Patent Publication Number: US-6903951-B1

Title: Content addressable memory (CAM) device decoder circuit

Description:
This application claims the benefit of provisional application Ser. No. 60/343,973 filed Dec. 27, 2001. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to decoder circuits, and more particularly to decoder circuits in content addressable memory (CAM) devices. 
     BACKGROUND OF THE INVENTION 
     Due to the increasing need for rapid matching capabilities, in networking hardware equipment for example, content addressable memories (CAMs) continue to proliferate. A CAM may perform matching functions by applying a search key or “comparand” to a table of stored data values. A CAM may then determine if any of the data values match a given search key. 
     CAM devices may take a variety of forms. As but a few of the possible examples, some CAM devices are based on particular types of CAM cells. Such cells may include storage circuits integrated with compare circuits. Examples of storage circuits may be static random access memory (SRAM) type cells or dynamic RAM (DRAM) type cells. Alternate approaches may include RAM arrays, or the like, with separate matching circuits and/or matching processes executed by a processor, or the like. 
     Conventional CAM devices may include both binary and ternary CAM devices. Binary CAM devices can provide a bit-by-bit comparison between a stored data value and a search key. Ternary CAM devices can provide maskable compare operations that can selectively exclude predetermined bits of a data value from a compare operation. 
     Typically, a conventional CAM device can generate match indications for each entry. That is, each entry can be compared with an applied search key value. If a search key value matches a stored data value, a match (or “hit”) indication may be generated for the entry. Conversely, if a search key value does not match a stored data value, a mismatch (or “miss”) indication may be generated for the entry. 
     Match results in a CAM device may include single match results, that can be generated when a single entry matches an applied key value, as well as multiple match results, that may be generated when more than one entry matches an applied key value. 
     While CAM entries can provide the above described match or search function, CAM entries may also include conventional data access functions, such as read or write operations, to read or write the data values that are compared to a search key. 
     Decoder circuits within CAM devices can conventionally select a CAM entry for a particular operation, such as a read or write. More specifically, conventional circuits may enable a particular CAM entry by coupling such a CAM entry to bit lines, or the like, so that data may be read from the entry or written into the entry. Mask data or similar data may be written for a given CAM entry in a similar fashion. 
     Conventional CAM decoder circuits can be conceptualized as being “one-hot” type decoders. That is, a typical conventional CAM decoder can receive a binary input value of N bits and output 2 N  pre-decode output values. In response to each particular input value, one of the pre-decoder output values can be activated. 
     While one-hot type decoders can be suitable for read and write operations in a CAM device, it may be desirable to employ decoders that can provide more advanced functions in order to provide additional features in a CAM device. 
     SUMMARY OF INVENTION 
     According to the present invention, a decoder circuit may include a first string decoder that activates a different first pre-decode signal in response to different first input data values in a first mode. In a second mode, a first string decoder can activate different numbers of the first pre-decode signals in response to each different first input data value. 
     According to one aspect of the embodiments, a decoder circuit may also include a first compare circuit that compares two or more of the first pre-decode signals to generate a first comparison result. 
     According to another aspect of the embodiments, a decoder circuit may also include first pre-decode signals that have an order with respect to one another. A first compare circuit can generate one comparison result when a lower order first pre-decode signal is active and a higher order first pre-decode signal is inactive, and generates another first comparison result when the lower order first pre-decode signal and higher order first pre-decode signal are both active. 
     According to another aspect of the embodiments, a decoder circuit may also include an enable circuit. In response to one first comparison result from a first comparator, an enable circuit can generate decoder output signals according to second pre-decode signals. In response to another first comparison result from the first comparator, an enable circuit can generate predetermined decoder output signals. 
     In one particular approach, an enable circuit may include a first set of gates that are enabled in response to one first comparison result. An enable circuit may also include a second set of gates having inputs coupled to the outputs of the first set of gates. Such a set of gates can be enabled in response to another first comparison result. 
     According to another aspect of the embodiments, a decoder circuit may also include a second string decoder that activates a different second pre-decode signal in response to different second input data values in a first mode. In a second mode, a second string decoder may activate a different number of the second pre-decode signals in response to each different second data value. In addition, a second compare circuit can compare at least two of the second pre-decode signals to generate a second comparison result. 
     According to another aspect of the embodiments, in a decoder circuit, a second comparison result can be provided to a first compare circuit. 
     According to another aspect of the embodiments, a second compare circuit can logically combine a first comparison result with a second comparison result. 
     According to another aspect of the embodiments, a decoder circuit can include a number of content addressable memory (CAM) entries arranged into groups of size N, a number of first compare circuits, and a number of second compare circuits. In a second mode of operation, first compare circuits can compare at least two of the first pre-decode signals to generate first comparison results that each enable a CAM entry group. Further, second compare circuits can compare at least two second pre-decode signals to generate second comparison results. Second comparison results can enable entries within a same CAM entry group. 
     According to another aspect of the embodiments, a decoder circuit can further include a number of CAM entries formed in a first area of a substrate and a number of first compare circuits formed in a second area of the substrate adjacent to the first area. Further, a first string decoder can be formed in a third area of the substrate that is different than the second area. 
