Content addressable memory (CAM) devices having dedicated mask cell sub-arrays therein and methods of operating same

[00002] This application, which claims priority to U.S. Provisional Application Ser. No. 60/364,694, filed Mar. 15, 2002, is a continuation-in-part (CIP) of U.S. application Ser. No. 10/323,236, filed Dec. 18, 2002, which claims priority to U.S. Provisional Application Ser. No. 60/371,491, filed Apr. 10, 2002. The disclosures of U.S. Provisional Application Ser. Nos. 60/364,694 and 60/371,491 and U.S. application Ser. No. 10/323,236 are hereby incorporated herein by reference.

[00003] The present invention relates to integrated circuit memory devices and, more particularly, to content addressable memory (CAM) devices and methods of operating same.

[00004] In many memory devices, including random access memory (RAM) devices, data is typically accessed by supplying an address to an array of memory cells and then reading data from the memory cells that reside at the supplied address. However, in content addressable memory (CAM) devices, data within a CAM array is not accessed by initially supplying an address, but rather by initially applying data (e.g., search words) to the array and then performing a search operation to identify one or more entries within the CAM array that contain data equivalent to the applied data and thereby represent a “match” condition. In this manner, data is accessed according to its content rather than its address. Upon completion of the search operation, the identified location(s) containing the equivalent data is typically encoded to provide an address (e.g., block address+row address within a block) at which the matching entry is located. If multiple matching entries are identified in response to the search operation, then local priority encoding operations may be performed to identify a location of a best or highest priority matching entry. Such priority encoding operations frequently utilize the relative physical locations of multiple matching entries within the CAM array to identify a highest priority matching entry. An exemplary CAM device that utilizes a priority encoder to identify a highest priority matching entry is disclosed in commonly assigned U.S. Pat. No. 6,370,613 to Diede et al., entitled “Content Addressable Memory with Longest Match Detect,” the disclosure of which is hereby incorporated herein by reference. Additional CAM devices are described in U.S. Pat. Nos. 5,706,224, 5,852,569 and 5,964,857 to Srinivasan et al. and in U.S. Pat. Nos. 6,101,116, 6,256,216, 6,128,207 and 6,262,907 to Lien et al., assigned to the present assignee, the disclosures of which are hereby incorporated herein by reference.

[00005] CAM cells are frequently configured as binary CAM cells that store only data bits (as “1” or “0” logic values) or as ternary CAM cells that store data bits and mask bits. As will be understood by those skilled in the art, when a mask bit within a ternary CAM cell is inactive (e.g., set to a logic 1 value), the ternary CAM cell may operate as a conventional binary CAM cell storing an “unmasked” data bit. When the mask bit is active (e.g., set to a logic 0 value), the ternary CAM cell is treated as storing a “don't care” (X) value, which means that all compare operations performed on the actively masked ternary CAM cell will result in a cell match condition. Thus, if a logic 0 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 1 data bit, the compare operation will indicate a cell match condition. A cell match condition will also be indicated if a logic 1 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 0 data bit. Accordingly, if a data word of length N, where N is an integer, is applied to a ternary CAM array having a plurality of entries therein of logical width N, then a compare operation will yield one or more match conditions whenever all the unmasked data bits of an entry in the ternary CAM array are identical to the corresponding data bits of the applied search word. This means that if the applied search word equals {1011}, the following entries will result in a match condition in a CAM comprising ternary CAM cells: {1011}, {X011}, {1X11}, {10X1}, {101X}, {XX11}, {1XX1}, . . . , {1XXX}, {XXXX}.

[00006] As illustrated by FIG. 1A, a conventional ternary CAM cell 10 may include a data SRAM cell 12, a mask SRAM cell 16, a compare circuit 14 and an NMOS transistor 18 that is electrically connected in series between the compare circuit 14 and a respective match line. As will be understood by those skilled in the art, data can be loaded into the data SRAM cell 12 by driving a pair of differential bit lines BIT and BITB with a rail-to-rail data signal (i.e., BIT=1, BITB=O or BIT=O, BITB=1) and driving a respective main word line at an active high level. These driving operations will cause the outputs D and DB of the data SRAM cell 12 to remain at or switch to levels consistent with the value of the rail-to-rail data signal established on the respective pair of bit lines. An active mask value can be loaded into the mask SRAM cell 16 by driving the pair of differential bit lines BIT and BITB with a rail-to-rail mask signal that reflects a logic 0 value (i.e., BIT=0, BITB=1) and driving a respective mask word line at an active high level. These driving operations will cause the true and complementary outputs M and MB of the mask SRAM cell 16 to remain at or switch to logic 0 and logic 1 levels, respectively. As will be understood by those skilled in the art, setting M=1 will operate to turn on NMOS transistor 18 and cause the CAM cell 10 to operate as a conventional binary CAM cell. However, setting M=0 will operate to turn off NMOS transistor 18 and thereby preclude the compare circuit 14 from pulling down the match line to indicate a “miss” (i.e., inequality) between the bit of data stored within the data SRAM cell 12 and a bit of a comparand residing on the corresponding pair of differential data lines DATA and DATAB. In this latter case, the CAM cell 10 is treated as being actively masked and storing an “X” state, which represents a “don't care” condition. In some applications, the pair of differential bit lines and the pair of differential data lines may be electrically connected together as a single pair of bit/data lines.

[00007] A pair of ternary CAM cells 10′ may also share a respective mask SRAM cell 16′, as illustrated by FIG. 1B. Accordingly, setting the true output M of the mask SRAM cell 16′ to an active logic 0 value will operate to locally mask both upper and lower data SRAM cells 12 within the illustrated pair of CAM cells 10′ during each consecutive search operation. Groups of four (4) or (8) CAM cells within a respective column may also be locally masked by a shared mask cell that is located within the same column. Accordingly, whenever the shared mask cell is active during a search operation, local masking of all CAM cells within the corresponding group will be masked.

[00008] The aforementioned commonly assigned '613 patent to Diede et al. also discloses a segmented CAM array that is configured to perform pipelined search operations. Moreover, FIG. 7 of the '613 patent illustrates an external mask register that is used in combination with splitter logic to globally mask bits of an applied search word before multiple segments of the search word are supplied to pipeline registers associated with each segment of the CAM array.

