METHODS AND APPARATUS FOR CORRECTING SOFT ERRORS IN DIGITAL DATA

Abstract: On data writes to a cache memory in a digital data processing system, the existing data currently stored on the desired cache storage line is read out and parity checked. The read-out data is modified with new data only if there is no parity error. If a parity error is detected, a cache miss is signaled and the read-out line of data is written back into the cache memory. Error correction code checking and error correction is performed on the defective line of data on the write-back path to the cache memory.

Description: METHODS AND APPARATUS FOR CORRECTING SOFT ERRORS IN DIGITAL DATA DESCRIPTION TECHNICAL FIELD This invention related to methods and apparatus for correcting soft errors in digital data and is particularly useful for correcting soft errors in a computer memory. BACKGROUND OF THE INVENTION Data stored on modern day integrated circuit memory chips is subject to so-called "soft errors" caused by gamma rays, cosmic rays, alpha particles and other environmental factors. The passage of a gamma ray through a memory chip, for example, will sometimes cause a disturbance which is sufficient to reverse the binary state of a stored data bit. This is called a "soft" error because no permanent damage is done to the structure of the chip and the disturbed memory cell is thereafter completely reusable for storing data. Soft errors are particularly bothersome for the case of small, high-speed cache memory chips. If store updates are made to a "dirty" cache regardless of the presence of soft errors, data integrity is soon lost, especially when the error is in the unmodified segment of the data. Left uncorrected, soft errors can turn into fatal double bit errors. Various error correction methods have been proposed for correcting soft errors. One proposed method is to generate and include with each line of stored data a set of error correcting code bits which can be used to detect and locate a bit which has been changed as a result of a soft error event. As each line of data is subsequently read out of memory, all data bits including the error correcting code bits are decoded as a group and the decoder output indicates which, if any, data bit is in error. The data is corrected by reversing the binary state of the erroneous bit. Unfortunately, this method of error correction is time consuming and adversely affects system performance and increases latency. SUMMARY OF THE INVENTION The present invention provides a solution to the data integrity problem without making a big compromise on latency and data throughput. In particular, error correction testing is not performed on every data sample or segment. It is performed only when necessary. A simple parity check is used to determine when error correction is needed. For the case of data stored in a cache memory, for example, on data writes to the cache memory, the existing data currently stored on the desired cache storage line is read out and parity checked. The read-out data is modified with new data only if there is no parity error. If a parity error is detected, a cache miss is signaled and the read-out line of data is written back into the cache memory. Error correction code checking and error correction is performed on the defective line of data on the write-back path to the cache memory. For a better understanding of the present invention, together with other and further advantages and features thereof, reference is made to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings: FIG. 1 is a block diagram of a portion of a data processing system showing a representative embodiment of the present invention; FIG. 2 is a diagram showing the contents of a line of data in the cache memory of FIG. 1; FIG. 3 is a flow chart showing the steps used in FIG. 1 for performing a representative embodiment of the present invention; FIG. 4 is a schematic circuit diagram of an exclusive-OR circuit tree used in one embodiment of the parity checker of FIG. 1; and FIG. 5 is a schematic circuit diagram showing a representative form of construction for the error corrector of FIG. 1. DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Referring to FIG. 1, there is shown a portion of a digital data processing system which includes apparatus for correcting soft errors in a memory. The memory in question is represented by a cache memory 10. This cache memory 10 stores multiple lines of data, the composition of one line of which is shown in FIG. 2. The data field in FIG. 2 includes several bytes of data, typically four or eight. The parity field includes one parity check bit for each byte in the data field. The ECC (error correction code) field contains error correction code bits for the line as a whole. These ECC bits enable the position of an erroneous bit in the cache storage line to be determined. The apparatus shown in FIG. 1 is manipulated and controlled by instructions and control signals issued by a digital data processor 11 which may, for example, take the form of a so-called "microprocessor". The apparatus of FIG. 1 further includes write circuitry 12 for storing lines of data into the cache 10, each such line of data having the composition shown in FIG. 2. This data is obtained from a data bus 13 and is supplied to the write circuitry 12 by way of data merge circuits 14. Read circuitry 15 is provided for reading out lines of data from the cache memory 10 and supplying same to the data bus 13. A parity checker 16 is coupled to the output of read circuitry 15 for checking the parity of each read-out byte and producing an error signal if a parity error is detected in any byte. This parity error signal is supplied to processor 11 as a "cache miss" signal which tells processor 11 that the attempted cache access failed. Selection of the particular storage line in cache 10 to be read or written is accomplished by an address signal supplied to cache 10 by processor 11. The apparatus of FIG. 1 also includes an error corrector 17 for correcting data having a parity error and producing error-free corrected data. The data to be corrected is obtained by way of data bus 13 and the corrected data is supplied to write