MEMORY CORRECTION SCHEME USING SPARE ARRAYS

Abstract: Spare chips (16, 16′) are employed together with a replacement algorithm to replace chips (12) in memory array when failure is generally more extensive then unrelated cell fails in the memory chips. That is, substitution will be made if an error condition is a result of the failure of a whole chip (chip-kill), a segment of a chip (island-kill), a column of bits of a chip or a row of bits of a chip but will not be performed when it is due to a single failed cell. The replacement of a chip with a chip-kill or with an island-kill is done on the fly and involves only a row of the memory chips or elements leaving other elements of the memory unaffected by the replacement. For each spare chip (16) there is provided a shift register (20) which can store a replacement chip *address, and a compare circuit (22) for comparing the chip address currently being accessed with the stored replace­ ment address and for issuing a spare chip enabling signal (C) when a match is found.

Description: Background of the Invention The present invention relates to correction of faulted memory using spare memory arrays. In EP-A-95,028 entitled "Fault Alignment Control System and Circuits"; EP-A-105,402 entitled "Memory Address Permutation Apparatus"; EP-A-90,219 entitled "Memory System Restructured by Deterministic Permutation Algorithm"; and EP-A-95,669 entitled "A Method of Memory Reconfiguration for Fault Tolerant Memory", the addresses supplied to the decoders of the memory elements storing the various bit positions of a code word are skewed relative to one another when an uncorrectable error condition is detected in a code word, to change which memory cells store the faulted code word. This scheme separates stuck bits which caused the uncorrectable error, so that they will cause correctable error conditions only. For instance, if the memory was protected with a single error correcting/double error detecting code, the two cells storing the faulted bits of the code word containing a detectable error would then be placed, so that they occur in two separate code words where the faults would produce two separate correctable error conditions. Such a scheme for separating stuck bits is suggested in the PCT Publication WO 83/02164. This publication especially relates to an apparatus for mapping and classifying the faulty bits of a large computer memory. By comparing test words read out from the memory to their original version bit errors can be detected. By counting the bit errors and relating the count to the addressing, it is possible to determine the type of fault, e.g. failures of entire bit lines or entire word lines, etc. This developed error information is used to scatter faulty memory bits among access data words. While such relocation schemes maintain memory availability, the available memory space becomes "dirty" as a result of the accumulation of bad bits within the space as time passes. This problem is particularly acute where the failed bits are grouped in a completely bad memory chip or are the result of a plurality of bit line or word line fails. Furthermore, with address permutation schemes, when a detectable uncorrectable error is detected the whole memory has to be flushed of data while the address skewing process is being performed resulting in a significant down time for the memory. It has been suggested that memories contain spare arrays which would be used to replace any bad bit in the memory. In one such memory, the locations of bad bits are stored separately and whenever memory access is made to an address containing an error, a bit from one of the spare arrays is substituted for the bad bit on a bit by bit basis. While this arrangement ends the accumulation of bad bits in the memory it results in significant increase in the access time of the memory. To get around this problem, it has been suggested that element substitution be done on a column or row basis as in U.S. Patent 4,051,354; U.S. Patent 3,633,175; U.S. Patent 4,038,648 and U.S. Patent 3,995,261. This involves use of whole spare rows or columns of chips which is quite an inefficient way to use spare memory capacity. A further example for a memory system with selective assignment of spare locations is described in EP-A-82,981. This memory system uses a small writable index to store control information and replacement address information. A subset of the main memory address is applied to the index and compared with the stored addresses. In the embodiment described in the publication a minimum replacement area of 1k bit cells or of a group of four columns of a chip is suggested. The Invention In accordance with the present invention, as claimed, spare chips are employed together with a replacement algorithm to replace chips in a memory array when failure is generally more extensive than unrelated cell fails in the memory chips. That is, substitution will be made if an error condition is a result of the failure of a whole chip (chip-kill), a segment of a chip (island-kill), a column of bits of a chip, or a row of bits of a chip but will not be performed when it is due to a single failed cell. The replacement of a chip with a chip-kill or with an