SYSTEM AND METHOD FOR PROTECTING DATA AND CORRECTING BIT ERRORS DUE TO COMPONENT FAILURES

Abstract: A system for protecting data and correcting bit errors due to component failures. A check bits generation unit partitions the data into a plurality of logical groups. The check bits generation unit generates a parity bit for each of the logical groups, and additionally generates a global error correction. The global error correction code is equivalent to the result of generating individual error correction codes for each logical group and combining them in a predetermined manner. In one particular embodiment, the data is divided into a total of X logical groups. An error correction unit is coupled to receive the plurality of data bits and the check bits following storage or transmission. It is configured to generate a parity error bit for each of the logical groups of data based on the received data bits, and to generate a regenerated global error correction code in the same manner in which the original global error correction code is derived. A global syndrome code is generated such that, with knowledge of the specific logical groups that have a single bit error, a value indicative of the location of the error in such groups may be derived from the global syndrome code.

Description: TITLE: SYSTEM AND METHOD FOR PROTECTING DATA AND CORRECTING BIT ERRORS DUE TO COMPONENT FAILURES BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to error correction in electronic systems and, more particularly, to systems that employ error correction codes to facilitate correction of bit errors due to, for example, component failures 2. Description of the Related Art Error codes are commonly used in electronic systems to detect and correct data errors, such as transmission errors or storage errors. For example, error codes may be used to detect and correct errors in data transmitted via a telephone line, a radio transmitter, or a compact disc laser. Error codes may additionally be used to detect and correct errors associated with data stored in the memory of computer systems. One common use of error codes is to detect and correct errors of data transmitted on a data bus of a computer system In such systems, error correction bits, or check bits, may be generated for the data prior to its transfer or storage When the data is received or retrieved, the check bits may be used to detect and correct errors withm the data. Component failures are a common source of error in electrical systems. Faulty components may include faulty memory chips or faulty data paths provided between devices of a system. Faulty data paths can result from, for example, faulty pms, faulty data traces, or faulty wires. Hamming codes are a commonly used type of error code. The check bits in a Hamming code are parity bits for portions of the data bits. Each check bit provides the parity for a unique subset of the data bits. If an error occurs (i.e. one or more of the data bits change state), one or more of the check bits upon regeneration will also change state (assuming the error is within the class of errors covered by the code). By determming the specific bits of the regenerated check bits that changed state, the location of the error within the data may be determined For example, if one data bit changes state, this data bit will cause one or more of the regenerated check bits to change state. Because each data bit contributes to a unique group of check bits, the check bits that are modified will identify the data bit that changed state. The error may be corrected by inverting the bit identified as being erroneous. One common use of Hamming codes is to correct single bit errors within a group of data Generally speakmg, the number of check bits must be large enough such that 2 k -l is greater than or equal to n+k where k is the number of check bits and n is the number of data bits Accordingly, seven check bits are typically required to implement a smgle error correcting Hamming code for 64 data bits A single error correcting Hamming code is capable of detecting and correcting a single error Figures 1-3 illustrate an example of a system employing a single-error correction (SEC) Hamming code In this example, four data bits (D4, D3, D2, and Dl) are protected usmg three check bits (PI, P2, and P3) The parity generator 10 (Figure 1) is used to encode the data block that contains the data bits and the check bits The encodmg process is performed prior to stoπng or communicating the data. Figure 2 shows an assignment of data bits to calculate the check bits. In this example, the check bit PI is generated by an XOR (exclusive OR) of the binary values m D4, D3, and Dl. Similarly, the check bit P2 is generated by an XOR of the binary values m D4, D2, and Dl, and the check bit P3 is generated by an XOR of the binary values in D3, D2 and Dl. Figure 3 shows the bit positions and the corresponding content of these positions withm the encoded data block. The data block, which includes the data bits and the generated check bits, may then be stored in a memory chip or communicated over a data communication path. At the point of receipt, the data block is retrieved and decoded. The decoding process involves performing a validity check on the received word, and executing an error correction technique if an error was detected. To check whether an error occurred in the storage (or transmission) of the data block, the check bits PI, P2, and P3 are effectively regenerated using the received data, and each regenerated check bit is XORed with the corresponding received check bit to generate a corresponding syndrome bit. Figure 4 is a table depicting a manner in which these syndrome bits may be generated. More particularly, syndrome bit SI may be generated by XORing the received binary values in PI , D4, D3, and D 1. If none of the received data bits (D4, D3, Dl) is erroneous, the value of the received check bit PI is effectively XORed with itself, and the syndrome bit SI will be 0 (assuming the original check bit PI is not erroneous). If one of the data bits (D4, D3, Dl) or the check bit PI is erroneous, the syndrome bit SI will be 1 (asserted), thus indicating an error Syndrome bits S2 and S3 may be generated similarly. Taken collectively, the syndrome bits SI, S2 and S3 may be used to identify the location of an erroneous bit. For example, the binary value of the syndrome bits in the order [S3, S2, SI] indicates the position of the erroneous bit within the 7 bit data block as depicted in Figure 3. If the syndrome code is all zeros (i.e. "000"), the data has no smgle bit error. Upon identification of the erroneous bit position, the error is corrected by invertmg the binary