[00009] Content addressable memory (CAM) devices according to embodiments of the present invention achieve high integration by utilizing a plurality of CAM arrays that each contain a CAM cell sub-array and a mask cell sub-array therein. According to a preferred aspect of these CAM devices, each row of mask cells within the mask cell sub-array can be selectively read onto global mask signal lines so that one or more columns of a corresponding binary CAM cell sub-array can be globally masked during a search operation. In particular, mask assertion circuitry, which extends within the mask cell sub-array, is provided to drive the global mask signal lines with global mask data. This global mask data is provided as input data to a bit/data line control circuit that drives bit and/or data lines of the CAM cell sub-array during read, write and search operations. In some embodiments, the CAM cell sub-array may comprise exclusively binary CAM cells, however, in other embodiments the CAM cells are ternary or quaternary CAM cells that have local mask cells therein.

[00010] To facilitate pipelined search operations, among other things, preferred embodiments include a CAM array having a segmented sub-array of dedicated mask cells and a segmented sub-array of CAM cells therein. For example, the segmented sub-array of dedicated mask cells may include a first segment of dedicated mask cells that spans xR columns of mask cells and a second segment of dedicated mask cells that spans xS columns of dedicated mask cells. Likewise, the sub-array of CAM cells may include a first segment of CAM cells that spans xR columns of CAM cells and a second segment of CAM cells that spans xS columns of CAM cells. The segmented sub-array of dedicated mask cells may also include a first plurality of latches that are configured to latch mask word line signals and read mask line signals generated by the first segment of dedicated mask cells. In a corresponding fashion, the segmented sub-array of CAM cells may include a second plurality of latches that are configured to latch match line signals and word line signals generated by the first segment of CAM cells.

[00011] In some other embodiments, a CAM array is provided having a segmented sub-array of dedicated mask cells and a segmented sub-array of binary, ternary and/or quaternary CAM cells therein. A mask assertion circuit is also provided. This mask assertion circuit is configured to read global mask data from the dedicated mask cells during a read mask operation. This read mask operation may be performed as a background operation that is performed in preparation for a subsequent search operation that uses the previously read mask.

[00012] In still other embodiments, a CAM device is provided with a CAM array having a segmented sub-array of dedicated mask cells and a segmented sub-array of binary, ternary and/or quaternary CAM cells therein. A bit/data line driver circuit is also provided. This driver circuit is electrically coupled to the CAM array by a plurality of pairs of bit lines and a plurality of pairs of data lines. The CAM array further includes means, responsive to at least one read mask signal, for reading global mask data from the dedicated mask cells during a first search operation. The driver circuit may also include means, responsive to a clock signal, for latching the global mask data during a second search operation that follows the first search operation.

[00013] Still further embodiments of the present invention include method of operating an integrated circuit device. These methods include comparing a xS segment of a first search word and a xR segment of a second search word with entries in a CAM array, while concurrently reading xS and xR segments of first and second mask words, respectively, stored within a mask cell sub-array. Following these operations, a xS segment of the second search word and a xR segment of a third search word are compared with entries in the CAM array. This latter comparison operation uses the xS and xR segments of the first and second mask words to globally mask one or more bit positions in the xS and xR segments of the second and third search words, respectively.

[00014] In other embodiments of the invention, a CAM device is provided that includes a plurality of CAM arrays therein. Each of these CAM arrays may have a first row of mask cells and a mask assertion circuit therein. The mask assertion circuit is electrically coupled to the first row of mask cells and receives a first mask retained by the first row of mask cells when the first mask is asserted in response to a respective read mask signal. The first row of mask cells may contain exclusively hard mask cells or exclusively soft mask cells (e.g., RAM mask cells) or combinations of hard and soft mask cells. The hard mask cells may be layout-fixed or one-time programmable cells. A bit/data line driver circuit is also provided. The bit/data line driver circuit has inputs that are electrically coupled to outputs of the mask assertion circuit. According to a preferred aspect of this embodiment, the bit/data line driver circuit drives a plurality of pairs of differential bit/data lines in the CAM array with a comparand containing one or more bits of an applied search word and one or more mask bits at locations specified by the first row of mask cells. Thus, if the applied search word is 1111 and the first row of mask cells is configured to retain a mask that equals (or is equivalent to) 1100, where “0” designates an actively mask bit position, then the comparand will be represented as 11XX during the search operation, where “X” designates a globally masked bit position that may be achieved by driving a corresponding pair of differential bit/data lines at logic 0 levels (i.e., BIT=BITB=0).

[00015] A preferred CAM array may also include a plurality of rows of hard and soft mask cells that define a respective mask cell sub-array. The mask assertion circuit may be configured so that each mask retained by a mask cell row may be asserted alone or in combination with a mask retained by another one of the plurality of rows of mask cells. This aspect of the mask assertion circuit may be achieved by including wired NOR circuitry having precharged high output nodes that are pulled low when one or more active mask bits in a corresponding column of mask cells are asserted.

[00035] The present invention now will be described more fully herein with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout and signal lines and signals thereon may be referred to by the same reference characters. Signals may also be synchronized and/or undergo minor boolean operations (e.g., inversion) without being considered different signals. The suffix B (or prefix symbol “/”) to a signal name may also denote a complementary data or information signal or an active low control signal, for example.

[00036] Referring now to FIG. 2, a CAM device 20 according to an embodiment of the present invention will be described. The CAM device 20 includes an input data buffer 22, a bit/data line control circuit 24, a CAM array 26 and a data output buffer 28, connected as illustrated. In the illustrated embodiment, the data output buffer 28 is positioned below the other illustrated elements, however, in alternative embodiments, the data output buffer 28 may be positioned above the illustrated embodiments. A row control circuit (not shown) that generates word line, match line and pseudo-ground lines signals may also be provided adjacent one side of the CAM array 26. Pseudo-ground line signals may also be referred to as “low” match line (LM) signals, as explained more fully in U.S. Pat. No. 6,262,907 to Lien et al., entitled “Ternary CAM Cell,” the disclosure of which is hereby incorporated herein by reference. In an alternative embodiment, the CAM array may comprise CAM cells that are not responsive to pseudo-ground line signals.

[00037] A bank of sense amplifiers (not shown) that receives match line signals and pseudo-ground line signals may be provided adjacent another side of the CAM array 26. A row priority encoder (not shown) may be electrically coupled to outputs of the bank of sense amplifiers. In alternative embodiments, inputs to the row priority encoder may be directly connected to the match lines within the CAM array 26 and the bank of sense amplifiers may be omitted. As will be understood by those skilled in the art, the row priority encoder may be configured to generate an address of a highest priority matching entry within the CAM array 26, in response to a search operation. Operations performed by a row control circuit and priority encoder are more fully described in U.S. application Ser. No. 10/084,842, to Lien et al., filed Feb. 27, 2002, and Ser. No. 10/306,799, to Lien et al., filed Nov. 27, 2002, the disclosures of which are hereby incorporated herein by reference.

