting a conventional ternary CAM array storing exemplary data words and associated mask words to depict the operation of the conventional ternary CAM device. 
       FIG. 3  is a table depicting a ternary CAM array storing according to an embodiment of the present invention showing data words, associated mask words, and control words. 
       FIG. 4A  is a table depicting a net mask length control word that may comprise a control word in accordance with an embodiment of the present invention. 
       FIG. 4B  is a table depicting a table identifier control word that may comprise a control word in accordance with an embodiment of the present invention. 
       FIG. 4C  is a table depicting a segmented word validity word that may comprise a control word in accordance with an embodiment of the present invention. 
       FIG. 5A  is a table showing a control word comprising of both a net mask length control word and a table identifier control word that may comprise a control word in accordance with an embodiment of the present invention. 
       FIG. 5B  is a table showing a control word comprising of a net mask length control word, a table identifier control word, and a segmented word validity word that may comprise a control word in accordance with an embodiment of the present invention. 
       FIG. 6  is a block diagram of a computing system using one or more ternary CAM devices in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention are directed to an improved ternary CAM system and method, improving efficiency and reducing power consumption of the ternary CAM. In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form part hereof, and in which are shown, by way of illustration, specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. Other embodiments may be utilized and modifications may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     FIG. 3  is a table representing an expanded ternary CAM array  300  in accordance with an embodiment of the present invention. Comparing  FIG. 3  with  FIGS. 2A and 2B , it can be seen that the expanded ternary CAM array  300 , in addition to having a data portion  308  and a mask portion  312 , further comprises a control portion  320 . Thus, for each data entry  324  in the expanded ternary CAM array  300 , in addition to an associated mask  328 , one or more control words  332  is included. The masks  328  can comprise of net mask values having ranges of contiguous ones and zeros as previously described, or can contain interleaved zeros and ones for other masking applications. In embodiments of the present invention, the control words  332  associated with each data entry  324 , may comprise net mask length control words, table address control words, sets of segmented word status flags, or other information. As will be further explained, inclusion of these control words can improve efficiency and reduce power consumption in ternary CAM applications. 
     FIG. 4A  shows a control word  332  ( FIG. 3 ) in the form of a net mask length control word  400 . In this embodiment, the net mask length control word  400  is used to facilitate priority matching. As previously described, priority matching schemes frequently determine the highest priority match among multiple matching candidates by choosing the data entry having the net mask with the most significant (i.e., “do care”) bits. Generally, net masks do not have interleaved zeros and ones, but comprise a number of ones listed from the left most significant bit positions to the left, and a number of zeros listed from the right in the least significant bit positions. As a result, the net masks with the greatest number of most significant bits, or “do care” bits, are the longest net masks. As also previously described, this scheme is facilitated by sorting data entries so that the data entries with the longest associated net masks appear in the lowest physical addresses in the memory space. Thus, if more than one data entry registers a match with the comparand, the highest priority match is determined as occupying the lowest memory address of all matching candidates. 
   Using the net mask length control word  400  to specify the length of the net mask, data entries no longer have to be resorted by net mask length. Instead, as part of the matching process, the priority encoder  140  ( FIG. 1 ) can compare the net mask length control words to determine the highest priority match from multiple matching data words. This saves the processing time and power required to sort the data entries, as well as usable time lost while this process is taking place. 
   More specifically, the net mask control word  400  comprises eight bits: a seven-bit net mask length specifier  404 , comprised of bits NL 0  through NL 6 , and a word validity bit  408 , which indicates whether the control word is valid. Bits NL 0  through NL 6  represent a value indicative of the bit boundary where the net mask changes from one to zero. The net mask length value would define the number of most significant bits that are compared in a matching operation, with the remaining bits being a forced match. Accordingly, the seven bits contained in the net mask length specifier  404  can indicate a net mask length of up to one hundred twenty eight bits in length, accommodating conventional net masks. For example, bits NL 0  through NL 6  contain the net mask length, starting bit position  127  of the net mask and extending towards bit position  0 . Note that a net mask length specifier of seven bits is sufficient to mask all one hundred twenty eight bits of an Internet Protocol version 6 (IPv6) address. Conventionally, there are a minimum number of significant bits of the network address that must be included in a match operation for the match operation to meet the requirements of the network protocols. 
   The validity bit  408  indicates whether the net mask length control word  400  is actually a valid net mask length. For example, as previously explained, net masks do not contain interleaved ones and zeros, as compared to conventional ternary CAM mask words, which may contain interleaved ones and zeros. If the mask word is a special purpose mask word containing interleaved ones and zeros, or is simply not used, the validity bit  408  will specify a zero, and the priority encoder  140  ( FIG. 1 ) will not use the net mask length control word in a matching operation. However, if the net mask length is valid, the validity bit  408  will specify a one, and the priority encoder  140  ( FIG. 1 ) will use the net mask length control word in matching operations to determine the highest priority match in the case of multiple matching candidates. 
