Abstract:
An apparatus and method is discussed using a parity check matrix in order to acheive correction and detection of errors particularly pertaining to detection data fetched from a wrong address. The code structure enhances utilization of chip reliability by encoding and decoding digital signals through the utilization of a parity check matrix and parity bits generated from system address bits of a computer system with k symbols and b bits per symbol.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally directed to error correction, decoding and encoding systems found in digital devices. The present invention incorporates by reference the invention disclosed in the application filed Oct. 3, 1995 U.S. Ser. No. 08/538,691 entitled &#34;error detection and correction code for four-bit-per-chip memory system&#34; and directed to multi-bit-per chip computer memory systems in which error correction and detection apparatus are provided to acheive correction of all single symbol errors and the detection of all single symbol errors occurring with a single bit error in a second symbol. More particularly, the present invention is directed to a memory assignment scheme, which assigns error correction code words to selected array modules, and incorporates address parity bits in this assignment scheme in order to provide an error correction and detection method and apparatus capable of detecting a variety of errors, including the detection of data fetched from an erroneous address. 
     In a computer system having b bits of memory available per chip, the failure of each chip can cause one to b erroneous bits in the output data depending upon the failure mode and the data originally stored. A b bit symbol error pattern is used to indicate one of all of the possible 2 b  -1 error patterns in the b bits for each computer chip. Normal error correction and detection systems employed in such situations would involve the use of a single error correction (SEC) and double error detection (DED) code. The SEC-DED code, however, is neither effective in correcting symbol errors generated from single chip failures, nor is effective in detecting symbol errors generated from double chip failures. On the other hand, a single symbol error correction and double symbol error detection code, a so-called SSC-DSD code, would provide an effective error correction and detection method to acheive high reliability in a b bit per chip memory. However, SSC-DSD codes require more check bits and more memory chips than are actually necessary for the problem. In particular, for a 64-bit data bus, the required SSC-DSD code would require four redundant memory chips for every 16 data chips to implement a (78,64) SSC-DSD code. Such codes would be 78 bits in length with 64 information bits and 14 check bits. Thus a memory system implementing this code would require 20 chips based upon the above memory organization. 
     Recognizing the stringent requirements of the SSC-DSD memory system, the present inventors devised a new coding scheme as described in detail in U.S. Ser. No. 08/538,691, in which the number of redundant chips are decreased from 4 to 3 chips for a (76,64) code for each set of 16 chips. This code is then used to correct all single symbol errors and detect all single symbol errors plus a single bit error in a second symbol. In certain instances, however, although the data in storage is error free, an error condition can occur if data is fetched from the wrong address in memory. However, the invention introduced in U.S. Ser. No. 08/538,691 never addressed the issue of detection of data fetched from an erroneous address. 
     The present invention utilizes the coding scheme suggested in U.S. Ser. No. 08/538,691, but improves upon the code by generating address parity bits from system address bits and later including these in the generation of ECC word. Accordingly, a more efficient and more reliable improved computer memory system is provided through the use of the present invention. All of these are desirable results. 
     This application is related in subject matter to two other applications, Ser. Nos. 08/570,446 and 08/570,447 both filed on the same day as this application. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, digital signal encoding and decoding is accomplished through the utilization of a parity check matrix and parity bits generated from system address bits of a computer system with m symbols and b bits per symbol. The method of encoding data bits which are b bits in length comprise generating first the address parity bit or bits from the system address data bits. The address parity bit or bits are then used in conjunction with other data bits in generating a multiplicity, k, of multisegment final check bit vectors in which each segment of said final check bit vector is b bits long. The data bits and the final check bit vectors are then stored in different locations in the memory array of the computer. 
     A similar but reverse methodology is used for decoding the electrical signals for correcting errors in symbols which are b bits in length. First, the information pertaining to the previously stored data bits as well as the final check bit vectors are retrieved from the memory arrays. The address parity bits are generated using the previously stored data. Similarly, using the previously stored data as well as the generated parity bits a multiplicity, k, of multisegment final syndrome vectors in which each segment in each of said k final syndrome vector is kb bits long is generated and the final syndrome vector is then decoded to determine any possible errors. If any errors are detected, the signal may be corrected according to the symbol bits and type of error involved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is an overall block diagram illustrating a memory system in which a memory word is stored on k separate chips each providing b bits to a computer memory system register; 
     FIG. 2 is a functional block diagram illustrating the 15 possible error patterns for the case in which there are four bits per chip; 
     FIG. 3A and 3B are functional block diagrams illustrating the limits of error detectability for the circuits of the present invention; 
     FIG. 4 is a general ECC (Error Correction Code) flow diagram, illustrating various staging registers and line buffers involved in the store and fetch operations for the present invention; 
     FIG. 5 is an illustration of the processing of bits during a data store operation; 
     FIG. 6 is a further illustration of the operation of a data store command according to the concepts presented by the present invention; 
     FIG. 7 is an illustration of the processing of bits during a data fetch operation; 
     FIG. 8 is a further illustration of the operation of a data fetch command according to the concepts presented by the present invention; 
     FIG. 9 illustrates the arrangement of data in memory modules on the SIMM array(s). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention described below is particularly shown with respect to a particular example, namely the construction of a (76,64) code for a four-bit per chip computer memory system. However, the ideas presented herein are extensible to the general b bit per chip situation. In particular, it is noted that the number of rows of b by b submatrices in the parity check matrix is determined by the error correction capabilities that are desired. In the particular instance of the codes constructed in accordance with the present invention, three rows of by b submatrices are sufficient to provide the desired correction and detection capabilities. Likewise in general, codes constructed in accordance with the present invention will have k symbols. Each symbol will be represented by b bits. Since three of the symbols are employed for redundancy, there are thus k-3 information symbols and 3 symbols which comprise check bits. 
     With respect to the present invention for a (76,64) code, the parity check matrix for this code including an embodiment utilizing two address parity bits is given below: 
     
                                           TABLE I__________________________________________________________________________                                         SYMBOL                                              ADDRESS                                         ID   PARITYDATA                                  ECC     (CHIP)                                              BITS 1   2   3   4   5   6   7   8   9   10   11   12   13   14   15   16                                  17   18   19                                         ID   20__________________________________________________________________________1000 1000 1000 1000 1000 1000 1000 1000 1001 1101 1111 0011 1100 11001110 0110                             1000 0000 0000                                              01                                              0100 0100 0100 0100                                              0100 0100 0100 0100                                              0111 1011 1010 0010                                              1000 1000 1001                                              1101 0100 0000 0000                                               11                                              0010 0010 0010 0010                                              0010 0010 0010 0010                                              0111 0110 1100 1111                                              0011 0011 1001                                              1011 0010 0000 0000                                               10                                              0001 0001 0001 0001                                              0001 0001 0001 0001                                              1110 1101 1000 1010                                              0010 0010 0111                                              0110 0001 0000 0000                                               11                                              0110 1000 0010 1011                                              1010 1101 0101 1110                                              1000 0001 1000 0111                                              1000 1110 1000                                              0100 0000 1000 0000                                               10                                              1101 0100 0001 0110                                              0101 1011 1111 1001                                              0100 0011 0100 1110                                              0100 1001 0100                                              1100 0000 0100 0000                                               01                                              1011 0010 1010 1101                                              1000 0110 0100 1001                                              0010 0101 0010 1110                                              0010 1001 0010                                              0001 0000 0010 0000                                               00                                              0110 0001 0101 1011                                              0100 1101 1100 0111                                              0001 1111 0001 1001                                              0001 0111 0001                                              0011 0000 0001 0000                                               00                                              1000 1110 0111 1010                                              1001 0010 0110 0001                                              0101 1000 1011 1000                                              0110 1000 0100                                              1000 0000 0000 1000                                               00                                              0100 1001 1110 0101                                              0111 0001 1101 0011                                              1111 0100 0110 0100                                              1101 0100 1100                                              0100 0000 0000 0100                                               00                                              0010 1001 1110 1000                                              0111 1010 1011 0101                                              0100 0010 1101 0010                                              1011 0010 0001                                              0010 0000 0000 0010                                               01                                              0001 0111 1001 0100                                              1110 0101 0110 1111                                              1100 0001 1011 0001                                              0110 0001 0011                                              0001 0000 0000 0001                                               10__________________________________________________________________________ 
    
     DATA STORE OPERATION 
     During a store operation, in accordance with the embodiment represented by the parity check matrix shown in Table I, each of the 12 ECC check bits is generated. The 12 ECC check bits are generated in two stages. During the first stage, a partial ECC check bit is generated by exclusive OR circuits which take a subset of 64 data bits as indicated by the 1&#39;s in each row of the parity check matrix in Table I. For example, the first partial check bit is generated by exclusive-ORing data found at positions: 1, 5, 9, 13, 17, 21, 25, 29, 33, 36, 37, 38, 40, 41, 42, 43, 44, 47, 48, 49, 50, 53, 54, 57, 58, 59, 62 and 63. 
     The address parity bits are generated from selective combination of data from system address bits as discussed later in detail. During the second stage the partial check bits are combined with the address parity bits to generate the final 12 ECC check bits. 
     The set of 64 data bits together with the 12 check bits form an ECC word. Therefore, it seems that in the embodiment presented in Table I, when the two address parity bits are included, the ECC word is really a (78,66) code word. However, since the address parity bits can always be generated from the system address bits, it is unnecessary for them to be stored as part of the ECC word. Therefore, the code is still a (76,64) code since only 76 bits per code word are stored into the memory array or a SIMM (Single-Inline-Memory-Module) as shown in FIG. 9. The address parity bits are regenetated separately during the fetch operation from the previously stored system address bits. 
     The ECC word is divided into 19 groups of 4 bit symbols. In storing data, the ECC word is stored in 19 differnt chips. That is each symbol is stored on a different chip. This situation is illustrated in FIG. 1 wherein memory system 10 is shown to be organized using m chips with b bits being delivered from each chip at a time. Thus, the ECC word in register 25 is mb bits long. For the example illustrated herein, m=19 and b=4. 
     When an array chip, say chip i, fails the data stored in that chip may or may not be in error depending on the data stored. If the data is in error, the number of errors may be one or more than one. FIG. 2 illustrates all of the possible error patterns 2 b  -1=15 for a failed chip i. FIG. 3A and 3B illustrates the limits of error detectability in the present invention. FIG. 3A particularly shows the situation in which chip i produces from one to four errors while only a single error occurs in chip j. Reference numeral 20 in FIGS. 1 through 3 refer to the individual memory chips. FIG. 3B shows the situation in which chip i produces from one to four errors while only a single error occurs in the address parity bit. 
     At the outset of a store operation data bits are stored in a plurality of storage buffers. In the example illustrated in Table II, given below, the 64 data bits are first stored in two storage buffers named STBUF --  H --  D(0 . . . 31) and STBUF --  L --  D(0 . . . 31), respectively. There are 32 bits of data in each storage buffer and these bits are renamed as SEC --  0 --  H(0 . . . 31) and SEC --  0 --  L(0 . . . 31) when wired into the memory modules, namely the modules of a SIMM. 
     
