Error detecting method and apparatus for computer memory having multi-bit output memory circuits

Abstract: An error correcting code and apparatus is used in conjunction with a main memory in which a data word is stored in a plurality of circuits each of which produces multiple outputs. A minimum number of check bits are stored together with the data word for detecting and correcting single bit errors and detecting the existence of multi-bit errors. A parity bit for the entire data word is also stored. For a 32-bit data word, at least 3 bits of the data word are stored in each of 10 memory circuits. Seven check bits and one parity bit are also stored in the 10 memory circuits wherein no more than one of the check bits or data bit is stored in any one memory circuit. Upon reading the data word from the memory a set of verify check bits and a verify parity bit are generated and compared to the stored check bits and stored parity bit to produce a check bit syndrome and a parity bit syndrome. The check bit syndrome is decoded to produce an output that is input to an error generator circuit together with a parity syndrome for producing error signals indicating occurrence of a single bit error, a multi-bit error, a triple bit error, or a check bit error.

Description: TECHNICAL DESCRIPTION The present invention pertains in general to error correction and detection and in particular to the application of error correction and detection codes to computer memories which utilize memory circuits having multi-bit outputs. BACKGROUND OF THE INVENTION Computer memories typically utilize dynamic random access memories (DRAMs) for the storage of data that is used in the CPU of the computer. This data is subject to corruption due to many possible causes. As a result, memory systems have been refined and developed so that the occurrence of random errors is relatively infrequent. But, in many applications the integrity of the data is so critical that even relatively infrequent errors cannot be tolerated. Various error correction and detection codes have been developed to detect, and in certain situations, correct such errors. The most well known is the Hamming Code, which typically provides detection for one or two-bit errors and correction of one-bit errors. Commercial circuits are available which can implement such a Hamming code in computer memory systems. The technology of the Hamming Code is well known and is widely described in the literature in this field of technology. The use of the Hamming code is quite effective in conventional memory systems in which one bit of data is stored in each memory device. Large memory systems typically include hundreds, or even thousands of individual memory devices, each of which may contain, for example, 1 megabit, 4 megabits or 16 megabits of data. DRAM devices have conventionally had a one-bit output per device such that a data word is distributed across the same number of memory devices as the number of bits in the word. Randomly occurring errors, generally referred to as soft errors, are most frequently one-bit errors. Further, if a particular memory device were to fail, either permanently or transiently, there would still be produced a one-bit error in the resulting data word. Therefore, by use of a Hamming Code a one-bit error can be detected and corrected. It has been assumed that the probability of multi-bit errors under these circumstances is remote. A Hamming Code can typically detect, but not correct, a two-bit error. However, it is very difficult to economically detect and correct a greater number of bit errors. It is heretofore been the accepted position that the probability of occurrence of multi-bit errors is so remote that it could be safely ignored. With the introduction of much larger main memory capacities, such as several hundred megabytes for a single computer main memory, it has become necessary to utilize memory circuits which output multiple bits from each device, rather than only a single bit. Typical DRAM memory devices of this type can provide a four-bit output in parallel for memory device sizes such as one and four megabits. It is anticipated that such practice will continue, and possibly have even greater multi-bit outputs when memory circuits go up in capacity to 16 and 64 megabits. But, when a single memory device produces four bits in a single data word, there is a much greater likelihood of having a multi-bit error should there be a failure of that particular memory device. Certain applications of computer systems have been developed which require an extremely high level of data integrity. The existence of undetected errors, even at the rate of only one or two per year, for such a computer system could result in serious consequences. In view of the greater likelihood of encountering multi-bit errors due to the use of multi-bit output memory devices and the increased criticality of data integrity, there exist a need for a method and apparatus to detect the occurrence of multi-bit errors for