value in that position, i.e. from 0 to 1 or from 1 to 0. It is a common practice to store data in, or communicate data through, multiple components. For example, a data block may be stored m a plurality of memory chips, or it may be communicated through a plurality of wires. An error may be introduced if one of the components is faulty. A Hamming code such as that descπbed above may be used to address error correction in such systems. For example, consider the case of storing D bits of data that are protected by C check bits using M memory chips. The data block therefore contains D+C bits. If the data block is to be evenly divided among the M memory chips, each memory chip will store X of the data and/or check bits of the data block, where X = (D+C)/M. The standard approach to providing error correction for chip failures is to divide the D+C data and check bits mto X logical groups each including M bits, and assigning 1 bit from each chip to each of the groups The check bits in each group form a SEC (single-error correcting) code such as a Hamming code. When any chip fails, it introduces at most one error into each group, and these errors are corrected independently us g the SEC codes. If a Hamming code is used m each group, a total of C = X*L check bits are required, where L is the smallest mteger such that 2 Λ L > M This standard approach is inefficient because each group is able to independently identify which bit (if any) withm the group is in error However, if the only failures considered are memory chip failures, the failures in different groups are highly correlated. It would be desirable to provide a system and method which allow for the reliable storage or transmission of data in environments wherem component failures are possible In particular, it would be desirable to provide a system and method which allows errors in data to be detected and corrected while reducmg the number of check bits which must be transmitted or stored. SUMMARY OF THE INVENTION The problems outlmed above may in large part be solved by a system and method for detectmg and correcting errors in a data block in accordance with the present mvention In one embodiment, a check bits generation unit receives and encodes data to be protected The check bits generation unit effectively partitions the data mto a plurality of logical groups The check bits generation unit generates a parity bit for each of the logical groups, and additionally generates a global error correction The global error correction code is indicative of a predetermined combmation of a set of error correction codes each mdividually associated with a corresponding logical group In various embodiments, the global error correction code is equivalent to the result of generating individual error correction codes for each logical group and combmmg them in a predetermined manner In one particular embodiment, the data is divided into a total of X logical groups The global error correction code may be equivalent to the result of (or may be derived by) shiftmg (either linearly or cyclically) the error correction code for a given ith group by l bit positions, wherein I = 0 to X-l, and by XORing corresponding columns of the resulting shifted error correction codes together The global error correction code, which in this case is referred to as having "twisted check" bits (by virtue of the selective shiftmg of the individual error correction codes), thus includes a separate bit indicating the parity for each aligned column formed by the collection of resulting shifted error correction codes The data along with the parity bit for each logical group and the global error correction code are then conveyed through a communication channel or are stored in memory Advantageously, the overall number of required check bits (the parity bits for the logical groups and the bits forming the global error correction code) is smaller than the overall number of bits needed for the collection of error correction codes associated mdividually with the logical groups In one implementation, data at corresponding bit positions withm the logical groups are conveyed through a common component, such as the same wire, or are stored in the same component, such as the same memory chip Additionally, data bits at different bit positions withm a given logical group are not conveyed through, or are not stored withm, a common component An error correction unit is coupled to receive the plurality of data bits and the check bits following storage or transmission. The error correction unit is configured to generate a parity error bit for each of the logical groups of data based on the received data bits The error correction unit may further be configured to generate a regenerated global error correction code m the same manner in which the oπgmal global error correction code is deπved Thus, m one embodiment, the regenerated global error correction code is equivalent to the result of (or may be deπved by) shifting the regenerated error correction code for a given ith group by I bit positions, wherem l = 0 to X-l, and by XORing corresponding columns of the resultmg shifted error correction codes together A global syndrome code may be generated by XORing the oπgmal global error correction code with the regenerated global error correction code This global syndrome code is indicative of a difference between the original global error correction code and the regenerated global error correction code In general, the global syndrome code is generated such that, with knowledge of the specific logical groups that have a single bit error, a value indicative of the location of the error m such groups may be denved from the global syndrome code. The overall number of bits forming the global syndrome code and the parity bits for each logical group is smaller than the overall number of bits needed for the error correction codes mdividually associated with the logical groups. In embodiments m which the error correction codes for various logical groups are shifted and XORed, the global syndrome code includes a bit corresponding to each aligned column of the regenerated shifted error correction codes which indicates whether a parity error for that column exists. The existence of a single bit error in any logical group can be determined using the oπgmal paπty bit and the data received for that group Additionally, smce the class of coπectable eπors considered by the system is restricted to single bit eπors that occur at the same position within the various logical groups (such errors may occur due to single component failures), the error coπection code for each logical group that has an eπor will reflect a change at exactly the same