                                           TABLE II__________________________________________________________________________STBUF.sub.-- H.sub.-- D(0..31)        11          1111    1111                     2222   2222                               22330123 4567   8901          2345    6789                     0123   4567                               8901__________________________________________________________________________1000 1000   1000          1000    1000                     1000   1000                               10000100 0100   0100          0100    0100                     0100   0100                               01000010 0010   0010          0010    0010                     0010   0010                               00100001 0001   0001          0001    0001                     0001   0001                               00010110 1000   0010          1011    1010                     1101   0101                               11101101 0100   0001          0110    0101                     1011   1111                               10011011 0010   1010          1101    1000                     0110   0100                               10010110 0001   0101          1011    0100                     1101   1100                               01111000 1110   0111          1010    1001                     0010   0110                               00010100 1001   1110          0101    0111                     0001   1101                               00110010 1001   1110          1000    0111                     1010   1011                               01010001 0111   1001          0100    1110                     0101   0110                               1111__________________________________________________________________________                          STORE.sub.-- ADDRP.sub.-- D(0..1)                            ∥STBUF.sub.-- L.sub.-- D(0..31)                   SEC.sub.-- O.sub.-- C(0..11)                            ∥  11    1111       1111          2222             2222                2233      11                            ∥0123   4567 8901    2345       6789          0123             4567                8901                   0123                      4567                         8901                            01__________________________________________________________________________1001   1101 1111    0011       1100          1100             1110                0110                   1000                      0000                         0000                            010111   1011 1010    0010       1000          1000             1001                1101                   0100                      0000                         0000                            110111   0110 1100    1111       0011          0011             1001                1011                   0010                      0000                         0000                            101110   1101 1000    1010       0010          0010             0111                0110                   0001                      0000                         0000                            111000   0001 1000    0111       1000          1110             1000                0100                   0000                      1000                         0000                            100100   0011 0100    1110       0100          1001             0100                1100                   0000                      0100                         0000                            010010   0101 0010    1110       0010          1001             0010                0001                   0000                      0010                         0000                            000001   1111 0001    1001       0001          0111             0001                0011                   0000                      0001                         0000                            000101   1000 1011    1000       0110          1000             0100                1000                   0000                      0000                         1000                            001111   0100 0110    0100       1101          0100             1100                0100                   0000                      0000                         0100                            000100   0010 1101    0010       1011          0010             0001                0010                   0000                      0000                         0010                            011100   0001 1011    0001       0110          0001             0011                0001                   0000                      0000                         0001                            10__________________________________________________________________________ 
    
     The address parity bits are located in address buffers indicated as STORE --  ADDRP --  D(0 . . . 1) in Table II above. The address parity bits are generated from selective combination of data from system address bits as shown in the example provided below in Table III. 
     
                                           TABLE III__________________________________________________________________________ADDRESS PARITY BITS0 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 - - - -xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxSYSTEM ADDRESS2 2 3 3 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 38 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1BYTE 3    | BYTE 4 |          BYTE 5 |                   BYTE 6 |                            BYTE 74 5 6 7    | 0 1 2 3 4 5 6 7        |          0 1 2 3 4 5 6 7                 |                   0 1 2 3 4 5 6 7                          |                            0 1 2 3 4 5 6 7xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx4 MbitR R R R R R R R R R R C C C C C C C C C E C|CAS|                              BYTE1 cardE 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 O 0|DEC|                              OFFSET16  MbitR R R R R R R R R R R R R C C C C C C C C C E C|CAS.vertline.                             BYTE1 cardE1110 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 O 0|DEC.vertline.                             OFFSETNOT USED64  MbitR R R C C R R R R R R R R R R C C C C C C C C C E C|CAS.vertline.                         BYTE1 cardE11101110 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 O 0|DEC.vertline.                         OFFSET4 MbitR C R R R R R R R R R R C C C C C C C C C E C|CAS|                              BYTE2 cardE 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 D O 0|DEC|                              OFFSET16  MbitR R R C R R R R R R R R R R C C C C C C C C C E C|CAS.vertline.                           BYTE2 cardE1011 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 D O 0|DEC.vertline.                           OFFSETNOT USED64  MbitR C R R C C R R R R R R R R R R C C C C C C C C C EC|CAS|      BYTE2 cardE10111011 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 D O0|DEC|      OFFSET4 MbitR C C R R R R R R R R R R C C C C C C C C C E C|CAS.vertline.                             BYTE4 cardE 2 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 D D O 0|DEC.vertline.                             OFFSET16  MbitR R R C C R R R R R R R R R R C C C C C C C C C E C|CAS.vertline.                         BYTE4 cardE1110 2 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 D D O 0|DEC.vertline.                         OFFSETNOT USED64  MbitR C C R R C C R R R R R R R R R R C C C C C C C C C EC|CAS|      BYTE4 cardE11101110 2 1 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 D D O0|DEC|      OFFSET__________________________________________________________________________ CD means memory card selection bit C0-C11 means Column Address 0-11 R0-R11 means Row Address 0-11 EO means even/odd lines (on STC interleaving) RE means RAS A/B decode 
    
     STORE --  ADDRP --  D(0) is generated from the parity of system address bits 1, 2, 5-14, 26, 27 and 30. Similarly, STORE --  ADDRP --  D(1) is the parity of system address bits 0, 3, 4, 5-25, and 31. As illustrated in table III, the mapping of system address bits to memory module depends on the size of the modules. 
     The final ECC check bits are then generated by combining the contents of the STORE --  ADDRP with the data stored in the storage buffers STBUF and the data obtained from the partial check bits generated from the Exclusive-Or circuitry. In short, during the first stage, the partial check bits SY0(0 . . . 11) are generated from the high word data word STBUF --  H --  D(0 . . . 31) and SY1(0 . . . 11) are generated from the low data word STBUF --  L --  D(0 . . . 31). Then SY0 and SY1 and address parity bits are combined to produce the final check bits. The final 12 ECC check bits are stored in SEC --  0 --  C(0 . . . 11) buffer. The following equation illustrates in detail the generation of the 12 ECC check bits and their final storage in the SEC --  0 --  C(0 . . . 11) buffer. 
     The notation OPA(X||Y||Z) is used to describe a logic function that implements the odd parity of bits X,Y and Z. The generation of check bits is divided in two stages as mentioned earlier. 
     