island-kill is done on the fly and involves only a row of the memory chips or elements, leaving other elements of the memory unaffected by the replacement. In the preferred embodiment, the amount of spares needed with the scheme is quite small relative to the total number of elements in the array. For instance, in one of the illustrated embodiments only two spares are needed for a matrix array of 64 x 4 memory array chips. This is because the two spare chips can be placed in any position in a column or row of the matrix of arrays. Therefore, it is the object of the present invention to provide a sparing scheme in which memory elements are replaced in an operating machine with a zero or absolute minimum down time. It is another object of the present invention to provide a memory element replacement scheme that requires a minimum of spare memory elements. It is a further object of the invention to provide a memory element replacement scheme that uses a minimum of logic and a single replacement algorithm. The Drawings These and other objects of the invention can best be understood by referring to the accompanying drawings of an illustrative embodiment of the invention of which: Fig. 1 is a schematic diagram of a portion of a computer memory system incorporating the present invention; Fig. 2 is a schematic diagram of a memory card in the memory of the computer system in Fig. 1; Fig. 3 is a schematic drawing of a column of arrays in the memory card of Fig. 2; Fig. 4 is a table showing the effect of various types of failures on the words in the memory; Fig. 5 is a diagram of the test algorithm used in the memory of Fig. 1; Fig. 6 is a diagram of a replacement algorithm used in the memory shown in Fig. 1; Fig. 7 is an alternative form of a memory that incorporates the present invention; and Fig. 8 is another alternative form of a memory using multi-bit array chip organization that incorporates the present invention. Detailed Description of the Embodiment Fig. 1 shows a memory 32 for storing 144-bit code words or quadwords on 36 cards 10 each containing four bit positions of the code word, where 128 of the bit positions are data bits and the remainder are ECC (error correction code) check bits of a double error correction/triple error detection (DEC/TED) code. As shown in Fig. 2, all the storage bits in each bit position B i to B i+3 of any array card 10 of the memory are arranged in a column of 64 separate 256 x 256 arrays 12 of bit storage cells, each array or chip being accessed by a separate row select line from a decoder 14 which receives a 6-bit row address and decodes it to produce an access signal on one of the 64 row access lines. This access signal selects one row of chips 12 on the card. Each of the cards 10 in Fig. 1 has a similar decode scheme, whereby a chip row address enables only one of the 64 chips in each of the 144 bit array columns of the memory. An X-Y coordinate address of 16 bits is supplied to all of the chips 12 on the cards 10. This address is supplied to a word decoder 11 and a bit decoder 13 to select a single storage cell in each of the arrays 12. Thus, as a result of a combination of a row select address and an X-Y coordinate address a single bit in each of the 144 columns shown in Fig. 1 is selected. This memory is a paging store, that is, data is read out of this store in pages of 256 code words each. The pages are interleaved among the arrays 12 in each column, with bits of 16 code words of a page in a given array 12. Each code word in a page has the same Y address, 1 of 16 different X addresses and of course any code word of a page may have 1 of 16 different chip row addresses. Fig. 3 shows three of the arrays 12a, 12b and 12p for bits in the first bit position of code words. The numbers on the bits correspond to page, and word position. Thus, the left hand bit in the top most row of array 12a is numbered 1-1 since it is in the first word of the first page. The next bit position in that row is numbered 1-17 since it is in the 17th word of the first page and so on until the last or right hand bit which is numbered 16-241 to indicate it is in the 241st word of the sixteenth page. As shown, the second row of the array contains the storage cells for the first bit position of words 1, 17, 33...241 for the 17th to 32nd pages, the third line contains the storage cells for the first bit position of words 1, 17, 33...241 of the 33rd to 48th pages and so on until the 4096th page. Cells for bit positions of words 2, 18, 34...242 for pages 1 to 4096 are contained on the second chip 12b and cells for the bit positions of words 3, 19, 35...243 of pages 1 to 4096 are contained on the third chip (not shown) and so on, so that all bits in the first bit position of all code words for the first 4096 pages are contained on the top 16 chips 12a-p of the first column. Fig. 4 shows the effect that different types of failure categories have on this memory. The first column describes the type of failure. The second column tells the number of pages affected by the type of failure listed alongside it in the first column. The third column tells how many words per page will be affected by the failures, while the fourth and fifth columns contain the word and page