data which is stored in such a computer memory. EP-A-0 166 269 discloses an error correction method for a computer memory. In this method for each byte of a multiple bit word a store parity check bit and a store check bit is generated, and the store parity bits and the store check bits belonging to the multiple bit word are stored in a separate memory device. During read out of the multiple bit word verify check bits and a verify parity check bit are generated and compared with the store check bits and store parity check bit, respectively. For example, in a four byte, 32 bit word eight bits of error correction, namely four parity bits and four check bits, can be used to correct up to four errors if the errors are restricted to corresponding bits in the four bytes. On the other hand, if more than one error occurs in one byte no correction is possible. This system is disadvantageous for computer memories having a plurality of memory devices, each having a multiple bit output so that multiple bits, eg. one byte, are read from each memory device. Since the most common failure is that an entire memory device goes bad, all multiple bits read from this memory device are unreliable which results in an unrecoverable error since the check bits are insufficient to a correct all bits from one memory device. EP-A-0 401 994 discloses a method for storing and retrieving multiple bit words in a computer memory having a plurality of storage devices. The multiple bit word is divided into subwords, and an error detection value, preferably a parity bit, is calculated and appended to each subword segment. Each subword is then stored into separate storage devices. As a result of this storage method, the word can easily be reconstructed if there is a single bit error in any subword or even if a both subwords in a storage device are in error. SUMMARY OF THE INVENTION A selected embodiment of the present invention is a method of error correction and detection as claimed in claim 1 for data which is stored in a computer memory. A minimum number of error detection and correction store check bits is generated for a data word by use of a selected error correction code. This code detects and corrects single bit errors and detects multiple bit errors in the data word. A parity store check bit is generated for the entirety of the data word. The data word, the store check bits and the store parity check bit are stored in the computer memory. The computer memory comprises a plurality of multi-bit output memory devices wherein multiple bits of the data word are stored in each of the memory devices. No more than one of the store check bits or store parity check bit is stored in any one of the memory devices. The memory is read to produce the data word, the store check bits and the store parity bit. Due to error conditions there may be one or more bit errors in the collection of the data word, the store check bits and the store parity check bit which were read from the computer memory. The selected error correction code is applied to the data word, which was read from the computer memory, to generate a set of verify check bits. A verify parity check bit is generated by examination of the data word read from the computer memory. The store check bits read from the computer memory are compared to the verify check bits to produce a check bit syndrome. The store parity check bit read from the computer memory is also compared to the verify parity check bit to produce a parity syndrome. Next, the check bit syndrome is decoded to: (a) either detect and identify single bit errors or detect triple bit errors which all occur within the data bits and check bits stored within one of said memory devices, wherein a first output is produced upon such an occurrence, or (b) detect double bit errors, wherein a second output is produced upon such an occurrence, or (c) detect triple bit errors which all occur within the data bits within one of the memory devices and produce a third output under such circumstances, and comparing the first output, if such a first output is produced, to the parity syndrome to produce either a fourth output which indicates a single bit error or a fifth output which indicates the existence of a triple bit error within one of the memory circuits. A further aspect of the present invention comprises an error detection circuit according to claim 4. Further enhancements are provided by the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS For more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which: FIGURE 1 is an illustration of a portion of a computer memory which comprises a collection of dynamic random access memory (DRAM) devices which store a plurality of bits for a single data word together with error correction and detection check bits, FIGURE 2 is a circuit diagram for an error correction and detection circuit in accordance with the present invention for detecting and correcting single bit errors and detecting various types of multi-bit errors, and FIGURES 