     The present invention may also include a method of activating decoder signals in a content addressable memory (CAM) device. Such a method may include activating one of a number of first pre-decode signals in response to different first input values in a first mode, and activating a different number of the first pre-decode signals in response to each different first input value in a second mode. 
     According to one aspect of the embodiments, a method may include the first pre-decode signals having an order with respect to one another. In addition, activating a different number of the first pre-decode signals can include activating a sequential number of the first pre-decode signals according to the order. 
     According to another aspect of the embodiments, a method may include accessing a single CAM entry in a first mode, and searching a predetermined number of CAM entries according to first input values in a second mode. 
     According to another aspect of the embodiments, a method may include activating a different number of second pre-decode signals in response to each different second input value in a second mode. 
     According to another aspect of the embodiments, a method may include comparing second pre-decode signals to enable one of a plurality of decoder output signal groups in response to different second input values in a first mode. In addition, in a second mode, comparing second pre-decode signals to enable a different number of the output signal groups in response to each different second input values. 
     According to another aspect of the embodiments, a method may include activating decoder output signals from one of the output signal groups according to the first pre-decode signals in both a first and second mode. 
     According to another aspect of the embodiments, a method may include the first pre-decode signals having a significance with respect to one another. A method may also include, in a second mode, in response to a particular first input value, activating all first pre-decode signals having greater significance than one first pre-decode signal activated in response to the same first input value in the first mode. 
     The present invention may also include an address decoder circuit that includes one or more circuits having at least two modes of operation. A first mode can produce a single active output signal for each possible input state combination. A second mode can produce multiple active output signals for each possible input state combination. 
     According to one aspect of the embodiments, multiple active output signals in a second mode are consecutive and range from the lowest order output signal to the same output signal as the single active signal in the first mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block schematic diagram of a decoder circuit according to a first embodiment. 
         FIG. 2  is a table showing the operation of a “string” decoder according to the present invention. 
         FIG. 3  is a block schematic diagram of a decoder circuit according to a second embodiment. 
         FIGS. 4A  to  4 C are block schematic diagrams showing the operation of a decoder circuit according to a second embodiment. 
         FIG. 5  is a block diagram of a string decoder according to one embodiment. 
         FIG. 6  is a block diagram of a string decoder according to another embodiment. 
         FIG. 7A  is a schematic diagram of a mode controlled string decoder circuit according to an embodiment. 
         FIG. 7B  is a truth table showing the operation of the circuit of FIG.  7 A. 
         FIG. 8A  is a schematic diagram of a one-hot decoder circuit and combining logic according to an embodiment. 
         FIG. 8B  is a truth table showing the operation of the one-hot decoder circuit of FIG.  8 A. 
         FIG. 9  is a plan view of a content addressable memory device according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments of the present invention will now be discussed in conjunction with a number of figures. The embodiments describe a decoder apparatus and method that may be included in a CAM device. A decoder apparatus and/or method according to the present invention may provide conventional one-hot decoding in a first mode, but also provide “string” decoding in a second mode. String decoding may activate a different number of pre-decode signals in response to each different input value. 
     Referring now to  FIG. 1 , a decoder circuit according to one embodiment is set forth in a block schematic diagram and designated by the general reference character  100 . A decoder circuit may include “string” decoders  102 - 0  and  102 - 1 . A decoder circuit  100  may also include a compare circuit  104  and an enable circuit  106 . 
     Each string decoder ( 102 - 0  and  102 - 1 ) may receive an input value (IN 0  or IN 1 ) and provide corresponding pre-decode output signals (P 0 -P(n−1) or Pn-P(2n−1)). In the embodiment of  FIG. 1 , each string decoder ( 102 - 0  and  102 - 1 ) may also receive a mode signal EXPL. In response to a mode signal EXPL, a string decoder ( 102 - 0  and  102 - 1 ) may provide one-hot or string decoding. Particular examples of such decoding will be described in more detail below. 
     A compare circuit  104  may receive at least two pre-decode signals from one string decoder  102 - 1  and compare such values to generate a comparison result CMP. A comparison result CMP may be provided to an enable circuit  106 . 
     An enable circuit  106  may receive pre-decode signals from one string decoder  102 - 0 , and provide decoder output signals DEC 0 -DEC(n−1). The generation of decoder output signals (DEC 0 -DEC(n−1)) by an enable circuit  106  can be controlled by a comparison result CMP. As but one example, if a comparison result has one particular value, decoder output signals (DEC 0 -DEC(n−1)) may be generated in response to received pre-decode signals (P 0  to P(n−1)). However, if a comparison result has another particular value, decoder output signals (DEC 0 -DEC(n−1)) may be predetermined values (e.g., all active and/or all inactive). 
     Referring now to  FIG. 2 , one example of a string decoder operation is set forth in table form. The table of  FIG. 2  includes columns INx, EXPL, and P 7 -P 0 . A column INx can represent a received input value. In the very particular example of  FIG. 2 , an input value can range from 0-7. 