   In one embodiment of the present invention, if the net mask control word  400  is a valid net mask length, the net mask corresponding to the net mask length is generated and used in the matching operation. Where the net mask control word  400  is an invalid net mask length, the corresponding ternary mask for that data word is loaded independently. It will be appreciated by those of ordinary skill in the art, however, that various modifications can be made to the embodiment previously described with respect to  FIG. 4A  without departing from the scope of the present invention. For example, alternative bit arrangements can be used, various number of bits representing the net mask length can be changed, and correspondence of bit positioning the net mask to the value of the net mask length can be modified and still remain within the present invention. 
   Alternatively, the control word ( FIG. 3 ) can specify a table address word  430 , as shown in  FIG. 4B , which specifies a table to which the associated address has been assigned. Thus, if one or more devices associated with a certain table identifier are the only devices for which a match would be desired, only data entries having that table identifier specified will be considered in the matching process. This affords two advantages: First, the table identifier information can be specified independently of the data word, therefore data word space need not be set aside for table identifier information. Second, if only data words having a particular table identifier are of interest in the matching process, only data entries included in that table need be evaluated in the matching process. As a result, fewer data entries need to be evaluated in the matching process, saving circuit switching and the power that would be consumed in that process. 
   As shown in  FIG. 4B , an embodiment of the table identifier control word  430  comprises a five-bit table identifier  434 , including bits T 0  through T 4 , resulting in potentially defining 32 different tables. The table identifier control word  430  also specifies the data word and corresponding mask word segmentation of the table as defined by the two table word bit widths  438 , WW 0  and WW 1 . The data word width is defined by specifying the number of equal-sized segments within a word. For example, where 144-bits are allocated for each data word, “00” might define one 144-bit segment. However, where WW 0  and WW 1  are “10”, there would be four 36-bit segments per data word, and “11” would be indicative of two 72-bit segments per word. Finally, the table identifier control word  430  comprises a word validity bit  442  to indicate with a zero or one whether the table identifier control word is invalid or valid, respectively. 
   It will be appreciated by one ordinarily skilled in the art that the table identifier control word  430  previously described is merely one embodiment of the present invention, and that modifications can be made without departing from the scope of the present invention. For example, where it is desirable for a greater number of tables to be potentially defined, the number of bits representing the table identifier can be increased accordingly. 
   Once all members of a specific table have been identified by the table identifier  434 , it is then possible to enable only those entries for a given search operation. That is, by knowing ahead of time to which table each word belongs, it is possible to exclude from a given search all data words that do not belong to the table being searched on a word-by-word basis. This eliminates the need for placing new entries within a specific range of the CAM address space, which is required in some conventional table management schemes. Moreover, use of the table address control word  430  eliminates the need for table identifications bits within the data word, thus recovering additional data bits for use in multi-table applications. 
   As shown in  FIG. 4C , an embodiment of the invention might include a segmented word validity word  470 . The segmented word validity word  470  can be used in conjunction with the table identifier control word  430  ( FIG. 4B ). A ternary CAM data word ordinarily might be 256-bits wide. However, the full 256 bits may not be required to store entries. Thus, the 256-bit data word can be segmented into multiple separate data words, some or all of which may be valid at any one time. The segmented word validity word  470 , therefore, allows for different validity flags to be stored for each of the segmented words stored in that data entry. As shown in an embodiment depicted in  FIG. 4C , if the data word can be segmented into four different data word segments, four pairs of validity bits  474 ,  478 , 482 , and  486 , can be specified. For example, validity bit pair  474  comprises bits W 0 V 0  and W 0 V 1 , specifying two validity bits for the first segment of the segmented word. Similarly, validity bit pair  478  comprises bits W 1 V 0  and W 1 V 1 , specifying two validity bits for the second segment of the segmented word, and so on. 
   Use of the segmented word validity word  470  can provide greater flexibility in how to use and manage the data stored in each table in a CAM array. For example, upon finding a parity error for one of the data segments of a table, it may not be desirable to simply mark that segment as “empty”. Having multiple validity states, as defined by the corresponding bits in the segmented word validity word  470 , allows an entry to be temporarily excluded from searches without risk of having that segment overwritten in a subsequent write operation. 