         __________________________________________________________________________/***************** STORE ECC ******************************************//********************************************************************//*&gt;CHECKBIT GENERATION &lt;*//**********************************************************************/SY0(0)  := OPA(STBUF.sub.-- H.sub.-- D(0) ∥STBUF.sub.-- H.sub.-- D(4)∥STBUF.sub.-- H.sub.-- D(8) ∥STBUF.sub.-- H.sub.--D(12)  ∥STBUF.sub.-- H.sub.-- D(16)∥STBUF.sub.-- H.sub.--D(20)∥STBUF.sub.-- H.sub.-- D(24)∥STBUF.sub.-- H.sub.--D(28));SY1(0)  := OPA(STBUF.sub.-- L.sub.-- D(0) ∥STBUF.sub.-- L.sub.-- D(3)∥STBUF.sub.-- L.sub.-- D(4) ∥STBUF.sub.-- L.sub.--D(5)  ∥STBUF.sub.-- L.sub.-- D(7) ∥STBUF.sub.-- L.sub.--D(8) ∥STBUF.sub.-- L.sub.-- D(9) ∥STBUF.sub.-- L.sub.--D(10)  ∥STBUF.sub.-- L.sub.-- D(11)∥STBUF.sub.-- L.sub.--D(14)∥STBUF.sub.-- L.sub.-- D(15)∥STBUF.sub.-- L.sub.--D(16)  ∥STBUF.sub.-- L.sub.-- D(17)∥STBUF.sub.-- L.sub.--D(20)∥STBUF.sub.-- L.sub.-- D(21)∥STBUF.sub.-- L.sub.--D(24)  ∥STBUF.sub.-- L.sub.-- D(25)∥STBUF.sub.-- L.sub.--D(26)∥STBUF.sub.-- L.sub.-- D(29)∥STBUF.sub.-- L.sub.--D(30));SY0(1)  := OPA(STBUF.sub.-- H.sub.-- D(1) ∥STBUF.sub.-- H.sub.-- D(5)∥STBUF.sub.-- H.sub.-- D(9) ∥STBUF.sub.-- H.sub.--D(13)  ∥STBUF.sub.-- H.sub.-- D(17)∥STBUF.sub.-- H.sub.--D(21)∥STBUF.sub.-- H.sub.-- D(25)∥STBUF.sub.-- H.sub.--D(29));SY1(1)  := OPA(STBUF.sub.-- L.sub.-- D(1) ∥STBUF.sub.-- L.sub.-- D(2)∥STBUF.sub.-- L.sub.-- D(3) ∥STBUF.sub.-- L.sub.--D(4)  ∥STBUF.sub.-- L.sub.-- D(6) ∥STBUF.sub.-- L.sub.--D(7) ∥STBUF.sub.-- L.sub.-- D(8) ∥STBUF.sub.-- L.sub.--D(10)  ∥STBUF.sub.-- L.sub.-- D(14)∥STBUF.sub.-- L.sub.--D(16)∥STBUF.sub.-- L.sub.-- D(20)∥STBUF.sub.-- L.sub.--D(24)  ∥STBUF.sub.-- L.sub.-- D(27)∥STBUF.sub.-- L.sub.--D(28)∥STBUF.sub.-- L.sub.-- D(29)∥STBUF.sub.-- L.sub.--D(31));SY0(2)  := OPA(STBUF.sub.-- H.sub.-- D(2) ∥STBUF.sub.-- H.sub.-- D(6)∥STBUF.sub.-- H.sub.-- D(10)∥STBUF.sub.-- H.sub.--D(14)  ∥STBUF.sub.-- H.sub.-- D(18)∥STBUF.sub.-- H.sub.--D(22)∥STBUF.sub.-- H.sub.-- D(26)∥STBUF.sub.-- H.sub.--D(30));SY1(2)  := OPA(STBUF.sub.-- L.sub.-- D(1) ∥STBUF.sub.-- L.sub.-- D(2)∥STBUF.sub.-- L.sub.-- D(3) ∥STBUF.sub.-- L.sub.--D(5)  ∥STBUF.sub.-- L.sub.-- D(6) ∥STBUF.sub.-- L.sub.--D(8) ∥STBUF.sub.-- L.sub.-- D(9) ∥STBUF.sub.-- L.sub.--D(12)  ∥STBUF.sub.-- L.sub.-- D(13)∥STBUF.sub.-- L.sub.--D(14)∥STBUF.sub.-- L.sub.-- D(15)∥STBUF.sub.-- L.sub.--D(18)  ∥STBUF.sub.-- L.sub.-- D(19)∥STBUF.sub.-- L.sub.--D(22)∥STBUF.sub.-- L.sub.-- D(23)∥STBUF.sub.-- L.sub.--D(24)  ∥STBUF.sub.-- L.sub.-- D(27)∥STBUF.sub.-- L.sub.--D(28)∥STBUF.sub.-- L.sub.-- D(30)∥STBUF.sub.-- L.sub.--D(31));SY0(3)  := OPA(STBUF.sub.-- H.sub.-- D(3) ∥STBUF.sub.-- H.sub.-- D(7)∥STBUF.sub.-- H.sub.-- D(11)∥STBUF.sub.-- H.sub.--D(15)  ∥STBUF.sub.-- H.sub.-- D(19)∥STBUF.sub.-- H.sub.--D(23)∥STBUF.sub.-- H.sub.-- D(27)∥STBUF.sub.-- H.sub.--D(31));SY1(3)  := OPA(STBUF.sub.-- L.sub.-- D(0) ∥STBUF.sub.-- L.sub.-- D(1)∥STBUF.sub.-- L.sub.-- D(2) ∥STBUF.sub.-- L.sub.--D(4)  ∥STBUF.sub.-- L.sub.-- D(5) ∥STBUF.sub.-- L.sub.--D(7) ∥STBUF.sub.-- L.sub.-- D(8) ∥STBUF.sub.-- L.sub.--D(12)  ∥STBUF.sub.-- L.sub.-- D(14)∥STBUF.sub.-- L.sub.--D(18)∥STBUF.sub.-- L.sub.-- D(22)∥STBUF.sub.-- L.sub.--D(25)  ∥STBUF.sub.-- L.sub.-- D(26)∥STBUF.sub.-- L.sub.--D(27)∥STBUF.sub.-- L.sub.-- D(29)∥STBUF.sub.-- L.sub.--D(30));SY0(4)  := OPA(STBUF.sub.-- H.sub.-- D(1) ∥STBUF.sub.-- H.sub.-- D(2)∥STBUF.sub.-- H.sub.-- D(4) ∥STBUF.sub.-- H.sub.--D(10)  ∥STBUF.sub.-- H.sub.-- D(12)∥STBUF.sub.-- H.sub.--D(14)∥STBUF.sub.-- H.sub.-- D(15)∥STBUF.sub.-- H.sub.--D(16)  ∥STBUF.sub.-- H.sub.-- D(18)∥STBUF.sub.-- H.sub.--D(20)∥STBUF.sub.-- H.sub.-- D(21)∥STBUF.sub.-- H.sub.--D(23)  ∥STBUF.sub.-- H.sub.-- D(25)∥STBUF.sub.-- H.sub.--D(27)∥STBUF.sub.-- H.sub.-- D(28)∥STBUF.sub.-- H.sub.--D(29)  ∥STBUF.sub.-- H.sub.-- D(30));SY1(4)  := OPA(STBUF.sub.-- L.sub.-- D(0) ∥STBUF.sub.-- L.sub.-- D(7)∥STBUF.sub.-- L.sub.-- D(8) ∥STBUF.sub.-- L.sub.--D(13)  ∥STBUF.sub.-- L.sub.-- D(14)∥STBUF.sub.-- L.sub.--D(15)∥STBUF.sub.-- L.sub.-- D(16)∥STBUF.sub.-- L.sub.--D(20)  ∥STBUF.sub.-- L.sub.-- D(21)∥STBUF.sub.-- L.sub.--D(22)∥STBUF.sub.-- L.sub.-- D(24)∥STBUF.sub.-- L.sub.--D(29));SY0(5)  := OPA(STBUF.sub.-- H.sub.-- D(0) ∥STBUF.sub.-- H.sub.-- D(1)∥STBUF.sub.-- H.sub.-- D(3) ∥STBUF.sub.-- H.sub.--D(5)  ∥STBUF.sub.-- H.sub.-- D(11)∥STBUF.sub.-- H.sub.--D(13)∥STBUF.sub.-- H.sub.-- D(14)∥STBUF.sub.-- H.sub.--D(17)  ∥STBUF.sub.-- H.sub.-- D(19)∥STBUF.sub.-- H.sub.--D(20)∥STBUF.sub.-- H.sub.-- D(22)∥STBUF.sub.-- H.sub.--D(23)  ∥STBUF.sub.-- H.sub.-- D(24)∥STBUF.sub.-- H.sub.--D(25)∥STBUF.sub.-- H.sub.-- D(26)∥STBUF.sub.-- H.sub.--D(27)  ∥STBUF.sub.-- H.sub.-- D(28)∥STBUF.sub.-- H.sub.--D(31));SY1(5)  := OPA(STBUF.sub.-- L.sub.-- D(1) ∥STBUF.sub.-- L.sub.-- D(6)∥STBUF.sub.-- L.sub.-- D(7) ∥STBUF.sub.-- L.sub.--D(9)  ∥STBUF.sub.-- L.sub.-- D(12)∥STBUF.sub.-- L.sub.--D(13)∥STBUF.sub.-- L.sub.-- D(14)∥STBUF.sub.-- L.sub.--D(17)  ∥STBUF.sub.-- L.sub.-- D(20)∥STBUF.sub.-- L.sub.--D(23)∥STBUF.sub.-- L.sub.-- D(25)∥STBUF.sub.-- L.sub.--D(28)  ∥STBUF.sub.-- L.sub.-- D(29));SY0(6)  := OPA(STBUF.sub.-- H.sub.-- D(0) ∥STBUF.sub.-- H.sub.-- D(2)∥STBUF.sub.-- H.sub.-- D(3) ∥STBUF.sub.-- H.sub.--D(6)  ∥STBUF.sub.-- H.sub.-- D(8) ∥STBUF.sub.-- H.sub.--D(10)∥STBUF.sub.-- H.sub.-- D(12)∥STBUF.sub.-- H.sub.--D(13)  ∥STBUF.sub.-- H.sub.-- D(15)∥STBUF.sub.-- H.sub.--D(16)∥STBUF.sub.-- H.sub.-- D(21)∥STBUF.sub.-- H.sub.--D(22)  ∥STBUF.sub.-- H.sub.-- D(25)∥STBUF.sub.-- H.sub.--D(28)∥STBUF.sub.-- H.sub.-- D(31));SY1(6)  := OPA(STBUF.sub.-- L.sub.-- D(2) ∥STBUF.sub.-- L.sub.-- D(5)∥STBUF.sub.-- L.sub.-- D(7) ∥STBUF.sub.-- L.sub.--D(10)  ∥STBUF.sub.-- L.sub.-- D(12)∥STBUF.sub.-- L.sub.--D(13)∥STBUF.sub.-- L.sub.-- D(14)∥STBUF.sub.-- L.sub.--D(18)  ∥STBUF.sub.