[00038] The input data buffer 22 is preferably configured to buffer entries to be written into the CAM array 26 during write cycles and buffer search words to be applied (at least in part) to the CAM array 26 during respective search or “look-up” cycles. As described more fully in the U.S. application Ser. No. 10/323,236, filed Dec. 18, 2002, the disclosure of which is hereby incorporated herein by reference, each of the write and search cycles may comprise a plurality of pipelined operations. These buffered entries and search words are provided as inputs to the bit/data line control circuit 24. Column driver circuitry that may be associated with each column of the bit/data line control circuit 24 is described more fully hereinbelow with respect to FIG. 4.

[00039] In the illustrated embodiment, the bit/data line control circuit 24 is electrically connected to the CAM array 26 by pairs of differential bit lines (shown as 72 pairs: (BIT0, BITB0)-(BIT71, BITB71)). These pairs of differential bit lines receive write data in the form of rail-to-rail signals during write operations and receive comparand data during search operations. The comparand data may be established using rail-to-rail signals and possibly one or more global mask bits. A global mask bit may be established by setting both true and complementary bit lines in a respective pair low (i.e., BIT=BITB=0). The pairs of differential bit lines may also be driven with logic 1 values upon commencement of a read operation (i.e., BIT=BITB=1).

[00040] A plurality of global mask signal lines MASKB, which are described herein as being active when low, are provided to communicate mask information from the CAM array 26 to the bit/data line control circuit 24. As illustrated, each of the 72 columns of the CAM array 26 generates a respective global mask signal (e.g., MASKB0-MASKB71). As described more fully hereinbelow with respect to FIG. 3A, these global mask signals may be generated by mask assertion circuitry that is located within a mask cell sub-array 26a. As illustrated by FIG. 2, the mask cell sub-array 26a may include multiple rows of hard mask cells, shown as two (2) rows of hard mask cells, and multiple rows of soft mask cells, shown as eight (8) rows of soft mask cells.

[00041] A CAM cell sub-array 26b is also provided within the CAM array 26. The CAM cell sub-array 26b is illustrated as having a capacity of 512×72 words (i.e., the CAM array 26 has a depth=512 and width=72). Additional columns (not shown) may also be provided for retaining parity bit information and/or check bit information that may be useful to perform soft error detection and correction when the entries in each row are read out and checked for soft errors. In preferred embodiments, each mask cell sub-array 26a shares signal lines (e.g., bit lines) with a respective CAM cell sub-array 26b. Nonetheless, in some alternative embodiments, the mask cell sub-array 26a may be positioned so that signal lines are not shared. For example, the mask cell sub-array 26a may be positioned between the input data buffer 22 and the bit/data line control circuit 24 or otherwise associated with each CAM array 26.

[00042] In the illustrated embodiment, the CAM cell sub-array 26b is a highly integrated array having relatively small CAM cells therein (e.g., binary CAM cells). However, in some applications, CAM cell sub-arrays 26b having a mixture of binary CAM cells and larger CAM cells (e.g., ternary CAM cells having local mask cells therein) or exclusively larger CAM cells may be used. The read data buffer 28 is electrically coupled by the plurality of pairs of differential bit lines to the CAM cell sub-array 26b. During a read operation, the bit/data line control circuit 24 drives the true and complementary bit lines within each pair to logic 1 values. If the read operation is dynamic, the outputs of the bit/data line control circuit 24 are then disposed in high impedance states. Alternatively, if the read operation is static, the outputs of the bit/data line control circuit 24 need not be disposed in high impedance states. A row of soft mask cells within the mask cell sub-array 26a or a row within the CAM cell sub-array 26b is then accessed during a read operation by driving a respective mask word line or data word line high. In response to this access, read data signals are established on the bit lines and received by sense amplifiers within the read data buffer 28 using conventional operations.

[00043] FIG. 3A illustrates an exemplary SRAM-based CAM array 26 having hard and soft mask cells in a mask cell sub-array 26a and exclusively binary CAM cells in a CAM cell sub-array 26b. Alternative technologies, including DRAM and nonvolatile storage technologies, may also be used within the CAM array 26. The CAM cell sub-array 26b is illustrated as comprising 6 CAM cells that span a block of two rows and three columns. The mask cell sub-array 26a is illustrated as comprising 12 soft mask cells (shown as SRAM cells) that span a block of four rows and three columns. The mask cell sub-array 26a may also include a plurality of hard mask cells. As illustrated on the left side of FIG. 3B, these hard mask cells 26d may be represented by a single transistor. However, in a more preferred embodiment, the single transistor hard mask cell 26d and a portion of a respective mask assertion circuit 26c are replaced by an electrical conductor 26e. The electrical conductor 26e may be represented by a direct wiring connection, as illustrated, or by a fuse in the event a hard mask is one-time programmable. In the first row of soft mask cells, three programmable SRAM cells are provided. The gate electrodes of the access transistors associated with these three SRAM cells are electrically connected to a respective mask line, which operates as a conventional word line when writing to and reading from a row of soft mask cells. This mask line may also be referred to as a mask word line.

[00044] Referring again to FIG. 3A, each column of the mask cell sub-array 26a preferably comprises a mask assertion circuit 26c. The mask assertion circuit 26c includes the global mask signal lines MASKB, which are illustrated as extending in a vertical direction in parallel with the bit lines, and PMOS pull-up transistors. The global mask signal lines MASKB may be precharged high by the PMOS pull-up transistors. These pull-up transistors are responsive to a precharge control signal, shown as PRCHGB. The precharge control signal PRCHGB may be generated as an active low pulse during search operations. The mask assertion circuit 26c also includes a pair of NMOS transistors associated with each soft mask cell. As illustrated, the NMOS transistors in each pair are electrically connected in series between a respective global mask signal line MASKB and a ground reference potential. Accordingly, each pair of NMOS transistors is connected in a wired-NOR manner to a respective global mask signal line MASKB. A gate electrode of one NMOS transistor in a pair is electrically coupled to a complementary output of respective soft mask cell and a gate electrode of another NMOS transistor in the pair is electrically coupled to a respective read soft mask signal line. The NMOS transistors within each pair will become conductive if the following three conditions are met: (i) the respective global mask signal line MASKB is precharged high, (ii) the respective soft mask cell is storing an active mask value (M=0, MB=1) and (iii) the respective read soft mask signal line is active at a high level.