bit positions. Thus, by shifting the various eπor coπection codes m a twisted manner (i.e., shiftmg some more than others), and XORing the resulting vertically aligned columns, the coπelation that exists with respect to the specific bits that change in each error coπection code for each logic group with an eπor (l e., the eπor coπection code for each logical group having an eπor will mcur a change at exactly the same bit positions) may be exploited to thereby extract information indicating the specific bit position of the eπors BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed descnption and upon reference to the accompanying drawings in which: Figure 1 (pnor art) is a block diagram illustrating a panty generator to encode data bits usmg a smgle eπor coπecting Hamming code; Figure 2 (prior art) is an assignment table to compute a set of check bits; Figure 3 (pnor art) is an illustration of bit positions and their content for a data block encoded by the paπty generator of Figure 1; Figure 4 (pnor art) is an assignment table for generating a syndrome code, Figure 5 is a block diagram of a system including an eπor control unit according to one embodiment of the present invention; Figure 6A illustrates an encodmg process including the generation of row parity bits and row eπor coπection codes; Figure 6B illustrates a manner in which a global eπor coπection code may be generated; Figure 7A illustrates a manner in which a set of row parity eπor bits and regenerated eπor coπection codes may be generated, Figure 7B illustrates a manner in which a regenerated global eπor coπection code may be generated, Figure 8A illustrates a manner m which a global syndrome code may be generated, Figure 8B illustrates a manner in which a set of row syndrome codes may be generated, Figure 9 is a block diagram of a system including a linear shift unit for decoding a data block; Figure 10 is a functional block diagram of a linear shift unit; Figures 11-17 illustrate an exemplary generation of a row syndrome code; Figures 18 and 19 illustrate a flow diagram illustrating one embodiment of a linear decode process, Figures 20-27 are assignment tables illustrating the operation of a set of check bits m accordance with another embodiment of the mvention; Figures 28-35 are assignment tables illustrating the generation of a set of row panty eπor bits and regenerated twisted check bits for the embodiment of Figures 20-27, Figure 36 illustrates a linear decode process; and Figure 37 is a block diagram of a system in accordance with another embodiment of the present invention. While the mvention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example m the drawings and will herein be descnbed in detail. It should be understood, however, that the drawings and detailed descnption thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION OF THE DRAWINGS Turning now to Figure 5, a block diagram of a system including an eπor control unit 105 is shown accordmg to one embodiment of the present invention. In this particular embodiment, the eπor control unit 105 is coupled to convey and receive data from storage 110. The storage 110 is configured to store a data block (data bits and check bits) that is communicated from/to the eπor control unit 105. The storage 110 mcludes a plurality of memory chips 1 lOA-110M. Each of the memory chips 1 lOA-110M is configured to store a portion of the data block. The memory chips 1 lOA-110M may be any type memory/ storage devices usable to store data. As will be descπbed in further detail below, eπor control unit 105 is configured to encode data to be stored in the storage 110, and to decode received encoded data following storage to coπect certain categoπes of bit eπors (when they occur) usmg a relatively small number of check bits as compared with traditional approaches, such as typical single eπor coπecting Hamming codes. The eπor control unit 105 includes a check bits generation unit 120 to perform the encoding process and an eπor coπection unit 130 to perform the decoding process and to perform eπor coπection. Data bits are received via data path 151 and are encoded by check bits generation unit 120 pnor to storing a data block (data bits and check bits) mto storage 110 via data path 152. Subsequently, the stored data block may be retneved by eπor coπection unit 130 via data path 153. Eπor coπection unit 130 decodes the data block to detect and coπect eπors in the data. If one or more eπors are detected, the eπors are coπected. The data bits (coπected, if necessary) are then output via data path 155 It is noted that check bits generation unit 120 and eπor coπection unit 130 may be implemented m hardware, in software, or using a combination thereof. Vanous specific implementations are possible. The operation of eπor control unit 105 will next be explained with reference to Figures 5-8 Figures 6A-8B illustrate an example wherem a total of 20 data bits to be stored m storage 110 are protected Figures 6A and 6B illustrate an encodmg process performed by the check bits generation unit 120 pnor to the storage of the data bits. Check bits generation unit 120 receives the 20 data bits (via data path 151) and, in this example, processes the data accordmg to five logical groups, each group forming a respective row RO - R4, as shown in Figure 6A. A set of columns denoted D4, D3, D2, and Dl is formed by coπespondmg bit positions of each row. As will be descnbed m further detail below, the data forming each column is stored m the same memory chip, and different bits of each row are stored m different memory chips. Check bits generation unit 120 is configured to generate a panty bit per row (PPR) and a global eπor coπection code (GECC). The global eπor coπection code is equivalent to, or may be denved from a predetermined logical combmation of eπor coπection codes each mdividually associated with a respective row The PPR and GECC bits are check bits employed to protect the data bits against at most one eπor per row, provided that all eπors occur in the same column. The set of parity bits may be calculated usmg either even or odd parity. Since one parity bit is calculated per row, or logical group, the number of PPR bits equals the number of rows. In the example of Figure 6A, even panty is used. Thus, for the exemplary data as illustrated, each of the panty bits (PPRs) for rows R0-R3 is 1. The panty bit for row R4 is 0. Figure 6A illustrates an eπor coπection code (ECC) associated with each row. In the example of Figure 6A, the eπor coπection code for each row is calculated accordmg to the assignment table shown in Figure 2. For example, the eπor coπection code calculated for the data m row R0 is [111], and