Patent Publication Number: US-6711703-B2

Title: Hard/soft error detection

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of application No. 09/769,958 filed on Jan. 25, 2001 which claims priority under 35 U.S.C §119(e) to provisional application Ser. No. 60/178,108 filed on Jan. 26, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to memory protection, and more specifically to a technique for detecting errors in a memory device. 
     2. Description of the Related Art 
     This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Semiconductor memory devices used in computer systems, such as dynamic random access memory (DRAM) devices, generally comprise a large number of capacitors which store binary data in each memory device in the form of a charge. These capacitors are inherently susceptible to errors. As memory devices get smaller and smaller, the capacitors used to store the charges also become smaller thereby providing a greater potential for errors. 
     Memory errors are generally classified as “hard errors” or “soft errors.” Hard errors are generally caused by issues such as poor solder joints, connector errors, and faulty capacitors in the memory device. Hard errors are reoccurring errors which generally require some type of hardware correction such as replacement of a connector or memory device. Soft errors, which cause the vast majority of errors in semiconductor memory, are transient events wherein extraneous charged particles cause a change in the charge stored in one of the capacitors in the memory device. When a charged particle, such as those present in cosmic rays, comes in contact with the memory circuit, the particle may change the charge of one or more memory cells, without actually damaging the device. Because these soft errors are transient events, generally caused by alpha particles or cosmic rays for example, the errors are not generally repeatable and are generally related to erroneous charge storage rather than hardware errors. For this reason, soft errors, if detected, may be corrected by rewriting the erroneous memory cell with correct data. Uncorrected soft errors will generally result in unnecessary system failures. Further, soft errors may be mistaken for more serious system errors and may lead to the unnecessary replacement of a memory device. By identifying soft errors in a memory device, the number of memory devices which are actually physically error free and are replaced due to mistaken error detection can be mitigated, and the errors may be easily corrected before any system failures occur. 
     Memory errors can be categorized as either single-bit or multi-bit errors. A single bit error refers to an error in a single memory cell. Single-bit errors can be detected and corrected by standard Error Code Correction (ECC) methods. However, in the case of multi-bit errors, which affect more than one bit, standard ECC methods may not be sufficient. In some instances, ECC methods may be able to detect multi-bit errors, but not correct them. In other instances, ECC methods may not even be sufficient to detect the error. Thus, multi-bit errors must be detected and corrected by a more complex means since a system failure will typically result if the multi-bit errors are not detected and corrected. 
     Regardless of the classification of memory error (hard/soft, single-bit/multi-bit), the current techniques for detecting the memory errors have several drawbacks. Typical error detection techniques rely on READ commands being issued by requesting devices, such as a peripheral disk drive. Once a READ command is issued to a memory sector, a copy of the data is read from the memory sector and tested for errors en route to delivery to the requesting device. Because the testing of the data in a memory sector only occurs if a READ command is issued to that sector, seldom accessed sectors may remain untested indefinitely. Harmless single-bit errors may align over time resulting in uncorrectable multi-bit errors. Once a READ request is finally issued to a seldom accessed sector, previously correctable errors may have evolved into uncorrectable errors thereby causing unnecessary data corruption or system failures. Early error detection may significantly reduce the occurrences of uncorrectable errors and prevent future system failures. 
     Further, in redundant memory systems, undetected memory errors may pose an additional threat. Certain operations, such as hot-plug events, may require that the system transition from a redundant to a non-redundant state. In a non-redundant state, memory errors which were of little concern during a redundant mode of operation, may become more significant since errors that were correctable during a redundant mode of operation may no longer be correctable while the system operates in a non-redundant state. 
     The present invention may address one or more of the concerns set forth above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a block diagram illustrating an exemplary computer system; 
     FIG. 2 illustrates an exemplary memory device used in the present system; 
     FIG. 3 generally illustrates a cache line and memory controller configuration in accordance with the present technique; 
     FIG. 4 generally illustrates the implementation of a RAID memory system to recreate erroneous data words; 
     FIG. 5 illustrates an exemplary memory sub-system in accordance with the present technique; and 
     FIG. 6 is a block diagram illustrating an exemplary architecture associated with a computer system in accordance with the present technique. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Turning now to the drawings, and referring initially to FIG. 1, a multiprocessor computer system, for example a Proliant 8500 PCI-X from Compaq Computer Corporation, is illustrated and designated by the reference numeral  10 . In this embodiment of the system  10 , multiple processors  11  control many of the functions of the system  10 . The processors  11  may be, for example, Pentium, Pentium Pro, Pentium II Xeon (Slot- 2 ), or Pentium III processors available from Intel Corporation. However, it should be understood that the number and type of processors are not critical to the technique described herein and are merely being provided by way of example. 