circuitry 12 for writing back into cache 10. An ECC (error correction code) generator 18 is provided for generating the ECC bits to be stored when a line of data is written into cache 10. ECC generator 18 examines the line of data supplied to write circuitry 12 and generates the appropriate ECC bits for storage in cache 10. A further and larger memory unit or storage unit 20 is also coupled to the data bus 13 by way of its appropriate write circuitry 21 and read circuitry 22. Accessing of storage unit 20 is accomplished by way of address signals supplied by processor 11. Referring to FIG. 3, there is shown a flow chart which explains a representative method for practicing the present invention. This method is used for correcting soft errors on writes to the cache 10. As indicated by block 30, processor 11 initiates a store to cache operation and sends an appropriate address to cache 10 for the particular cache line into which it is desired to write data. The next step, represented by block 31, is to read the existing data currently resident on the predetermined storage line which is addressed by processor 11. As indicated by block 32, parity checker 16 checks the parity of each byte of this existing data and produces a parity error signal if a parity error is detected for any byte of the existing data. As indicated by block 33, if no parity error is detected, the new data byte or bytes which it is desired to store are merged with the existing data and the resulting merged data is written into the cache 10 at the same line address from which the existing data was obtained. This merging is accomplished by data merge circuits 14, the new data bytes and existing data bytes being supplied thereto by way of data bus 13. The new bytes replace the existing bytes at the byte positions occupied by the new bytes. The resulting merged data is written into cache 10 by way of write circuitry 12. Following completion of a successful write, the ECC bits for the new line of data are stored in cache 10. If, on the other hand, a parity error is detected for the read-out line of data, then, as indicated by block 34, a cache miss is signaled to the processor 11 and no writing of merged data occurs. The defective read-out line of data is transferred to the storage unit 20 and, as indicated by block 35, processor 11 initiates a write-back of this defective line to the cache 10, such write-back being to the same cache line from which the defective data was obtained. As indicated by block 36, ECC checking and error correction occurs on this write-back. More particularly, the defective line of data is read from storage unit 20 and supplied to error corrector 17. In the present embodiment, error corrector 17 is a single-bit error corrector. As such, error corrector 17 does an ECC type decoding of the entire line of data including the ECC bits to determine the bit position of the erroneous bit. Error corrector 17 then reverses the binary state of the erroneous bit to thereby produce an error-free line of data. This corrected line of data is written into cache 10 at its previous address. In this manner, the soft error is corrected and processor 11 is free to re-initiate the aborted cache write request. An advantage of the present invention is that the ECC error correction operation is performed only when needed. It is not performed on every write to cache, but only on those relatively infrequent occasions when a soft error is encountered. Thus, data integrity is maintained with a minimum impact on system performance and rate of data throughput. As a consequence, cache stores can be readily pipelined, if desired. The simplicity of the present invention is noteworthy. A simple high-speed parity check is used to determine whether a more time-consuming error correction procedure should be performed. FIG. 4 shows an exclusive-OR circuit tree 40 which may be used in the parity checker 16 for checking the parity of a byte of data. Circuit tree 40 includes exclusive- OR circuits 41-48, with data bits 0-7 being supplied to XOR circuits 41-44 and the parity check bit P being supplied to XOR circuit 48. If the parity is wrong, an error signal is produced on output line 49. Additional XOR circuit trees, like the tree 40, are provided for the other bytes in the line of data read out of cache 10. FIG. 5 shows a representative form of construction for the error corrector 17 of FIG. 1. The error corrector embodiment of FIG. 5 includes a set of multiplexer circuits MQ , MI , , M N for individually reversing the binary states of different data bits. There is one multiplexer circuit for each bit in a cache line of data. These data bits are identified as DQ , Dj , , Djvj . Each data bit D is supplied to a first input of its multiplexer. An inverter circuit "I" supplies an inverted replica of the data bit to the second input of the multiplexer. A switching signal S determines which one of the two inputs is allowed to pass through to the output line of the multiplexer. If the data bit is not erroneous, the non-inverted bit is passed to the multiplexer output. Conversely, if the data bit is erroneous, the inverted replica is passed to the multiplexer output. In this manner, the binary state of an eπoneous data bit is reversed. The error corrector of FIG. 5 further includes a decoder 50 for doing an ECC type decoding of the entire cache line of data including the ECC bits to determine the bit position of an eπoneous bit. Decoder 50 has a set of switching signal output lines SQ , Si , . S j , there being one such output line for each bit in the cache line of data. These switching signal output lines run to respective ones of the multiplexers MQ , Mi , , MN for controlling the selection of the multiplexer inputs. If there is no erroneous D bit, all of the multiplexers will be set to select the non-inverted inputs. If, on the other hand, there is an erroneous D bit, the multiplexer for that D bit will be set by its S signal to select the inverted input. While there has been described what is at present considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, intended to cover all such changes and modifications as come within the true spirit and scope of the invention.