the eπor coπection code calculated for row Rl is [011], and so on. It is noted, however, that other specific eπor coπection codmg techniques may be employed, and that different eπor coπection coding techniques may be associated with differing rows. Figure 6B illustrates a manner in which the global eπor coπection code may be generated. More particularly, in this embodiment, the global eπor coπection code may be generated by selectively shiftmg certain row ECCs with respect to one another and XORing the resulting vertically aligned bits. Smce some of the row ECCs are shifted more than others, the bits forming the global eπor coπection code are refeπed to as "twisted" check bits. In the embodiment of Figure 6B, the global eπor coπection code is denved by linearly shiftmg the eπor coπection code for a given ith group by l bit positions wherein l = 0 to X-l, and where X is the total number of rows or logical groups The bits in each resultmg vertically aligned column are then XORed together, to thereby generate the global eπor coπection code (leadmg or trailing 0s may be added to the entπes of each row of the twisted ECCs, as necessary, pnor to this XOR operation). It is noted that m the discussion above relatmg to Figures 6A and 6B, the global eπor coπection code is calculated by first generating the mdividual eπor coπection codes for each row and logically combming them in a predetermined manner. In other embodiments, the global eπor coπection code may be generated directly from the data bits such that it is equivalent to the result of generating individual eπor coπection codes for each row and combming them m a predetermined manner For example, the global eπor coπection code illustrated in Figure 6B could be generated directly from the data bits using a predetermined function, without generatmg individual eπor coπection codes for each row Upon calculation of the global eπor coπection code, the data block mcludmg 20 bits of data, the panty bits (PPRs) and the global eπor coπection code (GECC) are conveyed to storage 110. It is noted that the bits forming each column (D4, D3, D2, and Dl) of data m the data block are stored m a common memory chip, and that no two data bits in differing bit positions of a particular row are stored m the same memory chip. It is further noted that the panty bits and global eπor coπection code (GECC) may be stored m additional memory chips. A further example of the manner in which the panty bits and the bits forming the global eπor coπection code (GECC) may be distnbuted for storage within vanous memory chips will be provided further below m conjunction with Figures 20-35 It is further noted that in the example of Figures 6 A and 6B, a total of 12 check bits (the panty bits (PPRs) and the bits of the global ECC) are generated for storage along with the 20 data bits This overall number of check bits is advantageously less than the number of check bits required usmg conventional methods (wherem, for example, a total of 15 check bits, coπesponding to the row eπor coπection codes, are necessary) Generally speaking, m embodiments which employ a linearly shifted global eπor coπection code generated accordmg to the example of Figure 6B, a total of only (2X+L-1) check bits are needed, where it is assumed that D bits of data are protected with C check bits using M memory chips (where M evenly divides (D+C)), and where X=(D+C)/M and C=X*L where L is the smallest mteger such that 2 Λ L≥M Referring now to Figures 7A - 8B, the decodmg process of the data block following storage will next be explained. As illustrated in Figure 7A, eπor coπection unit 130 processes the data accordmg to the same logical groups as formed by check bits generation unit 120 Eπor coπection unit 130 may be configured to generate a regenerated parity bit per row (PPR') using the received data The same parity type (even or odd) is used duπng both encoding and decoding In the example of Figure 7A, two eπors are introduced mto the received data set, both occurring m column D2, as shown with underlmes More particularly, the bits m rows Rl and R3 of column D2 have been changed to exemplify eπors. Such eπors could be the result of, for example, a faulty memory chip that stores the column D2 bits of the data set. Thus, for this example, the regenerated panty bits for rows Rl and R3 change state. A bit for each row indicating that a panty eπor exists may be generated by XOR g the ongmal parity bits with the regenerated panty bits. It is noted that generation of these row panty eπor bits (RPEs) may instead be generated directly by XORmg coπespondmg bits of the received data bits with the ongmal panty bits (PPRs) (without separately generating the regenerated panty bits (PPRs)) If any of the rows have a single bit eπor, as indicated by the row panty eπor bits (RPEs), the eπor coπection unit 130 generates a regenerated global eπor coπection code in the same manner m which the ongmal global eπor coπection code was generated, usmg the regenerated eπor coπection codes. Thus, in one embodiment, the regenerated global eπor coπection code may be denved by shiftmg the eπor coπection code for a given ith group by I bit positions, where I = 0 - X-l, and by XORmg the bits m the resulting vertically aligned columns, as depicted m Figure 7B Similar to the previous discussion, in other embodiments, the regenerated global eπor coπection code may be denved directly from a predetermined function of the received data bits, without generating an mdividual eπor coπection code for each row In such embodiments, however, the global eπor coπection code is equivalent to a predetermined combmation of a set of eπor coπection codes each individually associated with a coπespondmg row As shown m Figure 8 A, upon generation of the regenerated global eπor coπection code, a global syndrome code may be generated by XORing the ongmal global eπor coπection code with the regenerated global eπor coπection code. This global syndrome code is indicative of a difference between the oπgmal global eπor coπection code and the regenerated global eπor coπection code. In embodiments m which the eπor coπection codes for various logical groups are effectively shifted in a twisted manner and are XORed, the global syndrome code mcludes a bit coπespondmg to each aligned column of the regenerated shifted eπor coπection codes which indicates whether a paπty eπor for that column exists. For mstance, m the example of Figure 8, the bits m the global syndrome code for positions 2, 3, 4 and 5 are set This is the result of the fact that the parity in each of columns 2, 3, 4 and 5 of the shifted regenerated eπor coπection codes of Figure 7B is different from the parity of each of the