address patterns of the faults. Referring back to Fig. 2, it can be seen that two spare array chips 16 are placed on each of the cards 10. Either of the spare array chips 16 can be substituted for any one of the 256 chips on the card 10. To accomplish this substitution, a 10-bit substitution address R0-R9 is stored in one of two shift register strings 20. As shall be seen, the data in each shift register string controls the substitution of one of the spares 16 for one of the arrays 12. The first seven bits R0-R6 of the substitution address are fed into a compare circuit 22 where bit R0 operates as a compare circuit enable signal and bits R1 to R6 are compared with the chip row select address. If they match, a compare signal C is generated enabling the appropriate spare 16 so that it can be accessed through the data gating circuitry 18 in place of a bad chip. The output of the compare circuit 22 is also used to enable the one of two decoder circuits 19 which receives the other three substitution address bits R7-R9 fed into the particular shift register 20. The decode circuit selects one of the four bit or column positions B i to B i+3 for substituting the spare array 16 for the array 12 in that column position only when the spare 16 is activated by an enabling output from the compare circuit. To accomplish the substitution the output of the decoder 19 is fed to the data gating select circuit 18 controlling the output of the particular bit position. These gating select circuits 18 operate as follows. In each bit position B i to B i+3 on the card 10, one output line from each of the decoders 19 is fed into an OR gate 23. So long as the given decoder output of both decoders 19 is not up, the complemented output of OR gate 23 activates an AND gate 24 to pass data to and from the chips 12 along the bit line B i . When either decoder output in that bit position comes up the AND gate 24 becomes deactivated and an AND gate 26 is activated by the true of that decoder's output substituting one of the spares 16 for the disabled chip 12. Thus, it can be seen, so long as all the chips are good on the card, the address placed in either shift register 20 is all zeros, and replacement control signal C is zero preventing the decoders 19 from deselecting any chip 12 in one of the bit positions B i to B i+3 and substituting a spare 16 therefor. When there is a non-zero output in R0 and possibly in any of the other register positions R₁ to R₉ one of the active output lines of the decoder 19 will be selected permitting the substitution. The substitution of array chips 16 for a chip 12 is controlled by a count of the non-zero syndromes generated by the ECC logic 30 shown in Fig. 1. The ECC logic 30 receives a 144-bit code word from the memory 32 and generates error syndrome bits S1-S16 by comparing stored check bits with ECC generated check bits. A non-zero detector 36 generates an output each time the syndrome bits are not all zero and feeds this output to a counter 38. This counter is automatically reset to zero at regular time intervals. This counter 38 counts the outputs of the non-zero detector 36 between reset times and if the count exceeds a threshold count one or more times during this interval, then the counter provides a signal to the chip kill testing apparatus 40 and resets itself to zero. The testing apparatus 40 then tests the memory for a bad chip. If a completely bad chip 12 is detected, its address is sent by spare replacement logic control function 42 in the memory controller or maintenance console into one of the registers 20 of the card 10 on which the bad chip 12 is mounted to substitute a spare chip 16 for the failed chip 12. The system determines the address of the bad chip from the syndrome bits S1 to S16 of the code words or by other code words in the error pattern of Fig. 4 which update the counter. If the test does not turn up a completely bad chip it will not cause the substitution. Instead, it allows the faulty but still viable chip 12 to remain in the memory until the computer system containing the memory goes into diagnostic mode for some other reason. Then a complete diagnostic analysis of the whole memory system is performed and the locations of bad bit lines or word lines are detected and the spares are used to substitute for faulty chips, giving the highest priority for chip-kills causing UEs (uncorrectable errors), followed by island-kills and line-kills respectively. Because of the periodic resetting of the counter 38 it is unlikely that chips containing individual cell fails or line fails will invoke the test procedure because the count in the counter will never reach the count of the threshold level before the counter is cleared. Therefore, only when there is a chip with a chip-kill or island-kill type of fault, the test procedure is performed. A better understanding of the test procedure can be obtained from the flow diagram of Fig. 5. As shown, when the non-zero syndrome counter 38 indicates the number of non-zero syndromes has exceeded the threshold level, the page and word address P j A i of the addressed word containg the bad bit is stored and the bit location is generated by the bad bit locator 44 and also stored, in step 45. The next step 46 is to test the memory for killed chips. This is done by generating syndromes for words P j A (j+16) and P (j+16) A i . As can be seen from Fig. 3, the bit locations of words P j A (i+16) and P (j+16) A i are located on the same chip adjacent to the failed bit in P j A i on a common bit or word line with the failed bit. The assumption being made in the test is that if two bits on the same line are bad the line is bad and if both the tested word line and bit line are bad the chip is all bad. The testing is done by placing test words into the memory and generating the syndromes from those test words. 