-- L.sub.-- D(20)∥STBUF.sub.-- L.sub.--D(23)∥STBUF.sub.-- L.sub.-- D(26)∥STBUF.sub.-- L.sub.--D(31));SY0(7)  := OPA(STBUF.sub.-- H.sub.-- D(1) ∥STBUF.sub.-- H.sub.-- D(2)∥STBUF.sub.-- H.sub.-- D(7) ∥STBUF.sub.-- H.sub.--D(9)  ∥STBUF.sub.-- H.sub.-- D(11)∥STBUF.sub.-- H.sub.--D(12)∥STBUF.sub.-- H.sub.-- D(14)∥STBUF.sub.-- H.sub.--D(15)  ∥STBUF.sub.-- H.sub.-- D(17)∥STBUF.sub.-- H.sub.--D(20)∥STBUF.sub.-- H.sub.-- D(21)∥STBUF.sub.-- H.sub.--D(23)  ∥STBUF.sub.-- H.sub.-- D(24)∥STBUF.sub.-- H.sub.--D(25)∥STBUF.sub.-- H.sub.-- D(29)∥STBUF.sub.-- H.sub.--D(30)  ∥STBUF.sub.-- H.sub.-- D(31));SY1(7)  := OPA(STBUF.sub.-- L.sub.-- D(3) ∥STBUF.sub.-- L.sub.-- D(4)∥STBUF.sub.-- L.sub.-- D(5) ∥STBUF.sub.-- L.sub.--D(6)  ∥STBUF.sub.-- L.sub.-- D(7) ∥STBUF.sub.-- L.sub.--D(11)∥STBUF.sub.-- L.sub.-- D(12)∥STBUF.sub.-- L.sub.--D(15)  ∥STBUF.sub.-- L.sub.-- D(19)∥STBUF.sub.-- L.sub.--D(21)∥STBUF.sub.-- L.sub.-- D(22)∥STBUF.sub.-- L.sub.--D(23)  ∥STBUF.sub.-- L.sub.-- D(27)∥STBUF.sub.-- L.sub.--D(30)∥STBUF.sub.-- L.sub.-- D(31));SY0(8)  := OPA(STBUF.sub.-- H.sub.-- D(0) ∥STBUF.sub.-- H.sub.-- D(4)∥STBUF.sub.-- H.sub.-- D(5) ∥STBUF.sub.-- H.sub.--D(6)  ∥STBUF.sub.-- H.sub.-- D(9) ∥STBUF.sub.-- H.sub.--D(10)∥STBUF.sub.-- H.sub.-- D(11)∥STBUF.sub.-- H.sub.--D(12)  ∥STBUF.sub.-- H.sub.-- D(14)∥STBUF.sub.-- H.sub.--D(16)∥STBUF.sub.-- H.sub.-- D(19)∥STBUF.sub.-- H.sub.--D(22)  ∥STBUF.sub.-- H.sub.-- D(25)∥STBUF.sub.-- H.sub.--D(26)∥STBUF.sub.-- H.sub.-- D(31));SY1(8)  := OPA(STBUF.sub.-- L.sub.-- D(1) ∥STBUF.sub.-- L.sub.-- D(3)∥STBUF.sub.-- L.sub.-- D(4) ∥STBUF.sub.-- L.sub.--D(8)  ∥STBUF.sub.-- L.sub.-- D(10)∥STBUF.sub.-- L.sub.--D(11)∥STBUF.sub.-- L.sub.-- D(12)∥STBUF.sub.-- L.sub.--D(17)  ∥STBUF.sub.-- L.sub.-- D(18)∥STBUF.sub.-- L.sub.--D(20)∥STBUF.sub.-- L.sub.-- D(25)∥STBUF.sub.-- L.sub.--D(28));SY0(9)  := OPA(STBUF.sub.-- H.sub.-- D(1) ∥STBUF.sub.-- H.sub.-- D(4)∥STBUF.sub.-- H.sub.-- D(7) ∥STBUF.sub.-- H.sub.--D(8)  ∥STBUF.sub.-- H.sub.-- D(9) ∥STBUF.sub.-- H.sub.--D(10)∥STBUF.sub.-- H.sub.-- D(13)∥STBUF.sub.-- H.sub.--D(15)  ∥STBUF.sub.-- H.sub.-- D(17)∥STBUF.sub.-- H.sub.--D(18)∥STBUF.sub.-- H.sub.-- D(19)∥STBUF.sub.-- H.sub.--D(23)  ∥STBUF.sub.-- H.sub.-- D(24)∥STBUF.sub.-- H.sub.--D(25)∥STBUF.sub.-- H.sub.-- D(27)∥STBUF.sub.-- H.sub.--D(30)  ∥STBUF.sub.-- H.sub.-- D(31));SY1(9)  := OPA(STBUF.sub.-- L.sub.-- D(0) ∥STBUF.sub.-- L.sub.-- D(1)∥STBUF.sub.-- L.sub.-- D(2) ∥STBUF.sub.-- L.sub.--D(3)  ∥STBUF.sub.-- L.sub.-- D(5) ∥STBUF.sub.-- L.sub.--D(9) ∥STBUF.sub.-- L.sub.-- D(10)∥STBUF.sub.-- L.sub.--D(13)  ∥STBUF.sub.-- L.sub.-- D(16)∥STBUF.sub.-- L.sub.--D(17)∥STBUF.sub.-- L.sub.-- D(19)∥STBUF.sub.-- L.sub.--D(21)  ∥STBUF.sub.-- L.sub.-- D(24)∥STBUF.sub.-- L.sub.--D(25)∥STBUF.sub.-- L.sub.-- D(29));SY0(10) := OPA(STBUF.sub.-- H.sub.-- D(2) ∥STBUF.sub.-- H.sub.-- D(4)∥STBUF.sub.-- H.sub.-- D(7) ∥STBUF.sub.-- H.sub.--D(8)  ∥STBUF.sub.-- H.sub.-- D(9) ∥STBUF.sub.-- H.sub.--D(10)∥STBUF.sub.-- H.sub.-- D(12)∥STBUF.sub.-- H.sub.--D(17)  ∥STBUF.sub.-- H.sub.-- D(18)∥STBUF.sub.-- H.sub.--D(19)∥STBUF.sub.-- H.sub.-- D(20)∥STBUF.sub.-- H.sub.--D(22)  ∥STBUF.sub.-- H.sub.-- D(24)∥STBUF.sub.-- H.sub.--D(26)∥STBUF.sub.-- H.sub.-- D(27)∥STBUF.sub.-- H.sub.--D(29)  ∥STBUF.sub.-- H.sub.-- D(31));SY1(10) := OPA(STBUF.sub.-- L.sub.-- D(1) ∥STBUF.sub.-- L.sub.-- D(6)∥STBUF.sub.-- L.sub.-- D(8) ∥STBUF.sub.-- L.sub.--D(9)  ∥STBUF.sub.-- L.sub.-- D(11)∥STBUF.sub.-- L.sub.--D(14)∥STBUF.sub.-- L.sub.-- D(16)∥STBUF.sub.-- L.sub.--D(18)  ∥STBUF.sub.-- L.sub.-- D(19)∥STBUF.sub.-- L.sub.--D(22)∥STBUF.sub.-- L.sub.-- D(27)∥STBUF.sub.-- L.sub.--D(30));SY0(11) := OPA(STBUF.sub.-- H.sub.-- D(3) ∥STBUF.sub.-- H.sub.-- D(5)∥STBUF.sub.-- H.sub.-- D(6) ∥STBUF.sub.-- H.sub.--D(7)  ∥STBUF.sub.-- H.sub.-- D(8) ∥STBUF.sub.-- H.sub.--D(11)∥STBUF.sub.-- H.sub.-- D(13)∥STBUF.sub.-- H.sub.--D(16)  ∥STBUF.sub.-- H.sub.-- D(17)∥STBUF.sub.-- H.sub.--D(18)∥STBUF.sub.-- H.sub.-- D(21)∥STBUF.sub.-- H.sub.--D(23)  ∥STBUF.sub.-- H.sub.-- D(25)∥STBUF.sub.-- H.sub.--D(26)∥STBUF.sub.-- H.sub.-- D(28)∥STBUF.sub.-- H.sub.--D(29)  ∥STBUF.sub.-- H.sub.-- D(30)∥STBUF.sub.-- H.sub.--D(31));SY1(11) := OPA(STBUF.sub.-- L.sub.-- D(0) ∥STBUF.sub.-- L.sub.-- D(1)∥STBUF.sub.-- L.sub.-- D(7) ∥STBUF.sub.-- L.sub.--D(8)  ∥STBUF.sub.-- L.sub.-- D(10)∥STBUF.sub.-- L.sub.--D(11)∥STBUF.sub.-- L.sub.-- D(15)∥STBUF.sub.-- L.sub.--D(17)  ∥STBUF.sub.-- L.sub.-- D(18)∥STBUF.sub.-- L.sub.--D(23)∥STBUF.sub.-- L.sub.-- D(26)∥STBUF.sub.-- L.sub.--D(27)  ∥STBUF.sub.-- L.sub.-- D(31));SEC.sub.-- O.sub.-- C(0)   := SY0(0)         XOR SY1(0)                XOR STORE ADDRP.sub.-- D(1);SEC.sub.-- O.sub.-- C(1)   := SY0(1)         XOR SY1(1)                XOR STORE.sub.-- ADDRP.sub.-- D(0)                XOR STORE.sub.-- ADDRP.sub.-- D(1);SEC.sub.-- O.sub.-- C(2)   := SY0(2)         XOR SY1(2)                XOR STORE.sub.-- ADDRP.sub.-- D(0);SEC.sub.-- O.sub.-- C(3)   := SY0(3)         XOR SY1(3)                XOR STORE.sub.-- ADDRP.sub.-- D(0)                XOR STORE.sub.-- ADDRP.sub.-- D(1);SEC.sub.-- O.sub.-- C(4)   := SY0(4)         XOR SY1(4)                XOR STORE.sub.-- ADDRP.sub.-- D(0);SEC.sub.-- O.sub.-- C(5)   := SY0(5)         XOR SY1(5)                XOR STORE.sub.-- ADDRP.sub.-- D(1);SEC.sub.-- O.sub.-- C(6)   := SY0(6)         XOR SY1(6)                ;SEC.sub.-- O.sub.-- C(7)   := SY0(7)         XOR SY1(7)                ;SEC.sub.-- O.sub.-- C(8)   := SY0(8)         XOR SY1(8)                ;SEC.sub.-- O.sub.-- C(9)   := SY0(9)         XOR SY1(9)                ;SEC.sub.-- O.sub.-- C(10)   := SY0(10)         XOR SY1(10)                XOR STORE ADDRP D(1);SEC.sub.-- O.sub.-- C(11)   := SY0(11)         XOR SY1(11)                XOR STORE.sub.-- ADDRP.sub.-- D(0);__________________________________________________________________________ 
    