     A column EXPL shows a mode signal that can determine a mode of operation for a string decoder. A mode value EXPL of “0” can indicate a conventional (e.g., one-hot) mode of operation for a string decoder. Such a mode may include writes and reads to single entries in a memory device, such as a CAM device. In contrast, a mode value EXPL of “1” can indicate an unconventional (e.g., one or more hot) mode of operation for a string decoder. Such a mode may include an “explore” operation in a CAM device that may restrict a search operation to less than all available CAM entries. 
     Particular examples of explore operations (also referred to as restricted search and/or search-beyond operations) are set forth in co-pending U.S. patent application Ser. No. 10/281,814 filed on Oct. 28, 2002 and entitled “METHOD AND APPARATUS FOR RESTRICTED SEARCH OPERATION IN CONTENT ADDRESSABLE MEMORY (CAM) DEVICES” by James et al. The contents of this application are incorporated by reference herein. 
     As can be seen in  FIG. 2 , when a mode signal has a value of 0 (a one-hot mode), a string decoder may function in the same general fashion as a conventional decoder, activating a different pre-decode signal (P 7 -P 0 ) in response to each different input value INx. 
     Unlike a conventional decoder, when a mode signal has a value of 1 (a string mode), a string decoder may activate different “strings” of values. More particularly, in  FIG. 2 , in response to each different input value, a string decoder may activate a different number of pre-decode signal (P 7 -P 0 ) in response to each different input value INx. 
     In the very particular case of  FIG. 2 , in a string-hot mode, a string decoder may activate the same pre-decode signal as in the one-hot mode, but may also activate all pre-decode signals of higher significance. Thus, as shown in  FIG. 2 , when a mode signal EXPL is a “0”, a string decoder may activate pre-decode signal P 4  in response to an input value INx of “4”. However, when a mode signal EXPL is a “1”, a string decoder may activate pre-decode signal P 4 , and in addition, higher significant pre-decode signals P 5 -P 7  in response to an input value of “4”. 
     Looked at in another way, pre-decode signals may be conceptualized as having an order with respect to one another, in this case, a numerical order. In a one-hot mode a single pre-decode signal may be activated. However, in a string mode, multiple pre-decode signals may be activated in a consecutive order. In addition, a lowest signal in such an order may be the same signal that is activated in the one-hot mode. 
     A decoder circuit according to a second embodiment will now be described with reference to FIG.  3 .  FIG. 3  shows a decoder circuit, designated by the general reference character  300 , that includes string decoders  302 - 0  to  302 - 2 , higher order compare circuit  304 - 0 , lower order compare circuits  306 - 0  and  306 - 1 , and enable circuits  308 - 0  and  308 - 1 . The example of  FIG. 3  shows an example of 3-bit encoding, in that an output of each string decoder ( 302 - 0  to  302 - 2 ) may be generated from a 3-bit encoded value. 
     String decoders ( 302 - 0  to  302 - 2 ) may receive an input address ADD, and in response thereto, generate pre-decode signals P 0 -P 23 .  FIG. 3  shows string decoders ( 302 - 0  to  302 - 2 ) arranged to enable hierarchical string decoding. Such an arrangement may allow for an enabling or disabling of different numbers of decoder output signal groups in response to each different higher order address portion. Thus, in  FIG. 3 , an address may include multiple portions having a significance with respect to one another. Such portions are shown as “MORE”, “CORE” and “LESS”. A lowest significance portion LESS may be provided as an input value IN 0  to a string decoder  302 - 0 . A next significant portion CORE may be provided as an input value IN 1  to a string decoder  302 - 1 . An even higher significance portion MORE may be provided as an input value IN 2  to a string decoder  302 - 2 . 
     String decoders ( 302 - 0  to  302 - 2 ) may each operate in the same general fashion as shown in FIG.  2 . Thus, string decoder  302 - 0  may activate one of pre-decode signals P 0 -P 7  in a one-hot mode. However, in a string mode, a string decoder  302 - 0  may activate one to eight pre-decode signals P 0 -P 7  depending upon a particular input value IN 0 . Similarly, string decoder  302 - 1  may activate one of pre-decode signals P 8 -P 15  in a one-hot mode, and activate one to eight pre-decode signals P 8 -P 15  in a string hot mode depending upon a particular input value IN 1 . In the same general fashion, string decoder  302 - 2  may activate one, to one to eight of pre-decode signals P 16 -P 23 , according to a mode. 
     A higher order compare circuit  304 - 0  may compare predetermined higher order pre-decode signals (in this case P 16 -P 23 ) and provide such comparison results to lower order compare circuits ( 304 - 0  and  304 - 1 ). In one particular approach, a higher order compare circuit  304 - 0  may receive multiple pre-decode signals and determine if one such pre-decode signal is active, or if multiple such pre-decode signals are active. In the very particular example of  FIG. 3 , higher order compare circuit  304 - 0  may receive two pre-decode signals. If one particular pre-decode signal is active and the other is not, one comparison result GTA may be activated. However, if both pre-decode signals are active, another comparison result EQA may be activated. 