[00045] Based on the illustrated configuration of the soft mask cells and the mask assertion circuitry 26c within the mask cell sub-array 26a, operations to read a first soft mask associated with the first row of soft mask cells onto the global mask signal lines MASKB commence by driving the first read soft mask signal line (Read Soft Mask #1) high. If the soft mask retained by the first row of soft mask cells is {010}, the leftmost and rightmost of the three global mask signal lines MASKB illustrated by FIG. 3A will be pulled low (from precharged high levels). These low states will indicate that the leftmost and rightmost columns of the CAM cell sub-array 26b are to be masked during a subsequent (e.g., next following) search operation. A leading edge of a clock signal CLK can then be generated to latch the low or high logic levels on the global mask signal lines MASKB into a N-bit wide shift register (comprising flip-flops (FF)) (see, e.g., FIG. 4). This shift register may comprise D-type master-slave flip flops that provide valid output data on a trailing edge of the clock signal.

[00046] As another example, operations to simultaneously read a first soft mask associated with the first row of soft mask cells and a second soft mask associated with the second row of soft mask cells onto the global mask signal lines MASKB commence by driving the first and second read soft mask signal lines (Read Soft Mask #1, Read Soft Mask #2) high. If the soft mask retained by the first row of soft mask cells is {110} and the soft mask retained by the second row of soft mask cells is {100}, the center and rightmost of the three global mask signal lines MASKB illustrated by FIG. 3A will be pulled low (from precharged high levels) to thereby indicate that the center and rightmost columns of the CAM cell sub-array 26b are to be masked during a subsequent search operation.

[00047] As a final example, an operation to load a first hard mask associated with a first row of hard mask cells onto the global mask signal lines MASKB commences by driving the first read hard mask signal line (Read Hard Mask #1) high. In the illustrated embodiment, the operation to drive the first read hard mask signal line high will cause the center of the three global mask signal lines MASKB illustrated by FIG. 3A to be pulled low (from a precharged high level) to thereby indicate that the center column of the CAM cell sub-array 26b is to be masked during a subsequent search operation.

[00048] FIG. 4 is an electrical schematic of a portion of a bit/data line control circuit 24 that is responsive to a respective output of the mask assertion circuit 26c illustrated by FIG. 3A. In particular, FIG. 4 illustrates a column driver 24a that is electrically coupled to a respective pair of differential bit lines, shown as BIT and BITB, that span the CAM array 26. The column driver 24a is also configured to receive mask information from the mask cell sub-array 26a. As illustrated by the flip-flop (FF) at the bottom of FIG. 4, the mask information is loaded into the column driver 24a in-sync with a clock signal CLK. This column driver may also be referred to as a bit/data line driver circuit.

[00049] Based on the illustrated configuration of the column driver 24a, a read operation may include an operation to pull-up each pair of differential bit lines to logic 1 levels, by setting both the write control signal line W (/R) and the search control signal line SEARCH low. This read operation is performed while the output of the mask assertion circuit 26c, shown as MASKB, is maintained high. Conventional read operations may then include driving a word line associated with a desired row high and activating the read data buffer 28 to accept the read data established on the bit lines. Similarly, a write operation includes setting the write control signal line W high and the search control signal line SEARCH low (while the output of the mask assertion circuit 26c is maintained high). Under these conditions, the true and complementary bit lines BIT and BITB will be driven at levels that are consistent with the value of the input data DATA.

[00050] The performance of a search operation includes setting the write control signal line W low and the search control signal line SEARCH high. Under these conditions, the true and complementary bit lines BIT and BITB will be driven at levels that are consistent with the value of the input data DATA if the corresponding global mask signal line MASKB_OUT is high. However, if the global mask signal line MASKB_OUT is low, the true and complementary bit lines BIT and BITB will both be pulled low to indicate a global mask condition during the search operation. This global mask condition is typically read from the mask cell sub-array 26a during an immediately preceding search cycle.

[00051] Referring now to FIG. 5A, a partial column 50a of a CAM array according to another embodiment of the present invention includes a mask cell 52 that operates to “locally” mask a plurality of CAM cells within a respective column. The CAM cells each include a data cell 54 and a compare circuit 56. The partial column 50a of FIG. 5A is similar to the partial column 50b of FIG. 5B, however, the compare circuit 56 within each CAM cell in FIG. 5B is connected to a separate pair of data lines DATA and DATAB that receive a respective bit of a comparand during a search operation. The use of a separate pair of data lines DATA and DATAB can advantageously reduce parasitic capacitance on the bit lines BIT and BITB and also enable a row of CAM cells within the CAM array to be read while a search operation is being performed.

[00052] The mask cell 52 and the CAM cells within the partial column 50a are illustrated as SRAM cells. A mask assertion circuit 58 is provided that includes a global mask signal line MASKB and a pair of NMOS transistors connected in series, as illustrated. A PMOS transistor, which is responsive to an active low precharge signal (PRCHGB), is also provided. The PMOS transistor operates to pull-up the global mask signal line MASKB prior to each search cycle. The precharge signal is an active low pulse in the illustrated embodiment. The value of the mask stored within the mask cell 52 can be read onto the global mask signal line MASKB by driving a respective read mask signal line (READ MASK) high.

[00053] In particular, if the mask cell 52 is storing an active mask value (i.e., M=0) when the read mask signal line is driven high, then both series connected NMOS transistors within the mask assertion circuit 58 will become conductive to pull-down the global mask signal line MASKB. When the global mask signal line MASKB is pulled low to represent that an active mask condition has been read from the mask cell 52, the four illustrated CAM cells become locally masked by the single mask cell 52. This local masking is achieved by blocking each compare circuit within the four illustrated CAM cells from pulling a respective match line (MATCH LINE) low from a precharged high condition. In contrast, if the global mask signal line MASKB remains high during a search operation, either because (i) the mask cell 52 is storing an active mask value that has not been read by driving the read mask signal line high or (ii) the mask cell 52 has been read but is nonetheless storing an inactive mask value (i.e., M=1), the four illustrated CAM cells operate as conventional binary CAM cells when a search operation is performed. In alternative embodiments, the CAM cells may comprise ternary CAM cells.

[00054] FIG. 6A is a block diagram of a CAM array 60a according to another embodiment of the present invention. As illustrated, the CAM array 60a includes a binary CAM cell sub-array and a mask cell sub-array. The logical width of the CAM array 60a is shown as 32 bits. The mask cell sub-array includes a plurality of rows of hard mask cells (shown as 2) and a plurality of rows of soft mask cells (shown as 4). Each row of hard mask cells includes sixteen (16) actively masked bit positions that are segmented into three contiguous groups. These three contiguous groups may include xR, xS and xT segments of bits, where R, S and T are positive integers. For example, if a CAM array has a logical width of 72 bits, the groups may be partitioned as x3, x6 and x27 segments, where R=3, S=6 and T=27. In the illustrated embodiment of FIG. 6A, R=2, S=4 and T=10.