     FIG. 4 is an illustration of the general ECC flow diagram. Logic steps 1 through 4, 100 through 400, are the various staging registers and line buffers used in the data store and data fetch operations. Logic steps 1 and 2, 100 and 200, involve different operations that finally allow the generation of ECC store operation 220 into the computer memory DRAM array 500. This information is later fetched from the DRAM array 500 to regenerate and decode the bits during the fetch operation, 300 through 400. 
     During the store operation, the ECC check bits 57 are generated from the 64 data bits 50 and the system address bits 52 as shown FIG. 5. The entire ECC word 58 is then stored into a memory array 500 to be used later during a fetch operation. FIG. 6 shows the storage of data in storage arrays 61, 62 and 63 before the generation of check bits 66,67 and 68. 
     Table I above is an embodiment of the present invention utilizing two address parity bits. Other embodiments are conceivable, however, in accordance to the concepts of the present invention where the number of address parity bits, can either be reduced to one or increased to more than two. Table IV given below provides another embodiment of the present invention where eight address parity bits are generated and used. 
     
                                           TABLE IV__________________________________________________________________________                                         SYMBOL                                              ADDRESS                                         ID   PARITYDATA                                  ECC     (CHIP)                                              BITS 1   2   3   4   5   6   7   8   9   10   11   12   13   14   15   16                                  17   18   19                                         ID    20   21__________________________________________________________________________1000 1000 1000 1000 1000 1000 1000 1000 1001 1101 1111 0011 1100 11001110 0110                             1000 0000 0000                                              0111 1101                                              0100 0100 0100 0100                                              0100 0100 0100 0100                                              0111 1011 1010 0010                                              1000 1000 1001                                              1101 0100 0000 0000                                               1111 0010                                              0010 0010 0010 0010                                              0010 0010 0010 0010                                              0111 0110 1100 1111                                              0011 0011 1001                                              1011 0010 0000 0000                                               1011 0111                                              0001 0001 0001 0001                                              0001 0001 0001 0001                                              1110 1101 1000 1010                                              0010 0010 0111                                              0110 0001 0000 0000                                               1100 1101                                              0110 1000 0010 1011                                              1010 1101 0101 1110                                              1000 0001 1000 0111                                              1000 1110 1000                                              0100 0000 1000 0000                                               1000 0011                                              1101 0100 0001 0110                                              0101 1011 1111 1001                                              0100 0011 0100 1110                                              0100 1001 0100                                              1100 0000 0100 0000                                               0110 0111                                              1011 0010 1010 1101                                              1000 0110 0100 1001                                              0010 0101 0010 1110                                              0010 1001 0010                                              0001 0000 0010 0000                                               0001 0010                                              0110 0001 0101 1011                                              0100 1101 1100 0111                                              0001 1111 0001 1001                                              0001 0111 0001                                              0011 0000 0001 0000                                               0000 1110                                              1000 1110 0111 1010                                              1001 0010 0110 0001                                              0101 1000 1011 1000                                              0110 1000 0100                                              1000 0000 0000 1000                                               0000 1100                                              0100 1001 1110 0101                                              0111 0001 1101 0011                                              1111 0100 0110 0100                                              1101 0100 1100                                              0100 0000 0000 0100                                               0001 1010                                              0010 1001 1110 1000                                              0111 1010 1011 0101                                              0100 0010 1101 0010                                              1011 0010 0001                                              0010 0000 0000 0010                                               0110 1000                                              0001 0111 1001 0100                                              1110 0101 0110 1111                                              1100 0001 1011 0001                                              0110 0001 0011                                              0001 0000 0000 0001                                               1011__________________________________________________________________________                                              1001 
    
     DATA FETCH OPERATION 
     During a Fetch operation, the reverse of the store process implemented during the store operation is used as shown in FIGS. 7 and 8. Data 70, 72 and 55, including the ECC word, is first fetched from memory 500, as shown in FIG. 7. FIG. 9 illustrates the circuits used during an ECC fetch operation. Data fetched from the DRAM array chips are named MDRMUX --  H(0 . . . 31) 92 for modules 1 to 8, MDRMUX --  L(0 . . . 31) 94 for modules 9 to 16 and MDRMUX --  C(0 . . . 11) 96 for modules 17 to 19. 
     When data is fetched from storage, a set of partial syndrome bits SYH(0 . . . 11) and SYL(0 . . . 11) is generated from the fetched data word from the DRAM array chips or the MDRMUX as illustrated in detail by the following equation: 
     