3A and 3B are a detailed circuit diagram for the error generator circuit shown in FIGURE 2. DETAILED DESCRIPTION The present invention, for the embodiment described herein, includes an 8-bit error detection and correction (EDC) code for use in conjunction with a memory system in which multiple bits of a data word are stored in individual memory devices. The most common form of memory device now in use is a dynamic random access memory (DRAM). The configuration of current DRAMs, which are applicable to the memory system described herein, are designated as 256K by 4 or 1 megabit by 4. This designation means that upon occurrence of a memory read or write cycle, 4 bits of data are written or read in parallel during one cycle. It is becoming more common, particularly in large main memories, to utilize such multi-bit output memory devices. A typical data word may comprise 32 bits, which are distributed across multiple memory devices. When error correcting and detection check bits are used in conjunction with the data word, they also are stored in the main memory. For a 32-bit data word, a Hamming Code uses a minimum of 7 bits which can detect and correct a single bit error and detect the existence of a double bit error. A standard 7-bit Hamming Code can be implemented by the use of commercially available parts such as the AMD Am2nC660, National Semiconductor 74F632, or Texas Instruments SN74AS632. For the present invention, the detection and correction of single bit errors and the detection of double bit errors are carried out in the same method as implemented in these commercially available parts. In addition to the Figures which are utilized herein to describe the present invention, there are further included four tables which follow the Detailed Description. These four tables will be referenced in the description of the invention herein. A segment of a main memory, as utilized in the present invention, is shown in FIGURE 1. A memory 10, which is a small part of a much larger overall main memory, comprises ten dynamic random access memory devices 12, 14, 16, 18 20, 22, 24, 26, 28 and 30. Each of these memory devices produces a 4-bit output during a memory cycle. Representative memory devices may have a configuration of 256K by 4 bits or 1 megabit by 4 bits. Memory devices of this type are widely available and are described in detail in the literature of the industry. Multi-bit memory devices of greater capacity will undoubtedly become available in the near future. The memory 10 consists of ten memory devices which are utilized to store one data word. An overall main memory of a computer would utilize a large number of the memories 10 therein. However, the present invention can be fully described by reference only to the memory 10. An important aspect of the present invention is the storage locations for the data bits for the data word and the associated error correction check bits and parity bit in the memory 10. Each of the memory devices 12-30 includes at least 3 data bits, and no more than 1 check bit is stored in any one of the memory devices. In accordance with a selected embodiment of the present invention, memory device 12 stores data bits 2, 3, 5 and 31. Memory device 14 stores data bits 14, 20 and 24 together with check bit 1. Memory device 16 stores data bits 0, 4, 8 and 9. Memory device 18 stores data bits 11, 12 and 13 together with check bit 6. Memory device 20 stores data bits 6, 10 and 17 together with check bit 5. Memory device 22 stores data bits 15, 18 and 19 together with check bit 7. Memory device 24 stores data bits 7, 21 and 22 together with check bit 4. Memory device 26 stores data bits 16, 23, and 25 together with check bit 2. Memory device 28 stores data bits 26, 27 and 30 together with check bit 0. And last, memory device 30 stores data bits 1, 28 and 29 together with check bit 3. The check bits 0 through 6 comprise the 7 bits of a conventional 7-bit Hamming code as utilized in the described commercial parts noted above. The eighth check bit is a parity check bit which is produced for the entire 32-bits of the data word. Referring now to FIGURE 2 there is illustrated further apparatus for use in accordance with the present invention. An error control device 40 includes 7 syndrome generator circuits 42, 44, 46, 48, 50, 52 and 54 together with a parity generator circuit 56. Each of the syndrome generator circuits receives 16-bits of the 32-bit data word, and through an exclusive or function produces an error correction check bit. The syndrome generator circuits 42-54 respectively produce check bits 0 through 6. The parity generator circuit 56 uses an exclusive or function for all 32 bits of the data word to produce a verify check bit. The data inputs for each of the syndrome generator circuits 42-54 is a data word read from the memory 10. These inputs are as follows: SYNDROME GENERATOR CIRCUIT DATA BITS INPUT TO SYNDROME GENERATOR CIRCUIT 42 0, 4, 6, 7, 8, 9, 11, 14, 17, 18, 19, 21, 26, 28, 29, 31 44 0, 1, 2, 4, 6, 8, 10, 