     In the very particular example of  FIG. 3 , a higher order compare circuit  304 - 0  may include a first gate  310 - 0  that can generate one compare result GTA, and second gate  310 - 1  that can generate another compare result EQA. Of course, while  FIG. 3  shows a gate  310 - 0  as an AND gate and gate  310 - 1  as an AND gate with an inverting input, alternative logic may be employed to generate a same or similar result. Accordingly, the particular logic gate arrangement for higher order compare circuit  304 - 0  in  FIG. 3  should not be construed as limiting the invention thereto. 
     In  FIG. 3 , higher order compare result GTA can be conceptualized as a “greater than” address result. Such a “greater than” address result can indicate that an address corresponding to a collection of decoder output signals is greater than a received address value. Thus, all such decoder outputs can be activated in a second mode or deactivated in a first mode. 
     In contrast, higher order compare result EQA can be conceptualized as an “equal to” address result. Such an “equal to” address result can indicate that one of decoder output signals in a collection of decoder signals has an address equal to a received address. Thus, groups within such a collection of decoder signals may be selectively activated according to lower order pre-decode signals (e.g., P 8 -P 15 ). 
     Lower order compare circuits  306 - 0  and  306 - 1  may compare predetermined lower other pre-decode signals (in this case P 8 -P 15 ) and provide such comparison results to corresponding enable circuits  308 - 0  and  308 - 1 . Thus, in  FIG. 3 , lower order compare circuit  306 - 0  can provide compare results GTB and EQB to enable circuit  308 - 0 , while lower order compare circuit  306 - 1  provides compare results GTC and EQC to enable circuit  308 - 1 . 
     In a similar fashion to higher order compare circuit  304 - 0 , in a very particular approach, lower order compare circuits ( 306 - 0  and  306 - 1 ) can receive multiple pre-decode signals and determine if one such pre-decode signal is active, or if multiple such pre-decode signals are active. In the very particular example of  FIG. 3 , lower order compare circuits ( 306 - 0  and  306 - 1 ) can receive two pre-decode signals. If one particular pre-decode signal is active and the other is not, one comparison result (GTB or GTC) may be activated. However, if both pre-decode signals are active, another comparison result (EQB or EQC) may be activated. 
     The operation of lower order compare circuits ( 306 - 0  and  306 - 1 ) may be affected by a higher order compare circuit  304 - 0 . In particular, lower order compare circuits ( 306 - 0  and  306 - 1 ) may be enabled according to one comparison result from a higher order compare circuit  304 - 0 . Further, a lower order compare circuit ( 306 - 0  and  306 - 1 ) result may be controlled by another comparison result from a higher order compare circuit  304 - 0 . For example, in  FIG. 3 , lower order compare circuits ( 306 - 0  and  306 - 1 ) may be enabled when compare result EQA from higher order compare circuit  304 - 0  is active. Further, lower order compare circuit ( 306 - 0  and  306 - 1 ) may result in GTB and GTC being forced active when compare result GTA from higher order compare circuit  304 - 0  is active. 
     In  FIG. 3 , one example of a lower order compare circuit  306 - 0  is shown in detail. Such a lower order compare circuit  306 - 0  may include a third gate  310 - 2  that can generate an initial compare result GTB′, and a fourth gate  310 - 3  that can generate another compare result EQB. A lower order compare circuit  306 - 0  may also include a combining gate  310 - 4 , that can logically combine an initial compare result GTB′ with a higher order compare result GTA, to thereby generate a compare result GTB. Of course, while  FIG. 3  shows particular gate arrangements for lower order compare circuit  306 - 0 , alternative logic may be employed to generate a same or similar result. Accordingly, the particular logic gate arrangement for lower order compare circuit  306 - 0  in  FIG. 3  should not be construed as limiting the invention thereto. 
     In  FIG. 3 , lower order compare results (GTB and GTC) can also be conceptualized as a “greater than” address result. Such a “greater than” address result can indicate that an address corresponding to a group of decoder output signals is greater than a received address value. Thus, all such decoder outputs can be activated or deactivated. For example, if compare result GTB is active, decoder output signal group DEC 0 -DEC 7  can be activated. Similarly, if compare result GTC is active, decoder output signal group DEC 8 -DEC 15  can be activated. 
     Lower order compare results (EQB and EQC) can be conceptualized as “equal to” address results. Such “equal to” address results can indicate that one of the decoder output signals in a corresponding group has an address equal to a received address. Thus, groups within such a collection of decoder signals may be selectively activated according to lower order pre-decode signals (e.g., P 0 -P 7 ). 
     The very particular example of  FIG. 3  shows an enable circuit  308 - 0  that can include a first set of logic gates  310 - 5  and a second set of logic gates  310 - 6 . A first set of logic gates  310 - 5  can logically combine each lowest order pre-decode signal P 0 -P 7  with one lower order compare result EQB. In particular, if compare result EQB is active (high in this example), outputs of logic gates  310 - 5  can follow received pre-decode signal P 0 -P 7  values. However, if compare result EQB is inactive (low in this example), outputs of logic gates  310 - 5  can be forced to predetermined values (e.g., all low). 
     A second set of logic gates  310 - 6  can logically combine each output from the first set of logic gates with another lower order compare result GTB. In particular, if lower order compare result GTB is inactive (low in this example), outputs of logic gates  310 - 6  can follow those of logic gates  310 - 5 . However, if lower order compare result GTB is active (high in this example), outputs of logic gates  310 - 6  can be forced to predetermined values (e.g., all high). 