coπesponding columns of the original shifted eπor coπection codes, as depicted in Figure 6B Accordingly, in this embodiment the global syndrome code includes a bit coπespondmg to each aligned column of the regenerated shifted eπor coπection codes which indicates whether a paπty eπor for that column exists. Since the regenerated eπor coπection codes for the logical groups that have eπors will incur changes at exactly the same bit positions, by shifting in a twisted manner the various eπor coπection codes and XORing the resulting bit aligned columns, the coπelation that exists with respect to changes m the eπor coπection codes for logical groups that have eπors may be exploited to thereby extract information indicating the specific positions of the eπors. For example, Figure 8B illustrates a manner m which a set of row syndrome codes, each of which is indicative of the position of an eπor in a given row, may be generated using the global syndrome code and the row panty eπor information. As illustrated in Figure 8B, entries (initially unfilled) for each row syndrome code are provided by shifting the entries in the same manner m which the eπor coπection codes were shifted to generate the global eπor coπection code. First, the row syndrome codes for any rows that do not have eπors can be filled in with all "0" values (since rows without eπors could not contnbute to a parity eπor m any given column, as identified by the global syndrome code). Thus, the row syndrome code entries for rows R0, R2, and R4 can be filled in with all "0" values. As depicted in Figure 8B, the first row with an eπor is row Rl. Smce the first bit of the row syndrome code for row Rl is in column position 1, and smce the global syndrome value for that bit position is 0 and the only other contnbuting value for that bit position of the global syndrome code is denved from the row syndrome code for row R0 (which is already known to be "0"), the value of the first bit of the row syndrome code for row Rl can be filled in as "0" Additionally, since the row syndrome code for every row with an eπor is known to be identical, the first bit of the row syndrome code for row R3 can also be filled in as a "0". Subsequently, the second bit of the syndrome code for row Rl may be similarly determined by considering the value in bit position 2 of the global syndrome, and the values of the bits m position 2 of rows 0 and 2 (I e., the last bit of the row syndrome code for row R0 and the first bit of the row syndrome code for row R2). In this situation, the value of the second bit of the row syndrome code for row Rl may be filled in as a "1". Again, this value is also reflected m the second bit of the row syndrome code for row R3. The last bit of the row syndrome codes may be determined similarly The binary value of the row syndrome codes (taken in reverse order with respect to the depicted values m Figure 8B) thus indicates the position of the eπor for logical groups that have eπors Therefore, in this example, the value "110", which is binary 6, coπelates to bit position D2, as shown in Figure 3. This position coπesponds to the position of the eπors introduced m the example shown in Figure 7A. Upon determination of the eπoneous bits m the received data set, the eπor coπection unit 230 is configured to coπect the data, as needed. For example, m the example of Figures 6-8, the bits m column D2 of the rows Rl and R3 are identified as bemg eπoneous Accordingly, eπor coπection unit 230 inverts these bits and outputs the data as coπected via data output 155 Turning next to Figures 9-17, a manner in which the row panty information and the global syndrome information may be decoded to generate the row syndrome code is next considered More particularly, as illustrated m Figure 9, eπor coπection unit 130 may employ an RPE/global syndrome code generation unit 131 and a linear shift unit 132 for decoding a received data block which is encoded usmg the linear shiftmg methodology as described above A functional block diagram illustrating functional aspects of the linear shift unit 132 is illustrated m Figure 10 As shown, lmear shift unit 132 includes a control unit 135 coupled to a pair of shift registers 133 and 134 The various functionality illustrated in Figures 9 and 10 may be implemented m hardware, m software, or usmg a combination thereof The RPE/global syndrome code generation unit 131 generates the row parity eπor bits (RPEs) and the global syndrome code m accordance with the foregomg description using the received data block The row panty eπor bits (RPEs) and the global syndrome code are then provided to the lmear shift unit 132 The operation of the linear shift unit 132 is explained with reference to Figures 11-17, along with the flow diagrams of Figures 18-19 As illustrated in Figure 11, shift register 133 includes a plurality of cells RPE[0 4] for stor g the row parity eπor bits (RPEs) for rows 0-4, respectively Similarly, shift register 134 includes a plurality of cells GSC[0 6] for stonng the global syndrome code Referring collectively to Figures 11 and 18, initially, the row panty eπor bits RPEs are loaded into shift register 133, and control unit 135 determines whether the shift register contams all "0"s (steps 310 and 330) If shift register 133 contains all 0s, thus indicatmg that no single bit row eπors exist, no data eπor is reported, and the data is output from the eπor control unit (step 340) On the other hand, if one or more row panty eπor bits are set, the global syndrome code is loaded into shift register 134 (step 350) Figure 12 illustrates the contents of shift registers 133 and 134 at this point in the methodology for the exemplary RPEs and global syndrome code generated for the example associated with Figure 8B The content of the right-most cell of shift register 133 (cell RPE[0]) is then determined during step 360 If the value m the nght-most cell RPE[0] of the shift register 133 is "0", the contents of both the shift register 133 and the shift register 134 are shifted one bit position to the right (step 370) The left-most panty m each of these registers is padded with a "0", and the values which are shifted from the nght-most positions are discarded This process repeats until the value m the right-most cell RPE[0] of register 133 is a "1" Figure 13 illustrates the resultmg values in shift registers 133 and 134 following this operation for the exemplary codes At this pomt, the bit value in the right-most cell of the register 134 is recorded as row syndrome code bit RSC[0], as is also shown m Figure 13 (step 380) If all the row syndrome code bits have not yet been determined (step 390), a detenmnation is made as to whether the last recorded row