[00055] The use of 16 actively mask bit positions in the embodiment of FIG. 6A enables the performance of “short” x16 word searches (i.e., where the full logical width of an applied search word is less than a logical width of the CAM array). A first group of each row of hard mask cells contains two actively mask bit positions (shown as M) and a second group of each row contains four actively mask bit positions. The third group of each row contains ten actively masked bit positions. Each row of soft mask cells includes a different number of actively masked bit positions that extend adjacent a right side of the CAM array 60a. The soft masks retained by the rows of soft mask cells may be used during search operations to identify an address of a matching entry having a longest prefix match (LPM). Applications that require a CAM device to identify entries having longest prefix matches are more fully described in U.S. application Ser. No. 09/962,737 to Lindahl et al., entitled “Content Addressable Memory (CAM) Devices That Can Identify Highest Priority Matches in Non-Sectored CAM Arrays and Methods of Operating Same,” filed Sep. 25, 2001, the disclosure of which is hereby incorporated herein by reference.

[00056] According to a preferred aspect of this embodiment, a first x16 search operation may be performed on the CAM array 60a by masking columns 0-1, 4-7 and 12-21(or even columns) using a first hard mask that is applied during one search cycle within a pipelined search operation. Alternatively, a second x16 search operation may be performed by masking columns 2-3, 8-11 and 22-31 (or odd columns) using a second hard mask that is applied during another search cycle. These x16 search operations may be performed in a staged manner to conserve power. For example, the search cycle may be performed by applying the first hard mask and initially partially searching columns 2-3 of the CAM array during a first stage. If at least one x2 partial match is detected between the first two bits of the x16 applied search word and the data retained in columns 2-3 of the CAM array 60a during the first stage, then the search operation may continue to a second stage. Otherwise, the search operation terminates. During the second stage, the next four bits of the x16 search word are compared to the data contained within columns 8-11. If at least one x4 partial match is detected during the second stage, then the search operation continues to the final third stage. Otherwise, the search operation terminates after the second stage. During the final third stage, the remaining x10 segment of the x16 search word is compared to the data within columns 22-31 of the CAM array 60a. The performance of pipelined search operations in CAM arrays and a discussion of the bit line and match line power savings that may be achieved thereby are more fully described in U.S. Provisional Application Serial No. 60/371,491, filed Apr. 10, 2002, entitled “Content Addressable Memory (CAM) Devices That Perform Power Saving Lookup Operations and Methods of Operating Same,” and in the aforementioned U.S. application Ser. No. 10/323,236, filed Dec. 18, 2002, the disclosures of which are hereby incorporated herein by reference.

[00057] Referring now to FIG. 6B, a CAM array 60b according to another embodiment of the present invention will be described. Like the CAM array 60a of FIG. 6A, the CAM array 60b of FIG. 6B includes a mask cell sub-array therein. The mask cell sub-array is illustrated as comprising two rows of hard mask cells. These rows of hard mask cells are segmented into contiguous groups so that power saving segmented “short” word search operations may be performed in CAM cell sub-arrays having relatively small binary CAM cells therein and/or relatively large CAM cells (e.g., ternary CAM cells) therein that each retain local mask information.

[00058] Referring now to FIG. 7, a CAM array 70a according to another embodiment of the present invention is illustrated. In particular, the CAM array 70a is illustrated as having a logical width of 40 bits, with each row of the CAM cell sub-array segmented into x10 partial rows that are separated by respective columns of match line latches. A x40 search operation may include performing a pipelined series of x10 partial word search operations on four segments of a x40 search word. For example, during a first search operation, a first x10 segment of a first search word (word D) may be applied to the bit lines associated with columns 0-9 of the CAM array 70a. At the completion of this first search operation, the states of the match lines associated with the first x10 segment of the CAM cell sub-array may be latched and provided as inputs to the second x10 segment of the CAM cell sub-array. Then, during a second search operation, a first x10 segment of a second search word (word C) and a second x10 segment of the first search word (word D) are applied to the bit lines associated with columns 0-9 and 10-19 of the CAM array 70a, respectively. At the completion of this second search operation, the states of the match lines associated with the first x10 segment and the second x10 segment may be latched and provided to the next higher segment by the corresponding match lines latches.

[00059] Next, during a third search operation, a first x10 segment of a third search word (word B), a second x10 segment of the second search word (word C) and a third x10 segment of the first search word (word D) are applied to the bit lines associated with columns 0-9, 10-19 and 20-29 of the CAM array 70a, respectively. At the completion of this third search operation, the states of the match lines associated with the first, second and third x10 segments are latched to prepare for the next following search operation. Finally, during a fourth search operation, a first x10 segment of a fourth search word (word A), a second x10 segment of the third search word (word B), a third x10 segment of the second search word (word C) and a fourth x10 segment of the first search word (word D) are applied to the bit lines associated with columns 0-9, 10-19, 20-29 and 30-39 of the CAM array 70a, respectively. At the completion of this fourth search operation, the states of the match lines associated with the first, second and third x10 segments are latched to prepare for the next following search operation and the states of the match lines associated with the fourth x10 segment may be passed in a conventional manner to a match line sense amplifier or directly to a row priority encoder, for example. Thus, in the illustrated embodiment, four pipelined search operations are required to completely compare a x40 search word to entries in the CAM array 70a. However, once the pipeline is full, each search operation will produce final match line signals that identify whether at least one match is present between a x40 search word and entries in the CAM array 70a. Similarly, if one of the x20 hard masks is used, each search operation will produce final match line signals that correspond to a respective x20 search word.

[00060] In a preferred power saving embodiment that is more fully described in the aforementioned U.S. application Ser. No. 10/323,236, the second x10 segment (or higher segments) of a x40 search word will not be applied to the bit lines associated with columns 10-19 unless at least one partial match has been identified at the outputs of the match lines associated with the first x10 segment of the CAM array 70a. Operations to precharge match and/or pseudo-ground lines that span columns 10-19 may also be disabled unless at least one match has been detected in the first x10 segment during the preceding search operation. Similarly, the third x10 segment of a x40 search word (or higher segments) will not be applied to the bit lines associated with columns 20-29 unless at least one partial match has been identified at the outputs of the match lines associated with the second x10 segment of the CAM array 70a. Likewise, the fourth x10 segment of a x40 search word will not be applied to the bit lines associated with columns 30-39 unless at least one partial match has been identified at the outputs of the match lines associated with the third x10 segment of the CAM array 70a.