   Combinations of these control words may be used. For example, as briefly discussed above, the segmented word validity word  470  ( FIG. 4C ) and the table identifier control word  430  ( FIG. 4B ) provide additional flexibility when used in combination.  FIG. 5A  illustrates another combined use of control words.  FIG. 5A  shows a dual control word  500  that could comprise the control word  332  ( FIG. 3 ). The control word  500  is a dual width word  504  having a net mask length word  400 , like that shown in  FIG. 4A , and a table identifier word  430 , as shown in  FIG. 4B . Having both of these control words allows for advantages of both as previously described. For example, priority matching could be performed according to net mask length without having to resort data entries, and data entries belonging to only a specific table be included. Similarly, all three types of control words previously described could be used as shown in  FIG. 5B . A triple-width control word  550  has a triple-width field comprised of a net mask length word  400  ( FIG. 4A ), a table identifier word  430  ( FIG. 4B ), and a segmented word validity word  470  ( FIG. 5C ). This combined control word allows for all of the advantages previously described. 
     FIG. 6  is a block diagram of a computer networking system incorporating an embodiment of the present invention. In the computer networking system  600 , a port processor  602  is adapted with a preferred embodiment of the present invention (not shown) as previously described. The computer networking system  600 , including the ternary CAM  601 , utilizes a port processor  602  to perform various functions, calculations or tasks on the incoming and/or outgoing network traffic. In addition, the computer system  600  includes one or more input devices  604  that are generally coupled to the port processor  602  through a standard bus, such as MII (Media Independent Interface) or UTOPIA. 
     FIG. 6  is a block diagram for a subsystem  600  of a computer network device, such as a bridge, switch, router or access point, incorporating an embodiment of the present invention. The subsystem  600  consists of one or more PHY devices  604 , or physical layer adaptation devices which interface the network device to the rest of the network; a port processor  602 , which could be as simple as a collection of logic or as sophisticated as an Ethernet MAC (Media Access Controller) or network processor; a CAM device  601  in accordance with an embodiment of the present invention; packet or cell buffer memory  605 ; and optionally, associated data memory  603 . 
   The PHY device  604  (or devices), in the receive or ingress direction, decode the incoming network traffic and present this traffic to the port processor  602  over a standard bus  610 , such as MII (media independent interface) or UTOPIA. In the transmit or egress direction, the port processor  602  presents traffic destined for the network to the PHY device  604  over the data bus  610 . The PHY device encodes the information for transmission over the rest of the network. 
   In the receive or ingress direction, the port processor  602  parses the incoming cell or frame to extract pertinent information from the header of the cell or frame. This extracted information is presented to the CAM  601  over the data bus  630 , along with control information over the address and control bus  620 . The data packet is sent to the buffer memory  605  for later processing or transmission over the system bus  680  to the rest of the system. The extracted information presented to the CAM  601  over the data bus  630 , along with control information over the address and control bus  620  is compared with the data stored in the CAM device  601 . The result, which is the combination of a match indication and data, provided the search was successful, will aid the port processor  602  in the decision making process of whether to accept an incoming frame or cell, and what to do with a frame or cell that is accepted. This result data may be stored and retrieved from the CAM device  601  itself, or may reside in optional associated data memory  603 . If the result data is stored in the associated data memory  603 , it is typical that the CAM device  601  directly controls the associated data memory  603  via the control bus  640 , and the data itself is written to or read from the associated data memory  603  via the data bus  650  by the port processor. 
   In the egress or outbound direction, the port processor  602  receives the data packet and control information over the system bus  680 , and may temporarily store the data packet in the buffer memory  605 . Either a tag included in the data, or the control information presented to the port processor  602  will be presented to the CAM device  601  for a compare operation. The resulting data from the search operation will tell the port processor  602  which header to append onto the cell or frame before transmitting, and possibly the order in which the frames or cells leave the network device. 
   A host or system processor (not shown) is typically used to maintain the table or tables within the CAM device  601 . This processor may be connected directly to the CAM device  601  or may manage and maintain the CAM device  601  through the port processor  602 . As will be appreciated by one ordinarily skilled in the art, the more time spent on updating and managing the CAM  601 , the less time there is for processing network traffic. There is an upper limit to obtaining faster processors and CAM devices, along with the negative of increased power consumption of the faster devices, to achieve improved network traffic processing performance. It will be further appreciated that embodiments of the present invention utilize additional control words to segment and prioritize the data stored within the CAM array, which in turn, streamlines the update and management tasks and time, and reduces power consumption. 
   It is to be understood that, even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only. Changes may be made in detail, and yet remain within the broad principles of the invention. For example, other types of combinations of the different control words described in  FIGS. 4A ,  4 B, and  4 C could be used than shown in  FIGS. 5A and 5B . Similarly, the control words could be ordered differently than shown in  FIGS. 5A and 5B . This, and other embodiments could make use of and fall within the principles of the invention. Thus, although specific embodiments of the invention have been described herein for purposes of illustration, and the invention is not limited except as by the appended claims.

Technology Category: 3