12, 16, 17, 18, 20, 22, 24, 26, 28 46 0, 3, 4, 7, 9, 10, 13, 15, 16, 19, 20, 23, 25, 26, 29, 31 48 0, 1, 5, 6, 7, 11, 12, 13, 16, 17, 21, 22, 23, 27, 28, 29 50 2, 3, 4, 5, 6, 7, 14, 15, 1, 19, 20, 21, 22, 23, 30, 31 52 8, 9, 10, 11, 12, 13, 14, 15, 24, 23, 26, 27, 20, 29, 31 54 0, 1, 2, 3, 4, 5, 6, 7, 24, 25, 26, 27, 28, 29, 30, 31 All 32 data bits (0..31) are input to the parity generator circuit 56 to produce a parity bit which is designated as check bit 7. Each of the syndrome generator circuits 42-56 is an exclusive OR circuit. The outputs from the respective circuits 42-56 are provided as first inputs to exclusive OR logic gates 70, 72, 74, 76, 78, 80, 82 and 84. The check bits 0-7 read from the memory 10 are input respectively to the second inputs of logic gates 70-84. The outputs of the logic gates 70-84 are respective syndrome bits 0-7. The syndrome bits 0-6 are herein designated as a check bit syndrome and the syndrome bit 7 is designated herein as a parity bit syndrome. The check bit syndrome (bits 0-6) are input to a decoder 100 which decodes the 7 inputs into each of the possible 128 combinations to thereby activate a selected 1 of the 128 outputs from the decoder 100. The decoder 100 is a conventional logic design for such decoding. The outputs from the decoder 100 are transmitted through a group of lines 102 to a corresponding set of inputs to an error generator circuit 104. The check bit syndrome (bits 0-6) are also input to the error generator circuit 104. The parity bit syndrome, bit 7, produced at the output of gate 84 is likewise provided as an input to the error generator circuit 104. Upon receipt of the inputs to the circuit 104, this circuit produces one of 4 possible outputs at lines 106, 108, 110, 112. An output is generated at the line 106 for a single bit error, at line 108 for a multi-bit error, at line 110 for a triple bit error and at line 112 for a check bit error. The error generator circuit 104 is described in detail in FIGURES 3A and 3B. The 128 outputs of the circuit 100 in lines 102 are input respectively to an OR circuit 130, an OR circuit 132 and an OR circuit 134. The output from OR circuit 130 is provided as a first input to an OR gate 136 which has line 108 as the output thereof for indicating a multiple bit error. The output of the OR circuit 132 is provided as a first input an OR circuit 138 which has the output thereof provided as a second input to the OR circuit 136 and to the line 110 for indicating the existence of triple bit error. The output of the OR circuit 134 is provided to line 112 for indicating a check bit error and is also provided as an input to an inverter 140 which has the output thereof provided to a first input of an AND gate 142. The output AND gate 142 is provided as the second input of the OR gate 138. The syndrome bits 0-7 from the gates 70-84 are input to a OR gate 150 which has the output thereof connected to the third input of the AND gate 142. The syndrome bit 7 is connected through an inverter 152 to the second input of the AND gate 142. This syndrome bit 7 is further connected to the first input of an AND gate 154 which has the output of OR gate 150 connected to the second input thereof. The output of the AND gate 154 is connected to the third input of the OR gate 138. The inputs to the error generator circuit 104 are the 128 elements of lines 102 together with the 8 syndrome inputs (bits 0-7). The logic shown in FIGURES 3A and 3B produces the error output indications at lines 106, 108, 110 and 112. The operation of the present invention is now described in reference to FIGURES 1 and 2. When a computer main memory, which includes the memory 10, receives a 32-bit data word, that data word is processed through a syndrome generator, which is equivalent to the syndrome circuits 42-54 to produce a 7-bit Hamming Code in a conventional manner. The 32-bit data word is likewise input to a parity generator circuit, such as the circuit 56, to produce a parity check bit. The 7 check bits of the Hamming Code together with the parity check bit are then stored in the memory devices 12-30 in the particular organization described in reference to FIGURE 1 and shown therein. These are referred to as store check bits and a store parity bit. Therefore, for each data word there are stored 32 bits for the data word, 7 bits for the Hamming Code and 1 bit for the parity check bit. Eight of the memory devices store 3 data bits together with 1 check or parity bit and two of the memory devices store only 4 data bits. When a request is received to read the particular data word, which has previously been stored in the memory 10, it is read in parallel, together with the check bits and parity bit, as shown for the outputs of the memory devices in FIGURE 1. The data words read from the memory 10 are input to the syndrome generator circuits 42-54 in the combinations shown as inputs for these circuits in FIGURE 2. All 32 bits of the data word read from the memory 10 are input to the parity generator circuit 