Abstract:
A method and apparatus for encoding MATCH signal and MATCH ADDRESS signal generation for a content addressable memory (&#34;CAM&#34;) array is disclosed. Each CAM core has an output encoder for providing a MATCH signal and, if a MATCH is determined, a MATCH --  ADDRESS signal indicative of the location in the CAM of the data of interest. In order to speed the critical search path, each signal line for the MATCH and MATCH --  ADDRESS have the same number of transistors, with designated MATCH --  ADDRESS transistors being used to indicate a MATCH, that is, providing a substitute MATCH signal, if the MATCH line has no transistor. The output encoder output signals are encoded to provide a final MATCH signal and a final MATCH --  ADDRESS which adds a CAM core designator to the MATCH --  ADDRESS from the CAM core having the data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the encoding of digital data and, more specifically, to MATCH and MATCH --  ADDRESS signal generation in a content addressable memory encoder. 
     2. Description of Related Art 
     Random access memory (&#34;RAM&#34;) is perhaps the most common form of integrated circuit memory available in the state of the art. However, RAM devices are not suited for use in systems which process associative data. For example, the well known methodology of sequentially accessing data when addressing the RAM is inefficient for systems involving stored information involving pattern recognition, natural language recognition, sparse matrix processes, and data-base interrogation. The address associated with the desired stored data may not be known. For this type of data, it is more efficient to interrogate a memory by supplying a compressed subset of the desired data or a code representative of the full data set. The memory responds by indicating either absence of the desired data set by a MISMATCH signal, or presence of the data set by a MATCH signal, and its associated address in the memory bank for the data set. 
     In the 1980&#39;s, another type of memory device was developed to have ambiguous and non-contiguous addressing and was dubbed the content addressable memory (&#34;CAM&#34;). See e.g., U.S. Pat. No. 3,701,980 (Mundy). In essence, for this type of associative data search, the entire CAM can be searched in a single clock cycle, giving it a great advantage over the sequential search technique required when using a RAM device. 
     A string dictionary can be stored in a CAM and used in generating Lev-Zempel compressed output data (hereinafter &#34;LZ&#34;; generically used for any LZ data compression technique; see &#34;Compression of Individual Sequences Via Variable-Rate Coding&#34;, IEEE Transactions on Information Theory, 24(5):530-536, September 1978, incorporated herein by reference). The input data signal to the CAM would comprise a bit string representation of the data which is being searched for in the CAM. The output would be a signal indicative as to whether the data was found, the MATCH signal, and, if found, the location within the CAM, the MATCH --  ADDRESS. Obtaining this MATCH and MATCH --  ADDRESS information is done with what is called in the art a &#34;match encoder.&#34; For example, for color hard copy printing, a data base may be stored in a CAM where the data consists of bit strings comprising tristimulus space values--cyan, yellow, magenta (&#34;CYM&#34;). 
     The problem with CAM devices is that compared to RAM each individual cell structure is relatively complex. See e.g., U.S. Pat. No. 4,780,845 (Threewitt); incorporated herein by reference. Thus, for the same integrated circuit real estate, a CAM device can not match the density, speed, or low-power performance of a RAM device. Integrated circuit process improvements generally affect both types of devices equally, so that in relative terms, CAM architects can not do much to narrow the performance gap. 
     Perhaps the most critical path through the CAM is the search cycle; that is, the time from receipt of an input data signal, or code, to the encoder input to determine if the CAM has the desired data set to the time of the output of a match or mismatch indication, and, if a MATCH signal is generated, the MATCH --  ADDRESS. 
     Therefore, there is a need to improve the speed of a search cycle, that is, to shorten the clock cycle duration, without the need of a integrated circuit fabrication process improvement. 
     SUMMARY OF THE INVENTION 
     In its basic aspects, the present invention provides an encoder apparatus for a CAM, having a cell array of addressable cell means for storage of data bits therein and appropriate CAM output circuitry for transmitting selected data. The apparatus includes: a first encoding mechanism for receiving the CAM cell array output, the first encoding mechanism further including a MATCH signal line connected to each cell, a plurality of MATCH ADDRESS signal lines connected to the MATCH signal line, and wherein the MATCH signal line and each of the MATCH ADDRESS signal lines have an identical number of substantially identical switching mechanism for indicating a change in digital state value on the MATCH signal and MATCH ADDRESS signal lines, respectively, wherein the MATCH signal line includes one of the switching mechanism for one-half of the cells in the cell array and wherein a predetermined one of the switching mechanism on the MATCH ADDRESS signal lines designates both a binary digit of the MATCH ADDRESS signal and a MATCH signal on lines for MATCH signals not having a MATCH signal line switching mechanism; and first output bus mechanism for transmitting MATCH signals and MATCH ADDRESS signals. 