syndrome code bit was 1 If not, the contents of shift register 134 are agam shifted by one position to the right (step 44) , and the process repeats On the other hand, if the last row syndrome code bit recorded was 1, aligned bit positions of the first and second shift registers are XORed and the result is stored m the second shift register 134 (steps 420 and 430) In the illustrated example, the most recently recorded row syndrome code bit was "0", and thus the contents of the second shift register 134 are shifted by one position to the nght (step 440), and the process repeats by recordmg the value in the right-most position of register 134 as the next RSC bit, as illustrated m Figure 14. Duπng the next iteration, smce the last recorded RSC bit was "1", the value m shift register 133 is XORed with the value m shift register 134 (step 420) and the result is stored in shift register 134, as illustrated in Figure 15. The contents of register 134 are agam shifted by one position to the right as illustrated in Figure 16 (step 440) and the resulting bit in the right-most position of shift register 134 is recorded as the next RSC bit, as illustrated in Figure 17 (step 380) When all bits of the row syndrome code have been determined, the process ends (step 400). The row syndrome code is indicative of the position of the eπor for any rows with a single bit eπor. In the illustrated example, the binary value of the row syndrome code is 6. Refemng to Figure 3, bit position 6 coπesponds to column D2, which is the position of the eπors introduced in the example of Figure 7A It is noted that alternative embodiments of the linear shift decode methodology descnbed m conjunction with Figures 9-19 are possible For example, rather than nght shiftmg the values in shift registers 133 and 134, the values may be left shifted in a similar manner to denve the row syndrome code. It is also noted that the received check bits may be decoded usmg other mechanisms. For example, mechanisms employing Gaussian Elimination could be used to decode the row panty eπor bits and the global syndrome code to denve a row syndrome code. Figures 20-35 are tables which illustrate another embodiment of a system which employs the linearly shifted technique for global syndrome code generation as descnbed above. In the embodiment depicted by Figures 20-35, 267 bits of data are protected with 21 check bits. The code is defined m terms of 288-bit blocks. The data bits withm a block are denoted D[266:0]. The 21 check bits withm a block consist of 8 paπty bits P[7:0] and a global syndrome code consistmg of 13 twisted check bits T[12.0]. The 288-bit block is divided into eight groups, denoted G[0] through G[7], each of which contams 36 bits. This code can coπect one eπor per group G[0] through G[7], provided that all of the eπors occur in the same position withm each group. The panty bits P[7 0] and twisted check bits T[12.0] are calculated according to the tables of Figures 20-27. Each table lists the 36 bit positions within the group and the contents of each bit position. For example, G[0][0] contams P[0], G[0][29] contams D[23], and G[l][32] contams T[6], and so on. The remammg rows m each table give the rules for calculating the check bits. Each check bit is the exclusive-or of certain values. More particularly, the values which contribute to a particular check bit are mdicated usmg a "1" at the appropnate positions m the tables For example, parity bit P[0] is the exclusive-or of G[0][35: 1], Since the paπty bits P[7:0] depend upon certain ones of the twisted check bits T[12.0], the twisted check bits T[12 ] can be calculated first, and subsequently the paπty bits P[7:0]. It is noted that m this example, there are total of 36 entnes in each group Accordmgly, the system may be constructed such that a total of 36 memory chips are used, each stoπng a bit from a coπespondmg position of each group. When the data are received, panty eπor bits Q[7 0] and regenerated twisted check bits V[12.0] (the regenerated global ECC) are calculated by taking the exclusive-or of the received values, m accordance with the assignment tables of Figures 28-35. In addition, a global panty eπor bit Q may be calculated taking the exclusive-or of Q[7:0]. In certain cases, it is necessary to generate an untwisted syndrome U[5.0] (1 e., the row or group syndrome code). This untwisted syndrome is a function of V[12:0] and Q[6:0] As illustrated m Figure 36 and like the previous example, the untwisted syndrome U[5O] can be calculated usmg parallel shift registers, 333 and 334, which are similar to the registers 133 and 134 illustrated in Figure 10 Shift register 333 contams 7 cells, A[6 0], and shift register 334 contams 13 cells, B[12.0]. Each cell holds a single bit. The shift registers 333 and 334 are separately controlled. When a shift register is made to shift, it transfers the contents of each cell one position to the nght, outputs the value that was in its right-most cell, and loads a "0" mto its left-most cell. Similar to the previous description, it is also possible to calculate the XOR of shift register 333 and shift register 334 and store the contents mto shift register 334 (shift register 333 is logically padded with 0s in positions 12:7 when the XOR is computed). The untwisted syndrome U[5:0] is calculated as follows: 1 Load Q[ι] in cell A[ι], 0< l < 6, and load V[ι] in cell B[ι], 0 < I < 12. 2. While A[0] contams 0. a Shift both shift register 333 and shift register 334 right one position, discarding the values shifted out 3. For ι = 0 through 5: a. Set U[ι] = B[0]. b. If B[0] = 1 , calculate the XOR of shift register 333 and shift register 334 and place the result m shift register 334 (that is, B[j] = A[j] XOR B[j], 0 < j < 6). c. Shift shift register 334 nght one position, discardmg the value shifted out. Given Q, Q[7:0], and U[5.0], eπors are coπected as follows. The descnption of what type of eπor has occuned assumes that the eπors are withm the considered type (namely 0 to 8 eπors, all m the same position within their group) 1. IfQ = 0: a. If Q[7:0] = 00000000, no eπor has occuned. b. If Q[7:0] ≠ 00000000, a coπectable eπor has occuned. Complement the bits in all locations of the form G[ι]|j], 0 < I < 7, 0 < j < 35, where Q[ι] = 1 and j = U[5:0] (viewed as a binary number). (If U[5:0] > 35, an unconectable eπor not m the class of considered eπors has occuned) 2. If Q = 1. A coπectable eπor has occuned Complement the bits m all locations of the form G[ι]D 0 < l < 7, 0 <j < 35, where Q[ι] = 1 andj = U[5.0] (viewed as a binary number). (If U[5:0] > 35, an unconectable eπor not m the class of considered eπors has occuned) It is noted that no eπor has occuned if Q[7.0] = 