56. Each of the syndrome generator circuits 42-54 generates a respective check bit which is termed a verify check bit. The parity generator circuit 56 likewise produces what is termed a verify parity check bit. Each of the verify check bits is input to a respective one of the gates 70-84 together with its corresponding store check bit which was read from the memory 10. The logic gates 70-82 produce a check bit syndrome which is a function of the store check bits read from the memory 10 and the verify check bits produced by the syndrome generator circuits 42-54. If no errors have occurred in the data words or check bits or in the production of the new verify check bits, then the outputs of the logic gates 70-84 will all be a uniform logic (0). This syndrome state will produce a given logic output on one of the output lines of the decoder 100, which output is provided to the error generator circuit 104. For this condition, the error generator circuit 104 will not activate any one of the lines 106-112 since no error has occurred. When an error situation has occurred, the inputs to the logic gates 70-84 will not all be the same and a check bit syndrome, indicating the occurrence of an error, will be generated at the output of the logic gates 70-82. This 7-bit syndrome is input to the decoder 100 which generates an active state for one of its 128 outputs that corresponds to the input syndrome. This output is provided to the corresponding input of the error generator circuit 104. Further input to the circuit 104 is the parity syndrome produced by the logic gate 84 and the check bit syndrome (0-6). These inputs are logically compared to produce one of the 4 outputs at lines 106, 108, 110 and 112. The logic for producing the particular output is described below. The occurrence of a single bit error in the data word is termed a "soft error". The existence of a single bit error can be determined and the location of the bit in error in the data word can likewise be determined. This bit is then inverted to produced a corrected data word. This is a conventional Hamming error detection and correction process. When this occurs, an output signal is generated at line 106 indicating the occurrence of a single bit (soft) error. However, if a multi-bit error has occurred in the data word and check bits, any one of two other outputs may be generated. If a triple bit error is detected wherein all of the errors occur within one of the memory devices 12-30, an output is generated at line 110. If however, there is a multi-bit error and it cannot be determined that all of the bits in error occurred within one of the memory devices 12-30, an output is generated at line 108 indicating the occurrence of such a multi-bit error. If an error has occurred in one of the check bits read from the memory 10, an error signal is generated at line 112. Table 1 illustrates the 8-bit syndrome generated at the output of gates 70-84 for each single bit error condition. The data bits are identified as DB0-DB31 and the check bits are identified as CB0-CB7. For each single bit error there is shown the corresponding syndrome expressed in a hexadecimal value. Referring to Table 2, there are illustrated the 128 possible syndromes for the 7-bit check bit comprising syndrome bits 0-6. For each syndrome there is shown the corresponding error type. The possible error types are single bit, triple bit, multiple bit and quad bit. The decoder 100 receives the 7 bit syndrome input and activates one of its 128 outputs in response thereto. Table 3 illustrates the syndromes that are generated by the logic gates 70-84. A syndrome is shown for each of the 3- and 4-bit error conditions which can occur for each of the DRAM memory circuits 12-30. The syndromes are again listed as a hexadecimal value. These represent all 8 bits of the syndrome produced at the outputs of the logic gates 70-84. Table 4 illustrates each of the logic relationships between the possible syndrome values and the outputs produced at the lines 106, 108, 110 and 112. A single bit error is indicated by the letter "S", double bit error by the letter "D", a triple bit error by the letter "T" and a quad bit error by the letter "Q". The numbers following the letters are the number of error conditions represented by that entry. The data listings in the Table 1-4 taken in conjunction with the circuits shown in FIGURES 2 and 3 uniquely define the logic within the error generator circuit 104 for producing the selected 4 outputs at lines 106, 108, 110 and 112. The present invention provides detection of triple bit errors provided that such errors occur within a single one of the memory circuits 12-30. Triple bit errors which occur across multiple ones of the memory circuits 12-30 are not necessarily detected. However, since such errors will likely be due to the complete failure of one of the memory circuits, it is highly likely that any triple bit error will be detected. Although several embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.