         __________________________________________________________________________/*********** FETCH ****************************************************//**********************************************************************//*&gt;CHECKBIT REGENERATION &lt;*//**********************************************************************/SYH(0):= OPA(MDRMUX.sub.-- H(0) ∥MDRMUX.sub.-- H(4) ∥MDRMUX.sub.-- H(8) ∥MDRMUX.sub.-- H(12)   ∥MDRMUX.sub.-- H(16)∥MDRMUX.sub.-- H(20)81MDRMUX.sub.-- H(24)∥MDRMUX.sub.-- H(28));SYL(0):= OPA(MDRMUX.sub.-- L(0) ∥MDRMUX.sub.-- L(3) ∥MDRMUX.sub.-- L(4) ∥MDRMUX.sub.-- L(5)   ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(8) ∥MDRMUX.sub.-- L(9) ∥MDRMUX.sub.-- L(10)   ∥MDRMUX.sub.-- L(11)∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(15)∥MDRMUX.sub.-- L(16)   ∥MDRMUX.sub.-- L(17)∥MDRMUX.sub.-- L(20)∥MDRMUX.sub.-- L(21)∥MDRMUX.sub.-- L(24)   ∥MDRMUX.sub.-- L(25)∥MDRMUX.sub.-- L(26)∥MDRMUX.sub.-- L(29)∥MDRMUX.sub.-- L(30));SYH(1):= OPA(MDRMUX.sub.-- H(1) ∥MDRMUX.sub.-- H(5) ∥MDRMUX.sub.-- H(9) ∥MDRMUX.sub.-- H(13)   ∥MDRMUX.sub.-- H(17)∥MDRMUX.sub.-- H(21)∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(29));SYL(1):= OPA(MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(2) ∥MDRMUX.sub.-- L(3) ∥MDRMUX.sub.-- L(4)   ∥MDRMUX.sub.-- L(6) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(8) ∥.sub.-- MDRMUX.sub.-- L(10)   ∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(16)∥MDRMUX.sub.-- L(20)∥MDRMUX.sub.-- L(24)   ∥MDRMUX.sub.-- L(27)∥MDRMUX.sub.-- L(28)∥MDRMUX.sub.-- L(29)∥MDRMUX.sub.-- L(31));SYH(2):= OPA(MDRMUX.sub.-- H(2) ∥MDRMUX.sub.-- H(6) ∥MDRMUX.sub.-- H(10)∥MDRMUX.sub.-- H(14)   ∥MDRMUX.sub.-- H(18)∥MDRMUX.sub.-- H(22)∥MDRMUX.sub.-- H(26)∥MDRMUX.sub.-- H(30));SYL(2):= OPA(MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(2) ∥MDRMUX.sub.-- L(3) ∥MDRMUX.sub.-- L(5)   ∥MDRMUX.sub.-- L(6) ∥MDRMUX.sub.-- L(8) ∥MDRMUX.sub.-- L(9) ∥MDRMUX.sub.-- L(12)   ∥MDRMUX.sub.-- L(13)∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(15)∥MDRMUX.sub.-- L(18)   ∥MDRMUX.sub.-- L(19)∥MDRMUX.sub.-- L(22)∥MDRMUX.sub.-- L(23)∥MDRMUX.sub.-- L(24)   ∥MDRMUX.sub.-- L(27)∥MDRMUX.sub.-- L(28)∥MDRMUX.sub.-- L(30)∥MDRMUX.sub.-- L(31));SYH(3):= OPA(MDRMUX.sub.-- H(3) ∥MDRMUX.sub.-- H(7) ∥MDRMUX.sub.-- H(11)∥MDRMUX.sub.-- H(15)   ∥MDRMUX.sub.-- H(19)∥MDRMUX.sub.-- H(23)∥MDRMUX.sub.-- H(27)∥MDRMUX.sub.-- H(31));SYL(3):= OPA(MDRMUX.sub.-- L(0) ∥MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(2) ∥MDRMUX.sub.-- L(4)   ∥MDRMUX.sub.-- L(5) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(8) ∥MDRMUX.sub.-- L(12)   ∥MDRMUX.sub.-- L(14)∥MbRMUX.sub.-- L(18)∥MDRMUX.sub.-- L(22)∥MDRMUX.sub.-- L(25)   ∥MDRMUX.sub.-- L(26)∥MDRMUX.sub.-- L(27)∥MDRMUX.sub.-- L(29)∥MDRMUX.sub.-- L(30));SYH(4):= OPA(MDRMUX.sub.-- H(1) ∥MDRMUX.sub.-- H(2) ∥MDRMUX.sub.-- H(4) ∥MDRMUX.sub.-- H(10)   ∥MDRMUX.sub.-- H(12)∥MDRMUX.sub.-- H(14)∥MDRMUX.sub.-- H(15)∥MDRMUX.sub.-- H(16)   ∥MDRMUX.sub.-- H(18)∥MDRMUX.sub.-- H(20)∥MDRMUX.sub.-- H(21)∥MDRMUX.sub.-- H(23)   ∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(27)∥MDRMUX.sub.-- H(28)∥MDRMUX.sub.-- H(29)   ∥MDRMUX.sub.-- H(30));SYL(4):= OPA(MDRMUX.sub.-- L(0) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(8) ∥MDRMUX.sub.-- L(13)   ∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(15)∥MDRMUX.sub.-- L(16)∥MDRMUX.sub.-- L(20)   ∥MDRMUX.sub.-- L(21)∥MDRMUX.sub.-- L(22)∥MDRMUX.sub.-- L(24)∥MDRMUX.sub.-- L(29));SYH(5):= OPA(MDRMUX.sub.-- H(0) ∥MDRMUX.sub.-- H(1) ∥MDRMUX.sub.-- H(3) ∥MDRMUX.sub.-- H(5)   ∥MDRMUX.sub.-- H(11)∥MDRMUX.sub.-- H(13)∥MDRMUX.sub.-- H(14)∥MDRMUX.sub.-- H(17)   ∥MDRMUX.sub.-- H(19)∥MDRMUX.sub.-- H(20)∥MDRMUX.sub.-- H(22)∥MDRMUX.sub.-- H(23)   ∥MDRMUX.sub.-- H(24)∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(26)∥MDRMUX.sub.-- H(27)   ∥MDRMUX.sub.-- H(28)∥MDRMUX.sub.-- H(31));SYL(5):= OPA(MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(6) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(9)   ∥MDRMUX.sub.-- L(12)∥MDRMUx.sub.-- L(13)∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(17)   ∥MDRMUX.sub.-- L(20)∥MDRMUX.sub.-- L(23)∥MDRMUX.sub.-- L(25)∥MDRMUX.sub.-- L(28)   ∥MDRMUX.sub.-- L(29));SYH(6):= OPA(MDRMUX.sub.-- H(0) ∥MDRMUX.sub.-- H(2) ∥MDRMUX.sub.-- H(3) ∥MDRMUX.sub.-- H(6)   ∥MDRMUX.sub.-- H(8) ∥MDRMUX.sub.-- H(10)∥MDRMUX.sub.-- H(12)∥MDRMUX.sub.-- H(13)   ∥MDRMUX.sub.-- H(15)∥MDRMUX.sub.-- H(16)∥MDRMUX.sub.-- H(21)∥MDRMUX.sub.-- H(22)   ∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(28)∥MDRMUX.sub.-- H(31));SYL(6):= OPA(MDRMUX.sub.-- L(2) ∥MDRMUX.sub.-- L(5) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(10)   ∥MDRMUX.sub.-- L(12)∥MDRMUX.sub.-- L(13)∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(18)   ∥MDRMUX.sub.-- L(20)∥MDRMUX.sub.-- L(23)∥MDRMUX.sub.-- L(26)∥MDRMUX.sub.-- L(31));SYH(7):= OPA(MDRMUX.sub.-- H(1) ∥MDRMUX.sub.-- H(2) ∥MDRMUX.sub.-- H(7) ∥MDRMUX.sub.-- H(9)   ∥MDRMUX.sub.-- H(11)∥MDRMUX.sub.-- H(12)∥MDRMUX.sub.-- H(14)∥MDRMUX.sub.-- H(15)   ∥MDRMUX.sub.-- H(17)∥MDRMUX.sub.-- H(20)∥MDRMUX.sub.-- H(21)∥MDRMUX.sub.-- H(23)   ∥MDRMUX.sub.-- H(24)∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(29)∥MDRMUX.sub.-- H(30)   ∥MDRMUX.sub.-- H(31));SYL(7):= OPA(MDRMUX.sub.-- L(3) ∥MDRMUX.sub.-- L(4) ∥MDRMUX.sub.-- L(5) ∥MDRMUX.sub.-- L(6)   ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(11)∥MDRMUX.sub.-- L(12)∥MDRMUX.sub.-- L(15)   ∥MDRMUX.sub.-- L(19)∥MDRMUX.sub.-- L(21)∥MDRMUX.sub.-- L(22)∥MDRMUX.sub.-- L(23)   ∥MDRMUX.sub.-- L(27)∥MDRMUX.sub.-- L(30)∥MDRMUX.sub.-- L(31));SYH(8):= OPA(MDRMUX.sub.-- H(0) ∥MDRMUX.sub.-- H(4) ∥MDRMUX.sub.-- H(5) ∥MDRMUX.sub.-- H(6)   ∥MDRMUX.sub.-- H(9) ∥MDRMUX.sub.-- H(10)∥MDRMUX.sub.-- H(11)∥MDRMUX.sub.-- H(12)   ∥MDRMUX.sub.-- H(14)∥MDRMUX.sub.-- H(16)∥MDRMUX.sub.-- H(19)∥MDRMUX.sub.-- H(22)   ∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(26)∥MDRMUX.sub.-- H(31));SYL(8):= OPA(MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(3) ∥MDRMUX.sub.-- L(4) ∥MDRMUX.sub.-- L(8)   ∥MDRMUX.sub.-- L(10)∥MDRMUX.sub.-- L(11)∥MDRMUX.sub.-- L(12)∥MDRMUX.sub.-- L(17)   ∥MDRMUX.sub.-- L(18)∥MDRMUX.sub.-- L(20)∥MDRMUX.sub.-- L(25)∥MDRMUX.sub.-- L(28));SYH(9):= OPA(MDRMUX.sub.-- H(1) ∥MDRMUX.sub.-- H(4) ∥MDRMUX.sub.-- H(7) ∥MDRMUX.sub.-- H(8)   ∥MDRMUX.sub.-- H(9) ∥MDRMUX.sub.-- H(10)∥MDRMUX.sub.-- H(13)∥MDRMUX.sub.-- H(15)   ∥MDRMUX.sub.-- H(17)∥MDRMUX.sub.-- H(18)∥MDRMUX.sub.-- H(19)∥MDRMUX.sub.-- H(23)   ∥MDRMUX.sub.-- H(24)∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(27)∥MDRMUX.sub.-- H(30)   ∥MDRMUX.sub.-- H(31));SYL(9):= OPA(MDRMUX.sub.-- L(0) ∥MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(2) ∥MDRMUX.sub.-- L(3)   ∥MDRMUX.sub.-- L(5) ∥MDRMUX.sub.-- L(9) ∥MDRMUX.sub.-- L(10)∥MDRMUX.sub.-- L(13)   ∥MDRMUX.sub.-- L(16)∥MDRMUX.sub.-- L(17)∥MDRMUX.sub.-- L(19)∥MDRMUX.sub.-- L(21)   ∥MDRMUX.sub.-- L(24)∥MDRMUX.sub.-- L(25)∥MDRMUX.sub.-- L(29));SYH(10):= OPA(MDRMUX.sub.-- H(2) ∥MDRMUX.sub.-- H(4) ∥MDRMUX.sub.-- H(7) ∥MDRMUX.sub.-- H(8)   ∥MDRMUX.sub.-- H(9) ∥MDRMUX.sub.-- H(10)∥MDRMUX.sub.-- H(12)∥MDRMUX.sub.-- H(17)   ∥MDRMUX.sub.-- H(18)∥MDRMUX.sub.-- H(19)∥MDRMUX.sub.-- H(20)∥MDRMUX.sub.-- H(22)   ∥MDRMUX.sub.-- H(24)∥MDRMUX.sub.-- H(26)∥MDRMUX.sub.-- H(27)∥MDRMUX.sub.-- H(29)   ∥MDRMUX.sub.-- H(31));SYL(10):= OPA(MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(6) ∥MDRMUX.sub.-- L(8) ∥MDRMUX.sub.-- L(9)   ∥MDRMUX.sub.-- L(11)∥MDRMUX.sub.-- L(14)∥MDRMUX.sub.-- L(16)∥MDRMUX.sub.-- L(18)   ∥MDRMUX.sub.-- L(19)∥MDRMUX.sub.-- L(22)∥MDRMUX.sub.-- L(27)∥MDRMUX.sub.-- L(30));SYH(11):= OPA(MDRMUX.sub.-- H(3) ∥MDRMUX.sub.-- H(5) ∥MDRMUX.sub.-- H(6) ∥MDRMUX.sub.-- H(7)   ∥MDRMUX.sub.-- H(8) ∥MDRMUX.sub.-- H(11)∥MDRMUX.sub.-- H(13)∥MDRMUX.sub.-- H(16)   ∥MDRMUX.sub.-- H(17)∥MDRMUX.sub.-- H(18)∥MDRMUX.sub.-- H(21)∥MDRMUX.sub.-- H(23)   ∥MDRMUX.sub.-- H(25)∥MDRMUX.sub.-- H(26)∥MDRMUX.sub.-- H(28)∥MDRMUX.sub.-- H(29)   ∥MDRMUX.sub.-- H(30)∥MDRMUX.sub.-- H(31));SYL(11):= OPA(MDRMUX.sub.-- L(0) ∥MDRMUX.sub.-- L(1) ∥MDRMUX.sub.-- L(7) ∥MDRMUX.sub.-- L(8)   ∥MDRMUX.sub.-- L(10)∥MDRMUX.sub.-- L(11)∥MDRMUX.sub.-- L(15)∥MDRMUX.sub.-- L(17)   ∥MDRMUX.sub.-- L(18)∥MDRMUX.sub.-- L(23)∥MDRMUX.sub.-- L(26)∥MDRMUX.sub.-- L(27)   ∥MDRMUX.sub.-- L(31));__________________________________________________________________________ 
    
     The same circuits that are used in generating the partial check bits during the store operations are utilized again to regenerate the partial syndrome bits. So for example, partial syndrome bit i is the Exclusive-OR of the fetched data word at positions specified by 1&#39;s in the ith row of the parity check matrix. That is to say that the partial syndrome bits are generated by supplying specified signals to a set of exclusive --  OR gates. 
     The syndrome bits SY(0...11) are then generated from the combination of the fetched data from check bits MDRMUX --  C(0 . . . 11) and the address parity bits from Fetch --  ADDRP(0 . . . m) which are regenerated from the incoming system address bits. 
     