     Typically, the processors  11  are coupled to a processor bus  12 . As instructions are sent and received by the processors  11 , the processor bus  12  transmits the instructions and data between the individual processors  11  and a host controller  13 . The host controller  13  serves as an interface directing signals between the processors  11 , cache accelerators  14 , a memory controller  15  (which may be comprised of one or more memory control devices as discussed with reference to FIGS.  5  and  6 ), and an I/O controller  19 . Generally, ASICs are located within the host controller  13 . The host controller  13  may include address and data buffers, as well as arbitration and bus master control logic. The host controller  13  may also include miscellaneous logic, such as error detection and correction logic. Furthermore, the ASICs in the host controller may also contain logic specifying ordering rules, buffer allocation, specifying transaction type, and logic for receiving and delivering data. When the data is retrieved from the memory  16 , the instructions are sent from the memory controller  15  via a memory bus  17 . The memory controller  15  may comprise one or more suitable standard memory control devices or ASICs. 
     The memory  16  in the system  10  is generally divided into groups of bytes called cache lines. Bytes in a cache line may comprise several variable values. Cache lines in the memory  16  are moved to a cache for use by the processors  11  when the processors  11  request data stored in that particular cache line. 
     The host controller  13  is coupled to the memory controller  15  via a memory network bus  18 . As mentioned above, the host controller  13  directs data to and from the processors  11  through the processor bus  12 , to and from the memory controller  15  through the network memory bus  18 , and to and from the cache accelerator  14 . In addition, data may be sent to and from the I/O controller  19  for use by other systems or external devices. The I/O controller  19  may comprise a plurality of PCI-bridges, for example, and may include counters and timers as conventionally present in personal computer systems, an interrupt controller for both the memory network and I/O buses, and power management logic. Further, the I/O controller  19  is coupled to multiple I/O buses  20 . Finally, each I/O bus  20  terminates at a series of slots or I/O interface  21 . 
     Generally, a transaction is initiated by a requester, e.g., a peripheral device, via the I/O interface  21 . The transaction is then sent to one of the I/O buses  20  depending on the peripheral device utilized and the location of the I/O interface  21 . The transaction is then directed towards the I/O controller  19 . Logic devices within the I/O controller  19  generally allocate a buffer where data returned from the memory  16  may be stored. Once the buffer is allocated, the transaction request is directed towards the processor  11  and then to the memory  16 . Once the requested data is returned from the memory  16 , the data is stored within a buffer in the I/O controller  19 . The logic devices within the I/O controller  19  operate to read and deliver the data to the requesting peripheral device such as a tape drive, CD-ROM device or other storage device. 
     A system  10 , such as a computer system, generally comprises a plurality of memory modules, such as Dual Inline Memory Modules (DIMMs). A standard DIMM may include a plurality of memory devices such as Dynamic Random Access Memory Devices (DRAMs). In an exemplary configuration, a DIMM may comprise nine memory devices on each side of the DIMM  22 . FIG. 2 illustrates one side of a DIMM  22  which includes nine DRAMs  23 . The second side of the DIMM  22  may be identical to the first side and may comprise nine additional DRAM devices (not shown). Each DIMM  22  access generally accesses all DRAMs  23  on the DIMM  22  to produce a data word. For example, a DIMM  22  comprising ×4 DRAMs  23  (DRAMs passing 4-bits with each access) will produce 72-bit data words. System memory is generally accessed by CPUs and I/O devices as a cache line of data. A cache line generally comprises several 72-bit data words. Thus, each DIMM  22  accessed on a single memory bus provides a cache line of 72-bit data words  24 . 
     Each of the 72 bits in each of the data words  24  is susceptible to soft errors. Different methods of error detection may be used for different memory architectures. The present method and architecture incorporates a Redundant Array of Industry Standard DIMMs (RAID). As used herein, RAID memory refers to a “4+1 scheme” in which a parity word is created using an XOR module such that any one of the four data words can be re-created using the parity word if an error is detected in one of the data words. Similarly, if an error is detected in the parity word, the parity word can be re-created using the four data words. By using the present RAID memory architecture, not only can multi-bit errors be easily detected and corrected, but it also provides a system in which the memory module alone or the memory module and associated memory controller can be removed and/or replaced while the system is running (i.e. the memory modules and controllers are hot-pluggable). 