00000000 It is also noted that U[5 0] only needs to be calculated m cases l.b and 2 (which indicates a coπectable eπor has occuned). In the embodiment illustrated by Figures 20-35, the global eπor coπection code formed by twisted check bits T[12 0] are generated directly from a predetermined combmation of the data bits. Each twisted check bit T[12:0] is stored at a position within each group such that it is the only check bit that covers that position. For example, as illustrated m Figure 28, the twisted check bit TO is stored at position G[0][01], and is the only check bit which contnbutes to the regenerated twisted check bit V[0], and so. In addition, it is noted that the parity bits P[7.0] for each data- group mclude certain twisted check bits stored withm that group Smce the panty bits are stored withm position [00] of the various groups, further efficiency may be attained. Alternative embodiments are also possible. For example, m another contemplated embodiment, the eπor coπection codes generated for the logical groups are "cyclically" shifted (rather than linearly shifted) and are XORed together to derive the global eπor coπection code as well as the regenerated eπor coπection code Embodiments that produce P global eπor coπection bits are possible, where P is the smallest pnme number such that P > X and 2 Λ (P-1)>=M. In such embodiments, and where a total of C = X + P check bits are required. Such techniques may utilize fewer overall check bits than embodiments which employ linearly shifted techniques as described previously. Still additional embodiments may also be possible wherem a global syndrome code is generated usmg other methods In such embodiments, the stored or transmitted global eπor coπection code is dependent upon or equivalent to a logical combination of a set of eπor coπection codes associated with the logical groups of the ongmal data, while the regenerated global eπor coπection code is dependent upon or equivalent to a logical combination of a set of eπor coπection codes associated with the logical groups of the received or retneved data bits. The global syndrome code is generated such that, with knowledge of the specific logical groups that have a smgle bit eπor, a value indicative of the location of the eπor in such groups may be denved from the syndrome code. The overall number of bits forrmng the global syndrome code and the panty bits for each logical group is smaller than the overall number of bits needed for the eπor coπection codes mdividually associated with the logical groups. Finally, Figure 37 illustrates an embodiment wherem a data block protected m accordance with the foregoing techniques is conveyed through a plurality of communication channels 410A....410M. Each communication channel 410A-410M is illustrative of, for example, a wire or other transmission medium. Similar to the previous embodiments, different bits m each logical group are conveyed through different communication channels. While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the mvention scope is not so limited. Any variations, modifications, additions and improvements to the embodiments descπbed are possible. These variations, modifications, additions and improvements may fall withm the scope of the mvention as detailed withm the following claims.

Claims: Claims: 1. A system for coπectmg data eπors, compnsing. a check bits generation unit for receiving a plurality of data bits, wherem said check bits generation unit is configured to generate a parity bit coπesponding to each of a plurality of groupmgs of said data bits, and wherein said check bits generation unit is configured to generate a global eπor coπection code indicative of a predetermined combmation of a set of eπor coπection codes each mdividually associated with a coπespondmg one of said plurality of groupmgs; an eπor coπection unit coupled to receive said plurality of data bits, said paπty bit for each of said plurality of groupings, and said global eπor coπection code, wherem said eπor coπection unit is configured to generate a parity eπor bit for each of said plurality of groupmgs of said data bits as received, and wherein said eπor coπection unit is configured to generate a syndrome code indicatmg a position of an eπor withm any of said plurality of groupmgs, wherein said syndrome code depends upon a difference between said global eπor coπection code and a regenerated global eπor coπection code. 2. The system as recited in Claim 1 wherein said global eπor coπection code is equivalent to a predetermined function of said set of eπor coπection codes. 3 The system as recited m Claim 2 wherem each bit of said global eπor coπection code is equivalent to an exclusive OR of a predetermined set of bits of said set of eπor coπection codes. 4. The system as recited in Claim 1 wherein said global eπor coπection code is a function of said set of eπor coπection codes such that a combmation of said global eπor coπection code, said regenerated global eπor coπection code, and said panty eπor bits uniquely determine said syndrome code. 5. The system as recited in Claim 1 wherem said check bits generation unit is configured to generate an eπor coπection code for each of said plurality of groupings of said data bits 6 The system as recited in Claim 1 wherem said global eπor coπection code is denved by bit-wise shiftmg at least some of said eπor coπection codes and by XORmg aligned bits of a plurality of resultmg shifted eπor coπection codes. 7. The system as recited m Claim 6 wherem said plurality of data bits mclude a total of X groupmgs, and wherein said global eπor coπection code is derived by shifting said eπor coπection code for a given ith group by l bit positions, wherem I = 0 to X-l, and by XORmg a plurality of resultmg shifted eπor coπection codes together. 8. The system as recited m Claim 7 wherem said global eπor coπection code is denved by linearly shiftmg said eπor coπection code for said given ith group by I bit positions. 9 The system as recited m Claim 7 wherem said global eπor coπection code is derived by cyclically shifting said eπor coπection code for said given ith group by l bit positions. 10 The system as recited m Claim 6 wherein said eπor coπection unit is configured to generate a global syndrome code by XORing said global eπor coπection code with said regenerated global eπor coπection code. 11. The system as recited in Claim 10 wherem said plurality of data bits mclude a total of X groupmgs, and wherein said global eπor coπection code is derived by shifting said eπor coπection code for a given ith group by l bit positions, wherem I = 0 to X-l, and by XORmg a plurality of resultmg shifted eπor coπection codes together. 