1. From the first 16 quadwords in page P j determine the quadword address A i containing the stuck bit. As pointed out above, the error correcting code syndrome will identify the bit position. Note the word address Ai. From PjAi and bit position identify the faulty chip location. 2. Identify the possible stuck bit position in memory word PjA i+16 or PjA i+32 by noting the error syndrome bits from the ECC apparatus. Also identify the possible stuck bit position for word A i in page P j+16 . 3. Test for identical bit stuck positions by comparing error syndrome bits from these words and determine the fault type as shown in the flow diagram in Fig. 5: (a) Identical stuck bit positions in words PjAi, PjA i+16 and P j+16 Ai implies chipkill. (b) Identical stuck bit positions in words PjAi, PjA i+16 but not in P j+16 Ai implies a word linekill. (c) Identical stuck bit position in word PjAi and P j+16 Ai but not in PjA i+16 implies bit linekill. (d) Stuck bit position in PjAi alone implies a cell fail. Further verification of an already identified fault type is always possible by randomly selecting and testing for same stuck bit positions as shown in (a), (b), (c), and (d) on words such as PjA i+32 , PjA i+48 ...; and, P j+32 Ai, P j+48 Ai...respectively. It is noted in Fig. 4 that the occurrence of a chipkill affects ¼ of BSM (basic storage module) in the memory, i.e. 4K pages out the 16K pages provided in a BSM. If, in the replacement algorithm a chipkill pattern is found by step 56 of Fig. 6, the chip is replaced on the fly and the syndrome counter 38 is reset in step 60. If a chipkill is not found, the counter continues to count and automatically resets at a regular time interval. If the threshold is not exceeded again, normal operation is continued without substitution. If the threshold is exceeded in step 62, the page Pj is deallocated and normal operation is continued. The page is also deallocated when an uncorrectable error (UE) is detected by the ECC apparatus and normal operation is resumed. As explained previously, the next time the machine goes into diagnostic mode, all or some spare chips can be assigned in such a way so that UEs and correctable errors in the memory BSM are minimized. Above has been described one embodiment of the invention. This embodiment is of a paging store. However, as shown in EP Application No. of same priority date herewith and entitled "Reconfigurable Memory Using Both Address Permutation and Spare Memory Elements", the invention is not limited to backing stores with certain modifications. This sparing technique can be used in combination with address permutation apparatus in small main stores, or in combination with double complement apparatus using the same testing algorithm. However the testing algorithm is a function of memory organization. Before the testing techniques described herein can be used, the location of bits indicating chip kills or bits indicating word and bit line failures must be identified in the memory under consideration. Another memory using the same testing algorithm is shown in Fig. 7. Here the placement of the two spares 16 can be made into any one of the 144 chips on the card where each chip contains one bit B i of the code word. Such card organization is selected for obtaining high data rate from the BSM. A substitution is made when either select bit S1 or S2 is non-zero, activating one of the decoders. Then the bit position replacement address bits R₁ to R₈ in the particular shift register specifies which array is to be replaced. The decoders, data gating and shift registers operate in the manner described in connection with the previously described embodiment to accomplish the substitution. A third embodiment of card design is shown in Fig. 8 where multibit chips of 256K bit each are organized as 64 x 2 chip array. This also uses one or two spare arrays per card. The ECC code with this embodiment is single error correct/single package detect. Upon the detection of a 2-4 bit package error the spare is substituted for defective arrays, while single bit errors in BSM are corrected by the single bit error correction capability of the code. Above have been described three embodiments of the invention. It should be apparent that the invention is adaptable to varying memory configurations. The unit of replacement can also be multi-chip array modules on a card or complete array cards in a BSM. Such card and module replacements may be necessary in some cases to overcome a support logic fail. Also, the designer can choose to make replacements offline after each memory diagnostic step for simplification.