     In another basic aspect of the invention there is presented a method for encoding MATCH signal and MATCH ADDRESS signal generation for a content addressable memory cell array. The method includes the steps of: 
     generating a single-bit MATCH signal for one-half of the data locations in the array; 
     generating a multi-bit MATCH ADDRESS signal for every data location in the array; and 
     using a bit signal from a generated multi-bit MATCH ADDRESS signal as a substitute MATCH signal for every data location not generating a single-bit MATCH signal. 
     Embodiments of the apparatus and method are also explained for an implementation of an encoder apparatus for MATCH signal and MATCH ADDRESS signal generation for a bank of memory that includes a plurality of content addressable memory cores. 
     It is an advantage of the present invention that it improves the speed of a CAM search cycle without the need for integrated circuit fabrication process improvements. 
     It is an advantage of the present invention that it does not increase integrated circuit size or power requirements. 
     It is another advantage of the present invention that implementations can be designed to reduce integrated circuit size requirements. 
     It is still another advantage of the present invention that implementations can be designed to reduce power consumption. 
     Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a CAM system incorporating the present invention. 
     FIGS. 2A, 2B, 2C are comparison schematic diagrams of detail for CAM core encoders as shown in FIG. 1 in which: 
     FIG. 2A represents a traditional encoder design, FIG. 2B represents a encoder in accordance with the present invention, and FIG. 2C shows a detail of FIGS. 2A and 2B. 
     FIGS. 3A and 3B and FIGS. 4A and 4B are comparison schematic diagrams of final --  encoders as shown in FIG. 1 in which: 
     FIGS. 3A-3B represent a traditional final --  encoder design, and 
     FIGS. 4A-4B represent a final --  encoder in accordance with the present invention as shown in FIG. 1. 
     FIG. 5A is a detailed schematic of one final --  encoder subsection for a CAM --  CORES x , as shown in FIG. 4B. 
     FIG. 5B is a detail of FIG. 5A. 
    
    
     The drawings referred to in this specification should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventor for practicing the invention. Alternative embodiments are also briefly described as applicable. In order to assist the reader in understanding the best mode, a exemplary embodiment of a LZ compression system is provided. One implementation of such a system can be used in a commercial implementation for a color hard copy apparatus. There is no express intent to limit the applicability of the present invention to the exemplary embodiment nor should any such limitation be implied; the scope of the invention is defined by the claims set forth hereinafter. 
     Turning now to FIG. 1, SEARCH --  DATA on standard bus 101 is fed from the search engine (e.g., a central processing unit (not shown)) through a driver 103, comprising standard buffering hardware as would be known in the art to drive relatively the high capacitance of cam core cell architecture. 
     Each CAM --  CORE 105 1  -105 N  comprises an array of standard transistor-based cell circuitry and search circuitry as would also be known to a person skilled in the art. Each cell of the array stores one bit. In accordance with the exemplary embodiment, a total CAM of 768 --  words by 19 --  bits is described. It is assumed for the exemplary embodiment that due to integrated circuit layout constraints, that N=6; that is, six CAM --  CORES 105 1  -105 6  of 128 --  words by 19 bits each is provided. 
     The SEARCH --  DATA is input through standard buses 107 1  -107 N  to interrogate each CAM --  CORE 105 1  -105 N . While for implementation for certain algorithms more than one CAM --  CORE 105 1  -105 N  could have a MATCH, it is assumed in this exemplary embodiment implementation that only one cell on one CAM --  CORE 105 1  -105 N  contains the data set of interest. Thus, if any, there will be only one MATCH x  signal and one corresponding FIRST --  MATCH --  ADDRESS x . 