         __________________________________________________________________________/********************* Syndrome Generation ****************************/SY(0)  :=MDRMUX.sub.-- C(0)    XOR SYH(0)           XOR SYL(0)  XOR FETCH.sub.-- ADDRP(1);SY(1)  :=MDRMUX.sub.-- C(1)    XOR SYH(1)           XOR SYL(1)           XOR FETCH.sub.-- ADDRP(0)                       XOR FETCH.sub.-- ADDRP(1);SY(2)  :=MDRMUX.sub.-- C(2)    XOR SYH(2)           XOR SYL(2) XOR FETCH.sub.-- ADDRP(0)                                 ;SY(3)  :=MDRMUX.sub.-- C(3)    XOR SYH(3)           XOR SYL(3)           XOR FETCH.sub.-- ADDRP(0)                       XOR FETCH.sub.-- ADDRP(1);SY(4)  :=MDRMUX.sub.-- C(4)    XOR SYH(4)           XOR SYL(4) XOR FETCH.sub.-- ADDRP(0)                                 ;SY(5)  :=MDRMUX.sub.-- C(5)    XOR SYH(5)           XOR SYL(5)  XOR FETCH.sub.-- ADDRP(1);SY(6)  :=MDRMUX.sub.-- C(6)    XOR SYH(6)           XOR SYL(6)  ;SY(7)  :=MDRMUX.sub.-- C(7)    XOR SYH(7)           XOR SYL(7)  ;SY(8)  :=MDRMUX.sub.-- C(8)    XOR SYH(8)           XOR SYL(8)  ;SY(9)  :=MDRMUX.sub.-- C(9)    XOR SYH(9)           XOR SYL(9)  ;SY(10) :=MDRMUX.sub.-- C(10)    XOR SYH(10)           XOR SYL(10) XOR FETCH.sub.-- ADDRP(1);SY(11) :=MDRMUX.sub.-- C(11)    XOR SYH(11)           XOR SYL(11) XOR FETCH.sub.-- ADDRP(0)                                 ;__________________________________________________________________________ 
    
     The partial syndrome bits SY(0 . . . 11) 72 are then decoded 74 to identify any possible errors as illustrated in FIG. 7. Module error indicators are then generated according to the formula given below during the decoding process to detect and indicate errors. 
     
         __________________________________________________________________________/* **** Syndrome &amp; Module Decoding ***********************************//**/ /* MOD.sub.-- 00 means: The most left Module 0 is defective */MOD.sub.-- 00 := .sup. ( SY( 1)             XOR                SY( 2)     XOR                               SY( 4) ) &amp;( SY( 0)      XOR         SY( 1)     XOR                       SY( 3)                           XOR                               SY( 5) ) &amp;( SY( 0)   XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 6) ) &amp;(      SY( 1)             XOR                SY( 2)     XOR                               SY( 7) ) &amp;( SY( 0)                 XOR                               SY( 8) ) &amp;(      SY( 1)            XOR                               SY( 9) ) &amp;(             SY( 2)     XOR                               SY(10) ) &amp;(                    SY( 3)                           XOR                               SY(11) ) ;MOD.sub.-- 01 := .sup. ( SY( 0)            XOR                               SY( 4) ) &amp;(      SY( 1)            XOR                               SY( 5) ) &amp;(             SY( 2)     XOR                               SY( 6) ) &amp;(                    SY( 3)                           XOR                               SY( 7) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)     XOR                               SY( 8) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY( 9) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY(10) ) &amp;(      SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY(11) ) ;MOD.sub.-- 02 := .sup. (        SY( 2)     XOR                               SY( 4) ) &amp;(                    SY( 3)                           XOR                               SY( 5) ) &amp;( SY( 0)   XOR                SY( 2)     XOR                               SY( 6) ) &amp;(      SY( 1)     XOR                       SY( 3)                           XOR                               SY( 7) ) &amp;(      SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 8) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)     XOR                               SY( 9) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)     XOR                               SY(10) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY(11) ) ;MOD.sub.-- 03 := .sup. ( SY( 0)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 4) ) &amp;(      SY( 1)             XOR                SY( 2)     XOR                               SY( 5) ) &amp;( SY( 0)      XOR         SY( 1)     XOR                       SY( 3)                           XOR                               SY( 6) ) &amp;( SY( 0)   XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 7) ) &amp;( SY( 0)   XOR                SY( 2)     XOR                               SY( 8) ) &amp;(      SY( 1)     XOR                       SY(3)                           XOR                               SY( 9) ) &amp;( SY( 0)                 XOR                               SY(10) ) &amp;(      SY( 1)            XOR                               SY(11) ) ;MOD.sub.-- 04 := .sup. ( SY( 0)             XOR                SY( 2)     XOR                               SY( 4) ) &amp;(      SY( 1)     XOR                       SY( 3)                           XOR                               SY( 5) ) &amp;( SY( 0)                 XOR                               SY( 6) ) &amp;(      SY( 1)            XOR                               SY( 7) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY( 8) ) &amp;(      SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 9) ) &amp;(      SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY(10) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)     XOR                               SY(11) ) ;MOD.sub.-- 05 := .sup. ( SY( 0)      XOR         SY( 1)     XOR                       SY( 3)                           XOR                               SY( 4) ) &amp;( SY( 0)   XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 5) ) &amp;(      SY( 1)             XOR                SY( 2)     XOR                               SY( 6) ) &amp;( SY( 0)      XOR         SY( 1)     XOR                       SY( 3)                           XOR                               SY( 7) ) &amp;(             SY( 2)     XOR                               SY( 8) ) &amp;(                    SY( 3)                           XOR                               SY( 9) ) &amp;( SY( 0)   XOR                SY( 2)     XOR                               SY(10) ) &amp;(      SY( 1)     XOR                       SY( 3)                           XOR                               SY(11) ) ;MOD.sub.-- 06 := .sup. ( SY( 1)     XOR                       SY( 3)                           XOR                               SY( 4) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 5) ) &amp;(      SY( 1)            XOR                               SY( 6) ) &amp;( SY( 0)      XOR         SY( 1)            XOR                               SY( 7) ) &amp;(      SY( 1)             XOR                SY( 2)     XOR                               SY( 8) ) &amp;( SY( 0)      XOR         SY( 1)     XOR                       SY( 3)                           XOR                               SY( 9) ) &amp;( SY( 0)   XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY(10) ) &amp;(      SY( 1)             XOR                SY( 2)     XOR                               SY(11) ) ;MOD.sub.-- 07 := .sup. ( SY( 0)      XOR         SY( 1)             XOR                SY( 2)     XOR                               SY( 4) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY( 5) ) &amp;( SY( 0)          XOR                       SY( 3)                           XOR                               SY( 6) ) &amp;(      SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 7) ) &amp;(                    SY( 3)                           XOR                               SY( 8) ) &amp;(             SY( 2)                    XOR                       SY( 3)                           XOR                               SY( 9) ) &amp;(      SY( 1)     XOR                       SY( 3)                           XOR                               SY(10) ) &amp;( SY( 0)      XOR         SY( 1)             XOR                SY( 2)                    XOR                       SY( 3)                           XOR                               SY(11) ) ;MOD.sub.-- 08 := .sup. ( SY( 4)     XOR                       SY( 7)                           XOR                               SY( 0) ) &amp;(      SY( 5)             XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 1) ) &amp;(      SY( 5)             XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 2) ) &amp;( SY( 4)      XOR         SY( 5)             XOR                SY( 6)     XOR                               SY( 3) ) &amp;(      SY( 5)     XOR                       SY( 7)                           XOR                               SY( 8) ) &amp;( SY( 4)      XOR         SY( 5)             XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 9) ) &amp;(      SY( 5)            XOR                               SY(10) ) &amp;( SY( 4)      XOR         SY( 5)            XOR                               SY(11) ) ;MOD.sub.-- 09 := .sup. ( SY( 8)      XOR         SY( 9)     XOR                       SY(11)                           XOR                               SY( 0) ) &amp;( SY( 8)   XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 1) ) &amp;(      SY( 9)             XOR                SY(10)     XOR                               SY( 2) ) &amp;( SY( 8)      XOR         SY( 9)     XOR                       SY(11)                           XOR                               SY( 3) ) &amp;(                    SY(11)                           XOR                               SY( 4) ) &amp;(             SY(10)                    XOR                       SY(11)                           XOR                               SY( 5) ) &amp;(      SY( 9)     XOR                       SY(11)                           XOR                               SY( 6) ) &amp;( SY( 8)      XOR         SY( 9)             XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 7) ) ;MOD.sub.-- 10 := .sup. ( SY( 4)      XOR         SY( 5)             XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 0) ) &amp;( SY( 4)   XOR                SY( 6)     XOR                               SY( 1) ) &amp;( SY( 4)      XOR         SY( 5)            XOR                               SY( 2) ) &amp;( SY( 4)                 XOR                               SY( 3) ) &amp;( SY( 4)   XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 8) ) &amp;(      SY( 5)             XOR                SY( 6)     XOR                               SY( 9) ) &amp;( SY( 4)      XOR         SY( 5)     XOR                       SY( 7)                           XOR                               SY(10) ) &amp;( SY( 4)   XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY(11) ) ;MOD.sub.-- 11 := .sup. (        SY(10)                    XOR                       SY(11)                           XOR                               SY( 0) ) &amp;(             SY(10)     XOR                              SY( 1) ) &amp;( SY( 8)      XOR         SY( 9)             XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 2) ) &amp;( SY( 8)   XOR                SY(10)     XOR                               SY( 3) ) &amp;(      SY( 9)             XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 4) ) &amp;( SY( 8)      XOR         SY( 9)             XOR                SY(10)     XOR                               SY( 5) ) &amp;( SY( 8)      XOR         SY( 9)             XOR                SY(10)     XOR                               SY( 6) ) &amp;( SY( 8)          XOR                       SY(11)                           XOR                               SY( 7) )MOD.sub.-- 12 := .sup. ( SY( 4)      XOR         SY( 5)            XOR                               SY( 0) ) &amp;( SY( 4)                 XOR                               SY( 1) ) &amp;(             SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 2) ) &amp;(             SY( 6)     XOR                               SY( 3) ) &amp;(      SY( 5)             XOR                SY( 6)     XOR                               SY( 8) ) &amp;( SY( 4)      XOR         SY( 5)     XOR                       SY( 7)                           XOR                               SY( 9) ) &amp;( SY( 4)   XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY(10) ) &amp;(      SY( 5)             XOR                SY( 6)     XOR                               SY(11) ) ;MOD.sub.-- 13 := .sup. ( SY( 8)      XOR         SY( 9)            XOR                               SY( 0) ) &amp;( SY( 8)                 XOR                               SY( 1) ) &amp;(             SY(10)                    XOR                       SY(11)                           XOR                               SY( 2) ) &amp;(             SY(10)     XOR                               SY( 3) ) &amp;( SY( 8)      XOR         SY( 9)             XOR                SY(10)     XOR                               SY( 4) ) &amp;( SY( 8)          XOR                       SY(11)                           XOR                               SY( 5) ) &amp;( SY( 8)          XOR                       SY(11)                           XOR                               SY( 6) ) &amp;(      SY( 9)             XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 7) ) ;MOD.sub.-- 14 := .sup. ( SY( 4)      XOR         SY( 5)             XOR                SY( 6)     XOR                               SY( 0) ) &amp;( SY( 4)          XOR                       SY( 7)                           XOR                               SY( 1) ) &amp;( SY( 4)          XOR                       SY( 7)                           XOR                               SY( 2) ) &amp;(      SY( 5)             XOR                SY( 6)                    XOR                       SY( 7)                           XOR                               SY( 3) ) &amp;(      SY( 5)            XOR                               SY( 8) ) &amp;( SY( 4)      XOR         SY( 5)            XOR                               SY( 9) ) &amp;(                    SY( 7)                           XOR                               SY(10) ) &amp;(             SY( 6)                    XOR                       SY( 7)                           XOR                               SY(11) ) ;MOD.sub.-- 15 := .sup. ( SY( 9)             XOR                SY(10)     XOR                               SY( 0) ) &amp;( SY( 8)      XOR         SY( 9)     XOR                       SY(11)                           XOR                               SY( 1) ) &amp;( SY( 8)   XOR                SY(10)                    XOR                       SY(11)                           XOR                               SY( 2) ) &amp;(      SY( 9)             XOR                SY(10)     XOR                               SY( 3) ) &amp;(      SY( 9)            XOR                               SY( 4) ) &amp;( SY( 8)      XOR         SY( 9)            XOR                               SY( 5) ) &amp;(                    SY(11)                           XOR                               SY( 6) ) &amp;(             SY(10)                    XOR                       SY(11)                           XOR                               SY( 7) ) ;MOD.sub.-- 16 := .sup. (                       SY( 4) ) &amp;(                            SY( 5) ) &amp;(                            SY( 6) ) &amp;(                            SY( 7) ) &amp;(                            SY( 8) ) &amp;(                            SY( 9) ) &amp;(                            SY(10) ) &amp;(                            SY(11) ) ;MOD.sub.-- 17 := .sup. (                       SY( 0) ) &amp;(                            SY( 1) ) &amp;(                            SY( 2) ) &amp;(                            SY( 3) ) &amp;(                            SY( 8) ) &amp;(                            SY( 9) ) &amp;(                            SY(10) ) &amp;(                            SY(11) ) ;MOD.sub.-- 18 := .sup. (                       SY( 0) ) &amp;(                            SY( 1) ) &amp;(                            SY( 2) ) &amp;(                            SY( 3) ) &amp;(                            SY( 4) ) &amp;(                            SY( 5) ) &amp;(                            SY( 6) ) &amp;(                            SY( 7) ) ;__________________________________________________________________________ 
    