     Having described the general structure of a second embodiment, the operation of the second embodiment will now be described with reference to  FIGS. 4A  to  4 C. 
       FIGS. 4A  to  4 C show examples of a decoder circuit having the same general arrangement of that shown in FIG.  3 . Accordingly, like portions will be referred to by the same reference character but with the first digit being a “4” instead of a “3.” Thus,  FIGS. 4A  to  4 C show string decoders  402 - 0  to  402 - 2 , higher order compare circuit  404 - 0 , lower order compare circuits  406 - 1  to  406 - 4 , and enable circuits  408 - 1  to  408 - 4 . Also shown in  FIGS. 4A  to  4 C are pre-decode signals P 0  to P 23 , as well as decoder output signals DEC 72  to DEC 103 . 
     Unlike  FIG. 3 ,  FIGS. 4A and 4B  show a particular CAM application for a decoder circuit. Thus, decoder output signals DEC 72  to DEC 103  can be provided to CAM entries ENTRY 72  to ENTRY 103 . 
     It is understood that in the example illustrated, in a search operation, CAM entries (ENTRY 72  to ENTRY 103 ) may each compare a stored data value to an applied search key value KEY. In response to such a comparison result, a CAM entry may generate a corresponding match indication (M 72  to M 103 ). Still further, in a search operation, a CAM entry may be enabled according to a corresponding decoder input signal DEC 72  to DEC 103 . In contrast, in a read or write operation, a selected CAM entry (ENTRY 72  to ENTRY 103 ) may receive or output a data value. Such a CAM entry may be selected according to a corresponding decoder input signal DEC 72  to DEC 103 . 
     In the examples of  FIGS. 4A  to  4 C, it will be assumed that entries can be enabled when a corresponding decoder output is active and disabled when a corresponding decoder output is inactive. Further, in  FIGS. 4A  to  4 C, a disabled entry may be represented by hatching. 
       FIGS. 4A  to  4 C illustrate a decoding of two combinations for input values IN 0  to IN 2  (which may be portions of a single CAM entry address).  FIG. 4A  shows result in which input value IN 2  is string decoded to generate pre-decode signals P 23  to P 16  having the values “1111 1110,” respectively (i.e., only P 16  is inactive), input value IN 1  is string decoded to generate pre-decode signals P 15  to P 8  having the values “1111 1110,” respectively (i.e., of pre-decode signals P 15  to P 8 , only P 8  is inactive), and input value IN 0  is string decoded to generate pre-decode signals P 7  to P 0  having the values “1100 0000”, respectively (i.e., only P 7  and P 6  are inactive). 
       FIG. 4B  shows a result in which input value IN 2  is string decoded in the same fashion as FIG.  4 A. However, in  FIG. 4B , a different input value IN 1  is string decoded to generate pre-decode signals P 15  to P 8  having the values “1111 1000,” respectively (i.e., P 10  to P 8  are inactive), and a different input value IN 0  is string decoded to generate pre-decode signals P 7  to P 0  having the values “1111 0000,” respectively (i.e., P 3  to P 0  are inactive). 
       FIG. 4C  shows a result in which input values IN 2 , IN 1  and IN 0  are the same as those of FIG.  4 B. However, such values are “one-hot” decoded and not string decoded. Consequently, pre-decode signals P 23  to P 16  have the values “0000 0010,” respectively (i.e., only P 17  is active), pre-decode signals P 15  to P 8  have the values “0000 1000,” respectively (i.e., only P 11  is active), and pre-decode signals P 7  to P 0  have the values “0001 0000”, respectively (i.e., only P 4  is active). 
     The operation shown in  FIG. 4A  will now be described. 
     Higher order compare circuit  404 - 0  may receive pre-decode signals P 17  and P 16  and compare such values. Because P 17  and P 16  are not both active, a “greater than” result (e.g., GTA) can be inactive. However, because P 17  is inactive and P 16  (the signal of lesser significance) is active, an “equal to” result (e.g., EQA) can be active. Such higher order comparison results may be provided to lower order compare circuits ( 406 - 1  to  406 - 4 ). Because a higher order “equal to” compare result (EQA) is active, lower order compare circuits ( 406 - 1  to  406 - 4 ) can be enabled, thus providing lower order compare results according to lower order pre-decode signals P 15  to P 8 . 
     Lower order compare circuit  406 - 1  can receive pre-decode signals P 9  and P 8 . Because P 9  and P 8  are not both active, a “greater than” result (GTB) can be inactive. However, an “equal to” result (EQB) can be active. Consequently, activation of decoder outputs DEC 79  to DEC 72  can vary according to lowest order pre-decode signals P 7 -P 0 . In the example of  FIG. 4A , pre-decode signals P 7 -P 0  are 1100 0000, respectively. As a result, decoder outputs DEC 79  to DEC 72  are 1100 0000, respectively. 
     Thus, referring to  FIG. 4A , in response to decoder outputs DEC 79  to DEC 72  of 1100 0000, respectively, entries ENTRY 72  to ENTRY 77  may be disabled, while entries ENTRY 78  and ENTRY 79  can be enabled. 