Claims: A method of error correction and detection for data which is stored in a computer memory (10) comprising the steps of: generating an error detection and correction set of store check bits for a data word by use of a selected error correction code which detects and corrects a single bit error and detects multiple bit errors in said data word, generating a store parity check bit for said data word, storing said data word, said store check bits and said store parity check bit in a computer memory (10) which comprises a plurality of multi-bit memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30), wherein multiple bits of said data word are stored in each of said memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30) and no more than one of said store check bits or store parity check bit is stored in any one of said memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30), reading said data word, said store check bits and said store parity check bit from said computer memory (10), wherein there may be one or more bit errors in said data word, generating an error detection and correction set of verify check bits for said data word, which was read from said computer memory, by use of said selected error correction code, generating a verify parity check bit for said data word read from said computer memory, comparing said store check bits read from said memory to said verify check bits to produce a check bit syndrome, comparing said store parity check bit read from said computer memory (10) with said verify parity check bit to produce a parity syndrome, and decoding said check bit syndrome and said parity syndrome to: (a) detect and identify single bit errors in said data word read from said computer memory, wherein a first output (106) is produced, or (b) detect multiple bit errors which do not all occur within a single one of said memory devices, wherein a second output (108) is produced, or (c) detect triple bit errors which all occur within the data bits within one of said memory devices, wherein a third output (110) is produced, or (d) detect a check bit error, wherein a fourth output (112) is produced. A method of error correction and detection for data as recited in claim 1 wherein said data word has 32 bits, said set of store check bits has 7 bits and each of said memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30) stores 4 bits and at least 3 of said 4 bits are data bits of said data word. A method of error correction and detection for data as recited in claim 1 wherein said set of store check bits comprises a minimum number of bits for said data word for detecting and correcting a single bit error and detecting a double bit error. An error detection circuit (40) for a computer memory (10) in which a data word has multiple bits thereof stored in each of a plurality of memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30), together with a set of syndrome check bits and a parity check bit, for the stored data word, comprising: a plurality of syndrome generators (42-54) each connected to receive a separate subset of bits of said data word, which has been read from said memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30) of said computer memory (10), each of said syndrome generators (42-54) generating a corresponding check bit syndrome, a parity generator circuit (56) for generating a parity bit for said data word read from said computer memory, logic circuits (70-82) connected to receive said stored syndrome check bits, which are read from said memory, and said generated syndrome check bits, for generating a syndrome at the outputs thereof, a logic circuit (84) connected to receive said stored parity bit, which is read from said computer memory, and connected to receive said generated parity bit, for producing a parity syndrome bit therefrom, a decoder circuit (100) connected to receive said syndrome, decode said syndrome and generate therefrom a logic signal on at least one of a plurality of output lines (102) thereof, and an error generator circuit (104) connected to said output lines of said decoder circuit (100) for receiving said logic signal, connected to receive said syndrome and connected to receive said parity syndrome bit, for producing a plurality of error indication signals (106-112) including a single bit error signal (106) for indicating a single bit error in said data word, a multiple bit error signal (108) for indicating a plurality of bit errors distributed across more than one of said memory devices in which said data word and check bits were stored and a triple bit error signal (110) indicating at least three bit errors in one of said memory devices. An error detection circuit for a computer memory (10) as recited in claim 4 wherein each of said syndrome generator circuits (42-54) is an OR gate and said logic circuits (70-82) connected to receive said stored syndrome check bits are OR gates. An error detection circuit for a computer memory (10) as recited in claim 4 wherein said error generator circuit also generates a check bit error signal (112) indicating the existence of a bit error in the check bits stored in said plurality of memory devices (12, 14, 16, 18, 20, 22, 24, 26, 28, and 30).

Inventors: JACKSON JAMES A, LOWDERMAN KEVIN M, QUATTROMANI MARC A, JACKSON, JAMES A., LOWDERMAN, KEVIN M., QUATTROMANI, MARC A.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1992
Main classification: G06F11
IPC codes: G06F11, G06F12