[00061] Referring now to FIG. 8A, a CAM device 20′ according to another embodiment of the present invention will be described. This CAM device 20′ includes an input data buffer 22′ that receives data to be written as entries into rows within a CAM array 26′ during write operations. The input data buffer 22′ also receives search words that are applied as comparands during search operations. This data buffer 22′ may include circuitry (e.g., splitter logic and shift registers) that facilitate pipelined search and pipelined write operations within the CAM array 26′. The input data buffer 22′ is configured to pass data to a bit/data line control circuit 24′. This data may take the form of XY input data ((DATAX0, DATAY0)-(DATAX79, DATAY79)) that is compatible with ternary (or quaternay) CAM cells. As described more fully hereinbelow with respect to FIG. 10B, these CAM cells may include a pair of laterally spaced memory cells (e.g., SRAM cells) that collectively store two bits of data. In the case of ternary CAM cells, the two bits of data may have three valid combinations and one invalid combination. In the case of quaternary CAM cells, the two bits of data may have four valid combinations. Because two of these four valid combinations represent similar active mask conditions, quaternary CAM cells are frequently treated as another embodiment of ternary CAM cells.

[00062] In preferred embodiments, the CAM array 26′ is configured to include a mask cell sub-array 26a′ and a CAM cell sub-array 26b′ therein. The CAM cell sub-array 26b′ is illustrated as an array of ternary CAM (TCAM) cells having a logical width and depth of 80 columns and 2048 rows, respectively. Although not shown, additional redundant rows and columns of the CAM cell sub-array 26b′ may also be provided as replacements for defective rows and columns. The mask cell sub-array 26a′ is illustrated as having a capacity of eight (8) soft masks and two hard masks. As described more fully hereinbelow, the eight soft masks may be retained within four (4) rows of dedicated XY mask cells, with each row retaining an even mask word and an odd mask word. The two hard masks may be retained within a single row of XY hard mask cells (not shown), with each XY hard mask cell including two of the hard mask elements shown in FIG. 3B.

[00063] The bit/data line control circuit 24′ is electrically coupled to the CAM array 26′ by a plurality of pairs of differential bit lines (shown as (BX0, BXB0, BY0, BYB0)-(BX79, BXB79, BY79, BYB79)), a plurality of pairs of differential data lines (shown as (D0,DB0)-(D79,DB79)) and a plurality of global mask signal lines (shown as MASKB0-MASKB79), which are illustrated as active low signal lines. In the illustrated embodiment, the plurality of pairs of differential bit lines are electrically connected to the mask cells within the mask cell sub-array 26a′ and the CAM cells within the CAM cell sub-array 26b′. The plurality of pairs of differential data lines are electrically connected to the CAM cells within the CAM cell sub-array 26b′, but are not electrically connected to the mask cells because the mask cells do not include compare circuitry. As explained more fully hereinbelow, the global mask signal lines (shown as MASKB0-MASKB79) are configured to pass global mask words from the mask cell sub-array 26a′ to a bit/data line driver circuit within the bit/data line control circuit 24′. Exemplary bit/data line driver circuits are illustrated by FIGS. 9 and 11A-11B. In FIGS. 8A and 11A-11B, the control signal ALLMASKB is an active low signal that causes all pairs of the differential data lines (within a segment) to be pulled low during a search operation. The control signal ALLMASKB may be a signal that is pipelined and therefore latched by the segment latches illustrated in FIG. 8B. The control signal ENABLE is an active high signal that is used to enable the driver circuits. The signals CLK and CLKB represent clock signals that may be 180 degrees out of phase relative to each other in the event they have a 50% duty cycle. Alternatively, the signals CLK and CLKB may be represented by relatively short duration pulses having a less than 50% duty cycle. The control signal PRCHGB is an active low signal that is used to precharge the global mask signal lines (MASKB0-MASKB79).

[00064] A read data buffer 28′ is provided to support read operations. The read data buffer 28′ is electrically connected to the CAM cell sub-array 26b′ by the illustrated pairs of differential bit lines. The two pairs of bit lines associated with each column of ternary CAM cells within the CAM cell sub-array 26b′ (e.g., ((BXN, BXBN), (BYN, BYBN), where n is an integer) generate one of three possible data values during a read operation. These values are illustrated by TABLE 1:

[00002]

TABLE 1
READ OPERATION (TERNARY)
BXN	BYN	VALUE
0	0	MASKED
0	1	0
1	0	1

[00065] In the event the CAM cells within the CAM cell sub-array 26b′ are quaternary CAM cells, then the two pairs of bit lines associated with each column of the CAM cell sub-array 26b′ generate one of four possible data values during a read operation. These values are illustrated by TABLE 2:

[00002]

TABLE 2
READ OPERATION (QUATERNARY)
	0	0	0/Masked
	0	1	0/Unmasked
	1	0	1/Masked
	1	1	1/Unmasked

[00066] Referring now to FIG. 10B, a preferred ternary CAM cell 102, which may be used in the CAM cell sub-array 26b′ of FIG. 8A, includes a pair of SRAM memory cells that share a common word line (shown as WL0). These two SRAM memory cells are illustrated as an XSIDE memory cell, which is electrically coupled to bit lines BX0 and BXB0, and a YSIDE memory cell, which is electrically coupled to bit lines BY0 and BYB0. The true output (X) of the XSIDE memory cell and the true output (Y) of the YSIDE memory cell are electrically connected to inputs of a four transistor (4T) compare circuit that extends between a match line (ML0) and a pseudo-ground line (PGND01) The 4T compare circuit also has two inputs that are electrically connected to a pair of differential data lines (shown as D0 and DB0). These aspects of the illustrated ternary CAM cell 102 are more fully described in the aforementioned '799 application. As illustrated by TABLE 3, the 4T compare circuit is configured to compare a bit of a comparand against the value of the data stored within the pair of memory cells during a search operation and reflect the result of that comparison on the match line. This comparison operation is performed in-sync with a high-to-low transition of the pseudo-ground line (PGND01). To conserve power, this high-to-low transition of the pseudo-ground line may be a transition from a maximum precharged voltage of (Vdd−Vth) to a minimum voltage of Vss, where Vdd is a power supply voltage to the CAM cell, Vss is a ground reference potential and Vth is a threshold voltage of an NMOS transistor.