Claims: A computer system having a memory (32) protected by an error checking arrangement (30, 36, 44) in which data is stored in multi-bit position code words with different bit positions on different arrays (12), with spare array substitution means (16, 18, 20, 42) for substituting a spare array (16) for a faulty array (12), characterized by: counter means (38) for counting the number of occurrences of non-zero syndrome signals of a syndrome generator (30) of said error checking arrangement (36), said counter means being reset to zero at regular time intervals or upon reaching a threshold count, test means (40) for testing the memory for bad arrays by means of said error checking arrangement when the count in the counter exceeds that threshold count; and spare replacement logic means (42) controlled by said test means and by error location logic means (44) of said error checking arrangement. The system of claim 1 wherein said error checking arrangement is an error detection and correction system, and said threshold count is in excess of that likely to be caused by single cell failures in that arrays during that time interval but will be exceeded by and thus detect multiple failures of entire row and column type in a single array. The system of claim 2 wherein said test means (40) checks for multiple failures of row and column type and said spare replacement logic means (42) effects that substitution only when said test means detects a multiple row and column failure. The system of claim 1 wherein the bit positions of a code word are contained in a plurality of memory cards (10), each containing a number of arrays, and wherein each card comprises a small number of spare arrays (16) and for each said spare is provided a separate register (20) for storing an array substitution address, and a compare circuit (22) for comparing said stored substitution address with the corresponding part of the current array address and for issueing a spare array enabling signal (C) when the compared array addresses match. The system of claim 4 wherein each card (10) further comprises a decoder circuit (19) for each said spare (16), said decoder being enabled by said spare array enabling signal (C) from said compare circuit (22) to decode a number of low order array address bits (R7, R8) also stored in said register (20) and to select one of a plurality of data bit input/output gates (18) adapted to substitute data bits to/from said spare array (16) for data bits to/from said memory array (12) which is being replaced. The system of a previous claim including means (42) for substituting spare arrays for arrays with at least a single row or column failure when said arrays are not being used to substitute for a bad array. In a computer system having a memory protected by an error checking arrangement, in which data is stored in multibit code words with different bit positions on different arrays, the method comprising: counting during periodic time intervals of the operating time of the memory the number of occurrences of correctable errors detected by the error checking arrangement, testing (52-56, 45, 46) the memory for bad arrays by means of the syndrome signals generated by said error checking arrangement (30, 36, 44) when the count of said occurences exceeds a predetermined number in excess of those likely to be caused by unrelated single bit failures, and substituting (60, 66) a spare array for an array that the tests indicate contains failures other than unrelated single bit failures leaving those arrays containing unrelated stuck bits in the memory. The method of claim 7 wherein the replacement of arrays categorized as chip fails is effected (60) on-the-fly upon completion of the testing but arrays categorized as bit line fails or word line fails are replaced (66) at a later time when the memory goes into the next diagnostic mode, based on the available bits of spare arrays at that time. The method of claim 7 wherein said counting is done offline during memory diagnostics to detect the type of memory fault.

Inventors: SINGH, SHANKER, SINGH, VIJENDRA P.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1984
Main classification: G06F11
IPC codes: G06F12, G06F11, G11C29