     FIG. 8 illustrates a set of module errors indicators MOD --  00, MOD --  01, . . . MOD --  18 74. Any error will cause the setting of one of the MOD&#39;s. For example an error in module 0 will cause MOD --  00 to be set to 1. During the decoding operations, bit error indicators are also generated from circuit lines which are later used to invert the fetched data bits to correct certain errors. Invert lines 81 INV --  H(0 . . . 31) 84, INV --  L(0 . . . 31), and INV --  C(0 . . . 11) are used in conjunction with bit errors so that when a bit error indicator is on, the corresponding data bit is corrected by inversion of the data. Bit error indicators 82 are generated from the following equations. 
     
         ______________________________________/* ********** Invert Enabling *****************************/INV.sub.-- H(0..3)       :=      MOD.sub.-- 00                        GATE    SY(0..3);INV.sub.-- H(4..7)       :=      MOD.sub.-- 01                        GATE    SY(0..3);INV.sub.-- H(8..11)       :=      MOD.sub.-- 02                        GATE    SY(0..3);INV.sub.-- H(12..15)       :=      MOD.sub.-- 03                        GATE    SY(0..3);INV.sub.-- H(16..19)       :=      MOD.sub.-- 04                        GATE    SY(0..3);INV.sub.-- H(20..23)       :=      MOD.sub.-- 05                        GATE    SY(0..3);INV.sub.-- H(24..27)       :=      MOD.sub.-- 06                        GATE    SY(0..3);INV.sub.-- H(28..31)       :=      MOD.sub.-- 07                        GATE    SY(0..3);INV.sub.-- L(0..3)       :=      MOD.sub.-- 08                        GATE    SY(4..7);INV.sub.-- L(4..7)       :=      MOD.sub.-- 09                        GATE    SY(8..11);INV.sub.-- L(8..11)       :=      MOD.sub.-- 10                        GATE    SY(4..7);INV.sub.-- L(12..15)       :=      MOD.sub.-- 11                        GATE    SY(8..11);INV.sub.-- L(16..19)       :=      MOD.sub.-- 12                        GATE    SY(4..7);INV.sub.-- L(20..23)       :=      MOD.sub.-- 13                        GATE    SY(8..11);INV.sub.-- L(24..27)       :=      MOD.sub.-- 14                        GATE    SY(4..7);INV.sub.-- L(28..31)       :=      MOD.sub.-- 15                        GATE    SY(8..11);INV.sub.-- C(0..3)       :=      MOD.sub.-- 16                        GATE    SY(0..3);INV.sub.-- C(4..7)       :=      MOD.sub.-- 17                        GATE    SY(4..7);INV.sub.-- C(8..11)       :=      MOD.sub.-- 18                        GATE    SY(8..11);______________________________________ 
    
     In this manner, the status of the module error and the syndromes are to enable a plurality of invert lines and generate other signals pertaining to types of errors 78 including a UE (Uncorrectable error) signal 79 or a CE (Correctable Error) signal 77. 
     
         ______________________________________**************** Error Indication ******************************/ECC.sub.-- ANYE :=      SY( 0) | SY( 1) | SY( 2) | SY( 3)      SY( 4) | SY( 5) | SY( 6) | SY( 7)      SY( 8) | SY( 9) | SY(10) |SY(11) :ECC CE :=  ( MOD.sub.-- 00 | MOD 01 | MOD 02 |      MOD 03      MOD.sub.-- 04 | MOD.sub.-- 05 | MOD.sub.--      06 | MOD.sub.-- 07      MOD.sub.-- 08 | MOD.sub.-- 09 | MOD.sub.--      10 | MOD.sub.-- 11      MOD.sub.-- 12 | MOD 13 | MOD.sub.-- 14      | MOD.sub.-- 15      MOD.sub.-- 16 | MOD.sub.-- 17 | MOD.sub.--      18   );ECC.sub.-- UCE :=      ECC.sub.-- ANYE &amp;  ECC.sub.-- CE;/*******************************************************/ /*&gt;INVERT AND FETCH.sub.-- ECC.sub.-- OUTPUT &lt;*//********************************************************/STEP3.sub.-- IN.sub.-- H(0..31):=MDRMUX.sub.-- OUT.sub.-- H(0..31) XORINV.sub.-- H(0..31);STEP3.sub.-- IN.sub.-- L(0..31):=MDRMUX.sub.-- OUT.sub.-- L(0..31) XORINV.sub.-- L(0..31);STEP3.sub.-- IN.sub.-- C(0..11):=MDRMUX.sub.-- OUT.sub.-- C(0..11) XORINV.sub.-- C(0..11);______________________________________ 
    
     FIG. 9 illustrates the arrangement of data in the memory modules on the SIMMS. There are 19 memory modules in a SIMM; a number of SIMMs can therefore be configured in this way into the memory. As illustrated, the data bits and check bits are divided into 4 bit symbols to store in 19 memory modules. SEC --  0 --  H(0 . . . 31) are stored in modules 1 to 8, SEC --  0 --  L(0 . . . 31) are stored in modules 9 to 16, and SEC --  0 --  C(0 . . . 11) are stored in modules 17 to 19. The mapping of system address bits to memory modules depends on the module size as shown in Table IV and FIG. 9. 
     It should be seen from above that the coding method and circuits and apparatus of the present invention have provided a mechanism for improved reliability and availability of computer memory systems. 
     While the present invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.