[00002]

TABLE 3
SEARCH OPERATION
XSIDE	YSIDE	D0/DB0	RESULT	MASK
0	0	X	MATCH	(ML0 = 1)	ACTIVE
0	1	0/1	MATCH	(ML0 = 1)	INACTIVE
0	1	1/0	MISS	(ML0 = 0)	INACTIVE
1	0	1/0	MATCH	(ML0 = 1)	INACTIVE
1	0	0/1	MISS	(ML0 = 0)	INACTIVE

[00067] Referring now to FIG. 10C, an XY mask cell 104 according to a preferred aspect of the present invention will be described. The XY mask cell 104 of FIG. 1 ° C. includes two SRAM memory cells, shown as XMASK and YMASK. These two memory cells allow for the storage of two bits of mask data within each mask cell. Thus, each row of mask cells 104 may retain an “even” row mask word and an “odd” row mask word. The XY mask cell 104 also includes four transistors that operate as a portion of a mask assertion circuit 26c′. These four transistors and the XMASK and YMASK memory cells occupy the same layout footprint as the CAM cell 102 illustrated by FIG. 10B. The pair of access transistors within the XMASK memory cell have gate electrodes that are electrically connected to a respective mask word line MWLX0 and the pair of access transistors within the YMASK memory cell have gate electrodes that are electrically connected to a respective mask word line MWLY0. Accordingly, the XMASK memory cell and the YMASK memory cell can be written to (and read from) independently. For example, writing an active mask value into the XMASK memory cell includes driving the bit lines BX0 and BXB0 at a logic 0 and logic 1 levels, respectively, and strobing the mask word line MWLX0 with an active high pulse. Similarly, writing an active mask value into the YMASK memory cell includes driving the bit lines BY0 and BYB0 at a logic 0 and logic 1 levels, respectively, and strobing the mask word line MWLY0 with an active high pulse.

[00068] Each of the memory cells in the illustrated XY mask cell 104 generates a complementary output (shown as XB and YB). These complementary outputs are provided to the mask assertion circuit 26c′. The active or inactive state of the “even” mask bit within the XMASK memory cell may be read by switching an “even” read soft mask signal line RSMX0 low-to-high during a mask read operation. As explained more fully hereinbelow, when the signal line RSMX0 is switched low-to-high and the XMASK memory cell is storing an active mask value (i.e., XB=1), a previously precharged global mask signal line MASKB0 will be pulled high-to-low to indicate a global mask condition. However, if the XMASK memory cell is storing an inactive mask value (i.e., XB=0), then the global mask signal line MASKB0 will remain high at its precharged level, when signal line RSMX0 is switched low-to-high. Similarly, the active or inactive state of the “odd” mask bit within the YMASK memory cell may be read by switching an “odd” read soft mask signal line RSMY0 low-to-high during a mask read operation. When the signal line RSMY0 is switched low-to-high and the YMASK memory cell is storing an active mask value (i.e., YB=1), a previously precharged global mask signal line MASKB0 will be pulled high-to-low to indicate a global mask condition. However, if the YMASK memory cell is storing an inactive mask value (i.e., YB=0), then the global mask signal line MASKB0 will remain high at its precharged level when the read operation is performed.

[00069] A sub-array of XY ternary CAM cells and a sub-array of XY mask cells, such as those illustrated by FIGS. 10B and 10C, may be combined as illustrated by the CAM array 70b of FIG. 8B. This CAM array 70b is illustrated as including four rows of dedicated mask cells that retain eight (8) “soft” (i.e., programmable) mask words and 2048 rows of ternary CAM cells. The CAM array 70b has a logical width of 80 bits and is divided into two segments (shown as a lower x20 segment and an upper x60 segment). A vertical column of segment latches is provided. This column of latches divides the x20 and x60 segments from each other and facilitates pipelined operations with the CAM array 70b. As described more fully in the aforementioned '236 application, these segment latches are configured to latch “horizontal” word line and match line signals provided by the x20 segment of CAM cells during x20 write (and read) and x20 search operations. These segment latches are also configured to latch “horizontal” mask word line and read soft mask line signals provided by the x20 segment of dedicated XY mask cells. Other latches are also provided to latch the illustrated “horizontal” control signals (e.g., ALLMASKB).

[00070] The illustrated CAM array 26′ of FIG. 8A and the CAM array 70b of FIG. 8B both receive bit and data line signals from a bit/data line driver circuit that is configured to support the use of dedicated XY mask cells and XY ternary CAM cells that share bit lines.

[00071] As illustrated by FIG. 11A, the bit/data line driver circuit 24a′ is responsive to a plurality of control signals. These control signals include an all mask signal (ALLMASKB), which is an active low signal, and an enable signal (ENABLE), which is an active high signal. The bit/data line driver circuit 24a′ is also connected to a respective column of the CAM array 26′ by two pairs of bit lines (shown as (BX0, BXB0) and (BY0, BYB0)), a pair of data lines (D0, DB0) and a global mask signal line (MASKB0). The all mask signal ALLMASKB and the enable signal ENABLE are provided as inputs to a 3-input NAND gate 25. The 3-input NAND gate 25 is also configured to receive a global mask signal, shown as MASKB0, which is provided by the mask assertion circuit 26c′. The illustrated portion of the mask assertion circuit 26c′ includes a PMOS pull-up transistor 27 that is responsive to an active low precharge signal (PRCHGB). When the precharge signal PRCHGB is switched high-to-low to prepare for a mask read operation, the global mask signal line MASKB0 is precharged high. A relatively weak PMOS pull-up transistor 29 and inverter are also provided to support each corresponding global mask signal line in its precharged state.

[00072] The bit/data line driver circuit 24a′ includes three (3) data and mask latches, shown as LX, LM and LY. The latch LX latches an X data bit (DATAX0) in response to a pair of complementary clock signals CLK and CLKB. The latch LY latches a Y data bit (DATAY0) in response to the clock signals. The latch LM latches a signal at the output of the 3-input NAND gate 25 and generates an active low global mask output signal (shown as MASKB0_OUT). When the global mask output signal MASKB0_OUT is latched low, all four bit lines BX0, BXB0, BY0 and BYB0 are set high and both data lines D0 and DB0 are set low to indicate a global mask condition during a corresponding search operation.

[00073] However, when all inputs to the 3-input NAND gate 25 are set high (which means the driver circuit 24a′ is enabled and no global masking is present), then the bit lines and data lines at the outputs of the driver circuit 24a′ will be driven with the differential data signals illustrated by TABLE 4.