     In general it is known to precharge the CAM --  CORE outputs; in this example, the outputs are precharged to all HIGH. Each CAM --  CORE 105 1  -105 N  has an output bus 109 1  -109 N  with one line for each of the stored data words, viz. 128 --  words in the exemplary embodiment. If a mismatch occurs for any location, the output bit for that location is pulled to LOW to indicate a MISMATCH; thus, if an output stays HIGH, it indicates a MATCH. 
     If there is no match, all outputs go LOW. Thus, for each CAM --  CORE 105 1  -105 n , one hundred and twenty eight outputs on respective buses 109 1  -109 N  tell whether a particular address in that cell array is a MATCH or a MISMATCH. The output signal derivation for each CAM --  CORE output of the six device memory bank is accomplished using a memory FIRST --  ENCODER 111 1  -111 N . 
     The one hundred and twenty eight outputs of the six CAM --  COREs 105 1  -105 6  now need to be turned into a final match signal, MATCH --  SIGNAL F , and DATA --  MATCH --  ADDRESS F  signal, preferably in one clock cycle, where DATA --  MATCH --  ADDRESS F  is both the address of a particular CAM --  CORE 105 x  and its cell array address, FIRST --  MATCH --  ADDRESS x . Assuming again only one MATCH designation for one CORE --  CORE 105 1  -105 N  of the memory bank, CAM --  CORE 1  105 1  through CAM --  CORE N  105 N , a MATCH F  signal and an appropriate DATA --  MATCH --  ADDRESS F  is derived using FINAL --  ENCODER 113. 
     Turning now to FIGS. 2A and 2B, a standard CAM encoder 201, FIG. 2A, is shown. Such an encoder 201 is used in a CAM system such as shown in the assignee&#39;s U.S. Pat. No. 5,373,290 (Lempel et al.) as element 194, FIG. 5, explained beginning in column 12, line 28 et seq., incorporated herein by reference in its entirety. In the encoder 201, a MATCH line 203 has a pull down transistor 205, configured as in FIG. 2C, one for each of the one hundred and twenty eight data words in each CAM --  CORE 105 1  -105 N . Likewise, one hundred and twenty eight CORE --  MATCH lines 207 0000000  (location zero) through 207 1111111  (location 127) are multiplexed to the MATCH line 203, from a least significant bit (&#34;LSB&#34;) MATCH --  ADDRESS line 209 1  through a most significant bit (&#34;MSB) MATCH --  ADDRESS line 209 7 , in essence a multiplex wired OR configuration  note: as will be described hereinafter, seven bits will also form the lower address bits of a ten bit address from the FINAL --  ENCODER 113, FIG. 1!. Thus, the MATCH line 203 has one hundred and twenty eight pull down transistors 205 (counted vertically in FIG. 2A), but each of the MATCH --  ADDRESS lines 20 1  -209 7  has only sixty four pull down transistors. 
     Comparing this embodiment of the standard CAM encoder 201 in FIG. 2A to the FIRST --  ENCODER 201 in accordance with the present invention as shown in FIG. 2B, the difference lies in that on MATCH line 203, pull down transistors 205 are provided only for one half of CORE --  MATCH lines 207 0000000  (location zero) through 207 1111110  (location 126). For locations having no MATCH line 203 pull down transistors 205, a designated pull down transistor of the MATCH --  ADDRESS lines 209 1  -209 7  are used to serve double duty, that is, also indicating a match condition when switched. 
     For example, as shown where every other MATCH line 203 has a pull down transistor 205, if the DATA of interest of the SEARCH --  DATA is at location 0000011, a location having no MATCH line 203 pull down transistor 205 but using bit --  0 to do the double duty, since only one location of the CAM --  CORE is ever a match, no conflicts will occur. That is, if the CAM --  CORE has set the MATCH --  ADDRESS at location 0000011, bit --  0 has change state, indicating a MATCH. As another example, if the most significant MATCH --  ADDRESS bit is used for the double duty, only the top sixty-four MATCH lines 203 require pull down transistors 205. Thus, use of one of the MATCH --  ADDRESS bits as also indicating a MATCH when a true match has occurred in this manner reduces the number of pull down transistors on the MATCH line 203 to sixty-four. As a result, the MATCH line 203 will be as fast as the MATCH --  ADDRESS lines 209. In a commercial implementation having a search access time of approximately 6 nanoseconds, an improvement of approximately 0.5 nanosecond has been found to be achieved. 