     Lower order compare circuit  406 - 2  can receive pre-decode signals P 10  and P 9 . Because P 10  and P 9  are both active, a “greater than” result (GTC) can be active. Consequently, decoder outputs DEC 80  to DEC 87  can be forced to predetermined values (high in this example). Thus, referring to  FIG. 4A , in response to decoder outputs DEC 80  to DEC 87  of 1111 1111, respectively, entries ENTRY 80  to ENTRY  87  may all be enabled. 
     Lower order compare circuit  406 - 3  can receive pre-decode signals P 11  and P 10 . Because both such signals are active, lower order compare circuit  406 - 3  can operate in the same general fashion as lower order compare circuit  406 - 2 , resulting in decoder outputs DEC 88  to DEC 95  of 1111 1111, respectively. Thus, entries ENTRY 88  to ENTRY 95  may all be enabled. 
     Similarly, lower order compare circuit  406 - 4  can receive pre-decode signals P 12  and P 11 , which are both active. Thus, decoder outputs DEC 96  to DEC 103  can be 1111 1111, enabling corresponding entries ENTRY 96  to ENTRY 103 . 
     The operation shown in  FIG. 4B  will now be described. 
     Because pre-decode signals P 23 -P 16  are the same as  FIG. 4A , in  FIG. 4B  higher order compare circuit  404 - 0  can operate in the same fashion as  FIG. 4A , enabling lower order compare circuits ( 406 - 1  to  406 - 4 ). 
     Lower order compare circuit  406 - 1  can receive pre-decode signals P 9  and P 8 . Because P 9  and P 8  are both inactive, a “greater than” result (GTB) and an “equal to” result (EQB) can both be inactive. Consequently, activation of decoder outputs DEC 79  to DEC 72  can be disabled. As a result, decoder outputs DEC 79  to DEC 72  are 0000 0000, respectively. This can disable corresponding entries ENTRY 79  to ENTRY 72 . 
     Similarly, lower order compare circuit  406 - 2  can receive pre-decode signals P 10  and P 9 , which are both inactive. Thus, decoder outputs DEC 87  to DEC 80  can be 0000 0000, disabling corresponding entries ENTRY 87  to ENTRY 80 . 
     Lower order compare circuit  406 - 3  can receive pre-decode signals P 11  and P 10 . Because P 11  and P 10  are not both active, a “greater than” result (GTD) can be inactive. However, an “equal to” result (EQD) can be active. Consequently, activation of decoder outputs DEC 95  to DEC 88  can vary according to lowest order pre-decode signals P 7 -P 0 . In the example of  FIG. 4B , pre-decode signals P 7 -P 0  are 1111 0000, respectively. As a result, decoder outputs DEC 88  to DEC 95  are 1100 0000, respectively. Thus, entries ENTRY 88  to ENTRY 91  may be disabled, while entries ENTRY 92  to ENTRY 95  may be enabled. 
     Lower order compare circuit  406 - 4  can receive pre-decode signals P 12  and P 11 . Because P 12  and P 11  are both active, a “greater than” result (GTE) can be active. Consequently, a decoder outputs DEC 103  to DEC 96  can be forced to predetermined values (high in this example). Thus, referring to  FIG. 4B , entries ENTRY 103  to ENTRY  96  may all be enabled. 
     In this way, string decoding may enable only a portion of the entries within a CAM device to thereby conduct a restricted search (e.g., a search beyond, or explore). 
     As noted above, while a decoder circuit according to the present invention may provide string decoding, such a circuit may also provide conventional decoding results. 
     A decoding operation that provides conventional decoding results will now be described with reference to FIG.  4 C. 
       FIG. 4C  is essentially the same as  FIG. 4B , but a mode signal EXPL is “0” instead of “1.” Such a mode signal may result in conventional decoding results. As but one example, conventional decoding may allow data to be read from or written to one CAM entry. 
     In  FIG. 4C , input values IN 0  to IN 2  may be the same as those of FIG.  4 B. However, because mode signal EXPL is low, string decoders  402 - 0  to  402 - 2  may provide “one-hot” decode outputs. 
     As in the previous examples, higher order compare circuit  404 - 0  may receive pre-decode signals P 17  and P 16  and compare such values. Because P 17  and P 16  are not both active, a “greater than” result (e.g., GTA) can be inactive. However, because P 17  is inactive and P 16  (the signal of lesser significance) is active, an “equal to” result (e.g., EQA) can be active. Such higher order comparison results may be provided to lower order compare circuits ( 406 - 1  to  406 - 4 ). Because a higher order “equal to” compare result (EQA) is active, lower order compare circuits ( 406 - 1  to  406 - 4 ) can be enabled, thus providing lower order compare results according to lower order pre-decode signals P 15  to P 8 . 
     Lower order compare circuit  406 - 1  can receive pre-decode signals P 9  and P 8 . Because P 9  and P 8  are both inactive, a “greater than” result (GTB) and an “equal to” result (EQB) can both be inactive. Consequently, activation of decoder outputs DEC 79  to DEC 72  can be disabled. As a result, decoder outputs DEC 79  to DEC 72  are 0000 0000, respectively. This can disable corresponding entries ENTRY 79  to ENTRY 72 . 