[00002]

TABLE 4
INPUT DATA	DRIVER CIRCUIT OUTPUTS	OPERATION
DATAX0	DATAY0	D0	DB0	BX0	BXB0	BY0	BYB0	WRITE	SEARCH
0	0	0	1	0	1	0	1	MASK	—
0	1	0	1	0	1	1	0	0	0
1	0	1	0	1	0	0	1	1	1

[00074] FIG. 11B illustrates an alternative bit/data line driver circuit 24a′ that is similar to the driver circuit 24a′ illustrated by FIG. 11B. However, the 3-input NAND gate 25 illustrated in FIG. 11A is replaced by a 2-input NAND gate 25′. In addition, an all mask signal (ALLMASK) is treated as an active high signal that drives the gate of an NMOS pull-down transistor 31. The drain terminal of this NMOS pull-down transistor 31 is electrically connected to the global mask signal line MASKB0. Thus, when active, the all mask signal ALLMASK operates to pull-down all the global mask signal lines MASKBn (where n is an integer) associated with a respective segment of a CAM array. The data signals illustrated by TABLE 4 apply to the driver circuit 24a′ of FIG. 11B.

[00075] In the CAM circuit 90 of FIG. 9, the bit/data line driver circuit of FIG. 11A is combined with the CAM cells 102 and dedicated mask cells 104 illustrated by FIGS. 10B-10C, to form a portion of a CAM device according to an embodiment of the present invention. In particular, FIG. 9 illustrates two “cells” of a bit/data line driver circuit that are connected to a 1×2 sub-array of mask cells 104 and a 1×2 sub-array of CAM cells 102. The CAM circuit 100 of FIG. 10A illustrates a 2×4 sub-array of dedicated XY mask cells 26a′ in combination with a 4×4 sub-array of ternary CAM cells 26b′. The sub-array of CAM cells 26b′ also utilizes shared pseudo-ground lines (shown as PGND01 and PGND23).

[00076] The global mask signals MASB0-MASKB3, which originate from the mask cell sub-array 26a′ of FIG. 10A, are provided as inputs to a bit/data line driver circuit (see, e.g, FIG. 9 and FIGS. 11A-11B). Along with the data input signals (see, e.g., (DATAX0, DATAY0)-(DATAX79, DATAY79)), these global mask signals are latched into the driver circuit so that global masking operations may be performed during a corresponding search operation. As illustrated by TABLE 5, the CAM array 70b of FIG. 8B may undergo a plurality of consecutive pipelined search operations, which include loading respective x20 and x60 segments of search words (SW) and mask words (MW) into latches (see, e.g., latches LX, LM and LY in FIGS. 11A-11B) within the bit/data line driver circuit. The search words are illustrated as words Z, A, B, C, D, E and F. In TABLE 5, the x20 search operations and x60 search operations are illustrated as being offset in time by 0.5T, where T is a period of a clock signal (e.g., T=2ns). This timing of the pipelined search operations is more fully illustrated and described in the aforementioned '236 application.

[00002]

TABLE 5
CAM Operations
Foreground
2	T(−)	Load A20 SW & A20 MW
1.5	T(−)	Load Z60 SW & Z60 MW
3.5	T(−)	Load A60 SW & A60 MW
4	T(−)	Load B20 SW & B20 MW
5.5	T(−)	Load B60 SW & B60 MW
6	T(−)	Load C20 SW & C20 MW
7.5	T(−)	Load C60 SW & C60 MW
8	T(−)	Load D20 SW & D20 MW
9.5	T(−)	Load D60 SW & D60 MW
10	T(−)	Load E20 SW & E20 MW
11.5	T(−)	Load E60 SW & E60 MW
12	T(−)	Load F20 SW & F20 MW

[00077] In TABLE 6, the foreground operations illustrated by TABLE 5 are combined with background operations. These background operations illustrate the timing of operations to read the x20 and x60 masks (MASK), which are held within a dedicated mask cell sub-array, into the mask assertion circuits associated with each column of a CAM array. TABLE 6 also illustrates how pipeline bubbles (in foreground) can be eliminated by tightly interleaving segmented search operations with segmented read and write operations (including operations to write mask words into the mask cell sub-array). These interleaving operations are also described more fully in the aforementioned '236 application.

[00002]

TABLE 6
CAM Operations
	Foreground		Background
2T(4ns)	Search_A20	1.5T	Search_Z60	2T	Read B20 Mask	1.5T	Read A60 Mask
4T	Search_B20	3.5T	Search_A60	4T	Read C20 Mask	3.5T	Read B60 Mask
6T	Search_C20	5.5T	Search_B60	6T	Read D20 Mask	5.5T	Read C60 Mask
8T	Search_D20	7.5T	Search_C60	8T	Read E20 Mask	7.5T	Read D60 Mask
10T	Search_E20	9.5T	Search_D60	10T	Read F20 Mask	9.5T	Read E60 Mask
12T	Search_F20	11.5T	Search_E60	12T	Read G20 Mask	11.5T	Read F60 Mask
14T	Search_G20	13.5T	Search_F60	14T	Read H20 Mask	13.5T	Read G60 Mask
16T	Search_H20	15.5T	Search_G60	16T	No-Op	15.5T	Read H60 Mask
18T	Write_I20	17.5T	Search_H60	18T	Read J20 Mask	17.5T	No-Op
22T	Search_J20	19.5T	Write_I60	22T	Read K20 Mask	19.5T	Read J60 Mask
24T	Search_K20	23.5T	Search_J60	24T	Read L20 Mask	23.5T	Read K60 Mask
26T	Search_L20	25.5T	Search_K60	26T	No-Op	25.5T	Read L60 Mask
28T	Read_M20	27.5T	Search_L60	28T	Read N20 Mask	27.5T	No-Op
32T	Search_N20	29.5T	Read_M60	32T	No-Op	29.5T	Read N60 Mask
34T	Write_O20	33.5T	Search_N60	34T	No-Op	33.5T	No-Op
38T	Read_P20	35.5T	Write_O60	38T	Read Q20 Mask	35.5T	No-Op
42T	Search_Q20	39.5T	Read_P60	42T	Read R20 Mask	39.5T	Read Q60 Mask
44T	Search_R20	43.5T	Search_Q60	44T	Read S20 Mask	43.5T	Read R60 Mask
46T	Search_S20	45.5T	Search_R60	46T	Read T20 Mask	45.5T	Read S60 Mask
48T	Search_T20	47.5T	Search_S60	48T	No-Op	47.5T	Read T60 Mask
50T	Write_Mask1_20	49.5T	Search_T60	50T	No-Op	49.5T	No-Op
54T	Write_Mask2_20	51.5T	Write_Mask1_60	54T	Read U20 Mask	51.5T	No-Op
58T	Search_U20	55.5T	Write_Mask2_60	58T	Read V20 Mask	55.5T	Read U60 Mask
60T	Search_V20	59.5T	Search_U60	60T	Read W20 Mask	59.5T	Read V60 Mask
62T	Search_W20	61.5T	Search_V60	62T	Read X20 Mask	61.5T	Read W60 Mask

[00078] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.