12. The system as recited m claim 11 wherem each bit of said global syndrome code mdicates whether the paπty associated with an aligned column of bits of said plurality of resultmg shifted eπor coπection codes is the same as a panty associated with coπespondmg aligned bits of a plurality of shifted eπor coπection codes based upon said regenerated eπor coπection codes for said plurality of groupmgs. 13. The system as recited m claim 11 wherein said paπty eπor bit for each of said plurality of groups and said global syndrome code are used to generate said syndrome code indicatmg a position of said eπor withm any of said plurality of groupmgs. 14. The system as recited m claim 13 wherein said eπor coπection unit is further configured to coπect said eπor withm any of said plurality of groupmgs 15 The system as recited m claim 1 further compnsing a component into which said plurality of data bits, said parity bit for each of said plurality of groupmgs, and said global eπor coπection code are conveyed pnor to bemg provided to said eπor coπection unit. 16. The system as recited in claim 15 wherem said component includes a plurality of memory chips. 17. The system as recited in claim 15 wherein said component includes a plurality of communication paths 18. The system as recited in claim 16 wherem each bit withm a particular grouping of data bits is stored m a separate memory chip, whereby no two bits of any given groupmg are stored withm the same memory chip 19. The system as recited m claim 18 wherein bits at coπesponding positions withm the said plurality of groupmgs are stored withm the same memory chip 20. The system as recited m claim 1 wherein said eπor coπection code for each of said plurality of groupings is a single eπor coπection Hamming code. 21. A method for coπecting data eπors compnsing- receiving a plurality of data bits; generatmg a parity bit coπespondmg to each of a plurality of groupmgs of said data bits, generatmg a global eπor coπection code indicative of a predetermined combination of a set of eπor coπection codes each individually associated with a coπesponding one of said plurality of groupings; receiving said plurality of data bits, said parity bit for each of said plurality of groupmgs, and said global eπor coπection code; generating a paπty eπor bit for each of said plurality of groupmgs of said data bits as received; and generatmg a syndrome code indicating a position of an eπor within any of said plurality of groupmgs, wherein said syndrome code depends upon a difference between said global eπor coπection code and a regenerated global eπor coπection code. 22. The method as recited m Claim 21 wherem said global eπor coπection code is equivalent to a predetermined function of said set of eπor coπection codes. 23 The method as recited in Claim 22 wherein each bit of said global eπor coπection code is equivalent to an exclusive OR of a predetermined set of bits of said set of eπor coπection codes 24. The method as recited m Claim 21 wherem said global eπor coπection code is a function of said set of eπor conection codes such that a combination of said global eπor coπection code, said regenerated global eπor coπection code, and said panty eπor bits uniquely determine said syndrome code. 25. The method as recited m Claim 21 wherem said check bits generation unit is configured to generate an eπor coπection code for each of said plurality of groupmgs of said data bits. 26. The method as recited m Claim 21 wherem said global eπor coπection code is denved by bit- wise shiftmg at least some of said eπor coπection codes and by XORmg aligned bits of a plurality of resultmg shifted eπor conection codes. 27 The method as recited in Claim 26 wherem said plurality of data bits mclude a total of X groupmgs, and wherem said global eπor conection code is denved by shifting said eπor conection code for a given ith group by 1 bit positions, wherem 1 = 0 to X-l, and by XORmg a plurality of resultmg shifted eπor conection codes together. 28. The method as recited in Claim 27 wherein said global eπor coπection code is denved by linearly shiftmg said eπor coπection code for said given ith group by I bit positions. 29 The method as recited m Claim 27 wherein said global eπor conection code is derived by cyclically shifting said eπor coπection code for said given ith group by I bit positions. 30. The method as recited m Claim 26 further compnsing generating a global syndrome code by XORmg said global eπor coπection code with said regenerated global eπor conection code. 31. The method as recited m Claim 30 wherem said plurality of data bits include a total of X groupmgs, and wherein said global eπor coπection code is deπved by shiftmg said eπor coπection code for a given ith group by l bit positions, wherein i = 0 to X-l, and by XORing a plurality of resulting shifted eπor conection codes together. 32. The method as recited m claim 31 wherem each bit of said global syndrome code indicates whether the panty associated with an aligned column of bits of said plurality of resultmg shifted eπor coπection codes is the same as a panty associated with coπespondmg aligned bits of a plurality of shifted eπor coπection codes based upon said regenerated eπor conection codes for said plurality of groupmgs. 33. The method as recited in claim 32 wherem said panty eπor bit for each of said plurality of groups and said global syndrome code are used to generate said syndrome code indicatmg a position of said eπor withm any of said plurality of groupmgs 34. The method as recited m claim 33 further compnsing coπecting said eπor withm any of said plurality of groupings. 35. The method as recited m claim 21 further compnsmg conveymg said plurality of data bits, said panty bit for each of said plurality of groupmgs, and said global eπor coπection code mto a component 36 The method as recited in claim 35 wherem said component mcludes a plurality of memory chips. 37. The method as recited in claim 35 wherein said component includes a plurality of communication paths. 38. The method as recited m claim 36 wherem each bit withm a particular groupmg of data bits is stored m a separate memory chip, whereby no two bits of any given groupmg are stored withm the same memory chip. 39. The method as recited m claim 38 wherem bits at coπespondmg positions withm the said plurality of groupmgs are stored withm the same memory chip. 40. The method as recited m claim 21 wherein said eπor coπection code for each of said plurality of groupings is a single eπor coπection Hammmg code.

Inventors: CYPHER ROBERT, CYPHER, ROBERT
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 2000
Main classification: G06F11
IPC codes: G06F11, G06F12, G11B20, H03M13, H04L1