     Similarly, lower order compare circuit  406 - 2  can receive pre-decode signals P 10  and P 9 , which are both inactive. Thus, decoder outputs DEC 87  to DEC 80  can be 0000 0000, disabling corresponding entries ENTRY 87  to ENTRY 80 . 
     Lower order compare circuit  406 - 3  can receive pre-decode signals P 11  and P 10 . Because P 11  and P 10  are not both active, a “greater than” result (GTD) can be inactive. However, an “equal to” result (EQD) can be active. Consequently, activation of decoder outputs DEC 95  to DEC 88  can vary according to lowest order pre-decode signals P 7 -P 0 . In the example of  FIG. 4C , pre-decode signals P 7 -P 0  are 0001 0000, respectively. As a result, only decoder output DEC 92  can be activated. Thus, entry ENTRY 92  may be enabled while entries ENTRY 88  to ENTRY 91  and ENTRY  93  to ENTRY 95  can be disabled. 
     Lower order compare circuit  406 - 4  can receive pre-decode signals P 12  and P 11 . Because P 12  and P 11  are both inactive, a “greater than” result (GTE) and “equal to” result (EQE) can both be inactive. Consequently, decoder outputs DEC 103  to DEC 96  can be forced to inactive states. Thus, as shown in  FIG. 4C , entries ENTRY 103  to ENTRY  96  are all disabled. 
     In this way, in response to particular input value combinations, a single entry may be enabled. This can allow for conventional decoding results. 
     It is noted that it may also be desirable in an arrangement like that set forth in  FIGS. 4A  to  4 C, to activate all decoder signals to enable a search of all CAM entries. It follows from the above description, that the application of input values that decode into all active pre-decode signals may give such a result. If string decoding like that of  FIG. 2  is utilized, such input values may all be 0. That is, in an unrestricted search mode (e.g., all valid CAM entries active) a binary address of 000 000 000 can generate pre-decode signals of “1111 1111 1111 1111 1111 1111” thus providing a conventional CAM search function. 
     Having described various embodiments of decoder circuits that can include string decoders, various possible string decoder examples will now be described. It is understood that the below string decoders are but examples of various possible approaches to string decoding that would be understood by those skilled in the art. Accordingly, the below examples should not necessarily be construed as limiting the invention thereto. 
     Referring now to  FIG. 5 , a block schematic diagram shows a string decoder, designated by the general reference character  500 . A string decoder  500  may include a string decoder circuit  502 , a one-hot decoder circuit  504 , and a multiplexer (MUX)  506 . A string decoder  500  may provide string type decoding as has been described above. A one-hot decoder circuit  504  may provide one-hot type decoding as also noted above. A MUX  506  may selectively output a result from a string decode circuit  504  or a one-hot decoder circuit  504  according to a mode signal EXPL. 
     Referring now to  FIG. 6 , a second example of a string decoder is shown in a block schematic diagram and designated by the general reference character  600 . A string decoder may include a mode controlled string decoder circuit  602 , a conventional one-hot decoder circuit  604 , and combining logic  606 . 
     A mode controlled string decoder circuit  602  may provide string decode type outputs when a mode signal EXPL is active, and different type outputs when a mode signal is inactive. Such outputs can be combined within combining logic  606  with outputs from a one-hot decoder circuit  604  to generate desired pre-decode signals that vary according to mode. 
     Referring now to  FIG. 7A , one example of a mode controlled string decoder circuit is set forth in a block diagram and designated by the general reference character  700 . A mode controlled string decoder circuit  700  may receive a number of inputs A 0   —  to A 2 _, and in response thereto provide string outputs SD 7  to SD 0 . 
     The very particular example of  FIG. 7A  includes a decode section  702  and a mode set section  704 . A mode set section  704  may receive inputs A 0 _ to A 2 _ and provide mode controlled values A 0 ′ to A 2 ′ to a decode section  702 . Mode controlled values A 0 ′ to A 2 ′ may follow received inputs A 0 _ to A 2   —  in string decode mode (e.g., mode signal EXPL high). Alternatively, mode controlled inputs A 0 ′ to A 2 ′ may be set to predetermined values in a conventional decode mode (e.g., mode signal EXPL high). 
     In the particular example of  FIG. 7A , a mode set section  704  may include NAND gates G 8  to G 10  that each have one input that receives a mode control signal. Gates G 8 , G 9  and G 10  may receive input values A 0 _, A 1   —  and A 2 _, respectively, as another input. The outputs of gates G 8 , G 9  and G 10  can provide mode controlled values A 0 ′ to A 2 ′. Thus, when mode signal EXPL is low, mode controlled values A 0 ′ to A 2 ′ may all be forced high. However, when mode signal EXPL is high, mode controlled values A 0 ′ to A 2 ′ may be the inverse of received inputs A 0 _ to A 2 _, respectively. 