Claims: CLAIMS What is claimed is: 1. A method of correcting an error in a data sample comprising: checking the parity of a data sample; if a parity error is detected, performing an error correction on the data sample; and if no parity error is detected, using the data sample without correction in at least one further data processing operation. 2. A method of correcting an error in a plural-byte data sample comprising: checking the parity of each byte in a plural-byte data sample; if a parity error is detected in any byte, performing an error coπection on the complete data sample; and if no parity error is detected, using the data sample without correction in at least one further data processing operation. 3. A method of correcting soft errors in a memory comprising: reading data from a predetermined location in a memory; checking the parity of the data; and if a parity eπor is detected, performing an error correction on the data and writing the corrected data back into the predetermined memory location. 4. A method in accordance with Claim 3 wherein the data is a plural-byte segment of data, the parity of each byte is checked and the error correction is performed on the complete data segment if a parity eπor is detected in any byte. 5. A method of coπecting soft eπors in a memory comprising: storing data into a memory; storing parity bits for the data into the memory; storing eπor correction code bits for the data into the memory; reading the data and its parity bits and eπor coπection code bits out of the memory; using the parity bits to determine the validity of the data; and using the error coπection code bits to correct invalid data. 6. A method in accordance with Claim 5 wherein: the parity bits are used to check the parity of the data and to produce an eπor signal if a parity error is detected; and the error coπection on the data is performed only when a parity error is detected. 7. A method of coπecting soft eπors in a memory when writing data to the memory comprising: initiating a write to a predetermined storage line in a memory; reading the existing data cuπently resident on the predetermined storage line; checking the parity of the read-out existing data; if no parity error is detected, merging new data with the existing data and writing the merged data to the predetermined storage line in the memory; and if a parity eπor is detected, performing an eπor coπection on the existing data and writing the coπected data to the predetermined storage line in the memory. 8. A method in accordance with Claim 7 wherein the write request to the predetermined storage line is re-initiated after successful writing of the coπected existing data. 9. Apparatus for coπecting soft eπors in a memory comprising: a memory for storing data; circuitry for reading out data from the memory; a parity checker for checking the parity of the read-out data and producing an eπor signal if a parity eπor is detected; an eπor coπector for coπecting data having a parity eπor; circuitry responsive to the parity error signal for causing the read-out data to be supplied to the eπor coπector for producing coπected data; and circuitry for writing the corrected data back into the memory. 10. Apparatus in accordance with Claim 9 wherein the memory is a cache memory. 11. Apparatus in accordance with Claim 9 wherein the parity checker is an exclusive-OR circuit tree. 12. Apparatus in accordance with Claim 9 wherein the eπor coπector is a single- bit error coπector for locating an erroneous bit in the read-out data and reversing the binary state of such bit. 13. Apparatus for coπecting soft errors in a memory comprising: a memory for storing multiple lines of data; write circuitry for writing lines of data into the memory, each such line having a plurality of data bytes, parity bits for the data bytes and eπor correction code bits for the line as a whole; read circuitry for reading out lines of data from the memory; a parity checker coupled to the read circuitry for checking the parity of each readout byte and producing an eπor signal if a parity eπor is detected in any byte; an eπor corrector for performing an eπor correction on a line of read-out data; and control circuitry responsive to a parity error signal for causing the associated line of read-out data to be supplied to the error coπector for producing a corrected line of data and for supplying the corrected line of data to the write circuitry for storing back into the memory. 14. Apparatus in accordance with Claim 13 wherein: the memory is a cache memory; the parity checker is an exclusive-OR circuit tree; and the eπor coπector is a single-bit eπor coπector for locating an erroneous bit in a read-out line of data and reversing the binary state of such bit. 15. Apparatus in accordance with Claim 13 and further including data merge circuitry for merging new data with a read-out line of data and for supplying the merged data to the write circuitry for storing into the memory and wherein the control circuitry cancels the data merge operation when a parity eπor signal is detected. 16. A method of detecting and coπecting an error in a data sample, the method comprising: reading a data sample, including at least one data byte, at least one parity bit for each data byte and at least one eπor coπection code bit for the data sample, from a computer data cache; checking the parity of at least one data byte for the data sample; and, where a parity eπor is detected, performing an error coπection on the at least one data byte, using the at least one error correction code bit, and writing the coπected at least one data byte back into the cache, replacing the incorrect at least one data byte. 17. The method of Claim 16, further comprising: where no parity eπor is detected in the data sample, allowing the data sample to be modified by introduction of new data and writing the modified data sample back into the cache at the cache location from which the data sample was read. 18. The method of Claim 16, further comprising: where no parity eπor is detected in the data sample, allowing the data sample to be read from the cache and used for at least one computer operation.

Inventors: GOLDSBURY TIMOTHY G, HETHERINGTON RICK, MOUDGAL ANURADHA N, PETRY JOHN P, GOLDSBURY, TIMOTHY, G., HETHERINGTON, RICK, MOUDGAL, ANURADHA, N., PETRY, JOHN, P.
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 2000
Main classification: G06F11
IPC codes: G06F11, G06F12