     Recall that the present exemplary embodiment as shown in FIG. 1 uses a bank of six CAM --  CORES 105 1-6 , each with its own FIRST --  ENCODER 111 1-6 . Now each of the output signals MATCH 1-6  on each FIRST --  ENCODER --  MATCH output bus 115 1-6  and its appurtenant FIRST --  MATCH --  ADDRESS output bus 117 1-6  needs to be encoded in order to obtain both a final MATCH F  signal back to the CPU, indicating the data of interest has been found, and a DATA --  MATCH --  ADDRESS F  specifying both the FIRST --  MATCH --  ADDRESS on bus 117 x , where x=the CAM --  CORE 0-127  location which generated a MATCH signal, and a designation of which of the six CAM --  CORES 105 1-6  generated a MATCH signal. This function is accomplished in the FINAL --  ENCODER 113 by adding three upper address bits to the seven FIRST --  MATCH --  ADDRESS bits for the CAM --  CORE 105 location where the full data of interest resides. 
     Turning to FIGS. 3A-3B and 4A-4B, a FINAL --  ENCODER 113 for accomplishing this task is provided. 
     FIG. 3A again refers to an embodiment as shown in assignee&#39;s U.S. Pat. No. 5,373,290 as part of element 194, FIG. 5. In element 194, a final --  encoder 301 for an array of six cam --  cores has six sections, one designated for each cam --  core of the array. As stated earlier, each FIRST --  ENCODER 111 1-N , FIG. 1, has an output line  115   1-N  for a MATCH 1-N  signal and an output bus 117 1-N  for a FIRST --  MATCH --  ADDRESS 1-N . Looking to both FIGS. 3A-3B and 4A-4B for comparison, and focusing on the section of FINAL --  ENCODER 113, FIG. 1, for CAM --  CORE 6  as an example of each section, the MATCH 6  signal on line 115 6  provides an appropriate HIGH or LOW state signal to each respective FINAL --  ENCODER 113 input subsection, CAM --  CORE 1-N , 303 1-N  Each FIRST --  MATCH --  ADDRESS 7-bit bus 117 1-N  is likewise input to each FINAL --  ENCODER 113 input subsection, CAM --  CORE 1-N . That is to say, each CAM --  CORE x  has its respective FIRST --  ENCODER 111 x  output connected to a respective subsection of the FINAL --  ENCODER 113 which will in turn provide the actual MATCH F  signal and DATA --  MATCH --  ADDRESS F  for the data of interest based on the SEARCH --  DATA input. 
     Looking also to FIGS. 5A and 5B, detail for FINAL --  ENCODER 113 subsection CAM --  CORE 6  303 6  is depicted. The FINAL --  ENCODER 113 is multiplexed with the inputs 115, 117 from the FIRST --  ENCODER x . Match signal pull down transistors 501 are provided in a manner such that when a MATCH 6  and FIRST --  MATCH --  ADDRESS 6  is received from a FIRST --  ENCODER 6 , the FINAL --  ENCODER input subsection CAM --  CORE 6  will provide both a MATCH F  signal on FINAL --  MATCH --  LINE 401 and an expanded, 10-bit address for the data, DATA --  MATCH --  ADDRESS F . In the example, the DATA --  MATCH --  ADDRESS designates the CAM --  CORE 6  in its added upper three bits on DATA --  MATCH --  ADDRESS F  upper bit lines 403 1-3 , and pass through the FIRST --  MATCH --  ADDRESS 6  on DATA --  MATCH --  ADDRESS F  lower bit lines 405 1-7  (with reversal of all signal levels, HIGH to LOW and LOW to HIGH if necessary to use standard logic where HIGH=1). 
     Returning to FIGS. 3A-3B and 4A-4B, each CAM --  CORE, can be compared and it can be seen that the removal of half of the pull down transistors 205 on FIRST --  ENCODER --  MATCH lines 207 in FIG. 2B for providing the MATCH x  signal has been added back in the FINAL --  ENCODER 113 on MATCH E  lines 401. However, it has been found that this arrangement in the critical path in the present invention as shown in FIGS. 2B, 4A-4B, and 5A -5B provides an improvement of in reducing the cycle time approximate ten percent over the arrangement of FIGS. 2A, 3A -3B in a synergistic manner. 
     The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application to thereby enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.