     Decode section  702  may provide string outputs SD 7  to SD 0 . In the very particular example of  FIG. 7A , a decode section  702  may include an inverter INV 0  that can invert a mode signal EXPL to generate string output SD 7 . A NOR gate G 0  may receive mode controlled values A 0 ′ and A 1 ′ as inputs. A NAND gate G 1  may also receive mode controlled values A 0 ′ and A 1 ′ as inputs. An inverter INV 1  can invert mode controlled value A 1 ′, while an inverter INV 2  can invert mode controlled value A 2 ′. 
     Decode section  702  may also include a number of gate for providing string outputs. In particular, decode section  702  may include a NOR gate G 2  having one input connected to the output of NAND gate G 1  and another input connected to the output of inverter INV 2 . The output of NOR gate G 2  can be string output SD 6 . Two more NOR gates can provide string outputs SD 5  and SD 4 . In particular, a NOR gate G 3  can have one input connected to the output of inverter INV 1  and another input connected to the output of inverter INV 2 . The output of NOR gate G 3  can be string decode output SD 5 . A NOR gate G 4  can have one input connected to the output of NOR gate G 0  and another input connected to the output of inverter INV 2 . The output of NOR gate G 4  can be string decode output SD 4 . 
     In the very particular decode section of  FIG. 7A , a mode controlled value A 2 ′ can be provided as string output SD 3 . Further, three NAND gates G 5 -G 7  can provide string outputs SD 2  to SD 0 . In particular, NAND gates G 5 -G 7  may all have one input connected to the output of inverter INV 2 . Further, NAND gate G 5  may have another input connected to the output of NAND gate G 1  and provide string output SD 2 , NAND gate G 6  may have another input connected to the output of inverter INV 1  and provide string output SD 1 , and NAND gate G 7  may have another input connected to the output of NOR gate G 0  and provide string output SD 0 . 
       FIG. 7B  is a truth table showing the operation of the particular mode controlled string decoder circuit  700  of FIG.  7 A. 
     Referring now to  FIG. 8A , one example of a one-hot decoder circuit is set forth in a schematic diagram and designated by the general reference character  800 . A one-hot decoder circuit  800  may receive a number of inputs A 0 _ to A 2 _, and in response thereto provide one-hot outputs CD 7  to CD 0 . 
     In the very particular example of  FIG. 8A , a one-hot decoder  800  may include inverters INV 3  to INV 5  for generating inverses of the various inputs A 0 _ to A 2 _. Logic gates G 11  to G 18  can logically combine different combinations of inputs A 0 _ to A 2 _ (and their inverses) to provide one-hot outputs CD 7  to CD 0 . 
       FIG. 8B  is a truth table showing the operation of the particular one-hot decoder circuit  800  of FIG.  8 A. 
       FIG. 8A  also includes combining logic  802  that may combine outputs from one-hot decoder circuit  800  with string outputs from a string decoder, like that shown in  FIG. 7A , to generate pre-decode output signals P 0 -P 7 . In the very particular example of  FIG. 8A , combining logic  802  may include a number of NAND gates G 19  to G 26 . NAND gate G 19  may receive a one-hot output CD 7  and a string output SD 7 . In a similar fashion, NAND gates G 20  to G 26  may receive one-hot outputs CD 6  to CD 0 , respectively, and string outputs SD 6  to SD 0 , respectively. NAND gates G 19  to G 26  may provide pre-decode signals P 0  to P 7  as outputs. 
     It follows from  FIGS. 7B and 8B , that resulting pre-decode signals P 0 -P 7  may the same response shown in FIG.  2 . 
     Of course one skilled in the art could arrive at alternate logic implementations for string decoder circuits, one-hot decoder circuits, and/or combining logic. Further, the term logic gate is not intended to exclude logic based on enabling and disabling passgates, or other such logic approaches. 
     Referring now to  FIG. 9 , a semiconductor device according to an embodiment is set forth in plan view and designated by the general reference character  900 . A semiconductor device  900  may be formed in a semiconductor substrate  902  and include a first area  904 , a second area  908 , and a third area  906 . A first area may include CAM entries  910 . A second area may include compare circuits  912 , like first and/or second compare circuits shown as  104  in  FIG. 1 , and/or  304 - 0 ,  306 - 0  and  306 - 1  of  FIG. 3 , and/or  404 - 0  and  406 - 1  to  406 - 4  in  FIGS. 4A-4C . A third area  906  may include string decoders  914 , like those shown as  102 - 0  and  102 - 1  of  FIG. 1 , and/or  302 - 0  to  302 - 2  in  FIG. 3 , and/or  402 - 0  to  402 - 2  in  FIGS. 4A  to  4 C. 
     In an arrangement like that shown in  FIG. 9 , string decoder circuitry may be situated outside of a second area  908  adjacent to CAM entries  910 . Such a second area  906  may be a more critical area in the layout of a semiconductor device, in that reducing such an area may lead to greater freedom in layout. 
     In this way pre-decoder circuitry may be situated more remote from CAM entries than other decoder circuitry. 
     While the embodiments set forth herein have been described in detail, it should be understood that the present invention could be subject to various changes, substitutions, and alterations without departing from the spirit and scope of the invention. Accordingly, the present invention is intended to be limited only as defined by the appended claims.