     FIG. 3 illustrates one implementation of RAID memory. RAID memory stripes a cache line of data  25  such that each of the four 72-bit data words  26 ,  27 ,  28 , and  29  is transmitted through a separate memory control device  30 ,  31 ,  32 , and  33 . A fifth parity data word  34  is generated from the original cache line  25 . Each parity word  34  is also transmitted through a separate memory control device  35 . The generation of the parity data word  34  from the original cache line  25  of data words  26 ,  27 ,  28 , and  29  can be illustrated by way of example. For simplicity, four-bit data words are illustrated. However, it should be understood that these principals are applicable to 72-bit data words, as in the present system, or any other useful word lengths. Consider the following four data words: 
     DATA WORD 1: 1 0 1 1 
     DATA WORD 2: 0 0 1 0 
     DATA WORD 3: 1 0 0 1 
     DATA WORD 4: 0 1 1 1 
     A parity word can be either even or odd. To create an even parity word, common bits are simply added together. If the sum of the common bits is odd, a “1” is placed in the common bit location of the parity word. Conversely, if the sum of the bits is even, a zero is placed in the common bit location of the parity word. In the present example, the bits may be summed as follows: 
     DATA WORD 1: 1 0 1 1 
     DATA WORD 2: 0 0 1 0 
     DATA WORD 3: 1 0 0 1 
     DATA WORD 4: 0 1 1 1 
     Parity Word: 0 1 1 1 
     When summed with the four exemplary data words, the parity word 0111 will provide an even number of active bits (or “1&#39;s”) in every common bit. This parity word can be used to re-create any of the data words (1-4) if a soft error is detected in one of the data words as further explained with reference to FIG.  4 . 
     FIG. 4 illustrates the re-creation of a data word in which a soft error has been detected in a RAID memory system. As in FIG. 3, the original cache line  25  comprises four data words  26 ,  27 ,  28 , and  29  and a parity word  34 . Further, the memory control device  30 ,  31 ,  32 ,  33 , and  35  corresponding to each data word and parity word are illustrated. In this example, a data error has been detected in the data word  28 . A new cache line  36  can be created using data words  26 ,  27 , and  29  along with the parity word  34  using an exclusive-OR (XOR) module  37 . By combining each data word  26 ,  27 ,  29  and the parity word  34  in the XOR module  37 , the data word  28  can be re-created. The new and correct cache line  34  thus comprises data words  26 ,  27 , and  29  copied directly from the original cache line  25  and data word  28   a  (which is the re-created data word  28 ) which is produced by the XOR module  37  using the error-free data words ( 26 ,  27 ,  29 ) and the parity word  34 . It should also be clear that the same process may be used to re-create a parity word  34  if an error is detected therein using the four error-free data words. 
     Similarly, if the memory control device  32 , which is associated with the data word  28 , is removed during operation (i.e. hot-plugging) the data word  28  can similarly be re-created. Thus, any single memory control device can be removed while the system is running or any single memory control device can return a bad data word and the data can be re-created from the other four memory words using an XOR module. 
     FIG. 5 illustrates one embodiment of a memory sub-system  40 , which incorporates a redundant (4+1) scheme. The memory sub-system  40  comprises five memory cartridges  42   a-e . Memory cartridge  42   e , for example, may be used for parity storage. The memory cartridge  42   a  includes eight DIMMs  44  mounted thereon. Each DIMM  44  includes nine memory devices, such as DRAMs  46  on each side of the DIMM substrate. (FIG. 5 illustrates only one side of the DIMM  44 .) Further, the memory cartridge  42   a  has a memory control device  48   a  mounted thereon. It should be understood that each memory cartridge  42   a-e  includes a plurality of DIMMs  44  and a corresponding memory control device  48 . The memory cartridges  42   a-e  may be mounted on a memory system board  50  via connectors  52   a-e  to create the memory sub-system  40 . The memory sub-system  40  can be incorporated into a computer system via an edge connector  54  or by any suitable means of providing a data path from the computer system to the memory storage devices  46 . It should be evident that each of the memory cartridges  42   a-e  may be removed (hot-plugged) from the memory sub-system  40 . By removing a memory cartridge such as memory cartridge  42   a  from the memory sub-system  40 , the computer system will transition from a redundant mode of operation (implementing the fifth memory cartridge) to a non-redundant state. When transitioning from a redundant to a non-redundant mode of operation during a hot-plug memory event, it may be advantageous to verify that no errors exist in the remaining memory cartridges  42   b-e . Thus, immediately proceeding the removal of the memory cartridge  42   a , a verify procedure may be advantageously implemented. 
     Further, a verify procedure may be advantageous in checking for memory errors in certain areas of memory which may sit idle for an extended period of time, allowing accumulation of errors or the growth of a single bit error to an uncorrectable multi-bit error. The verify procedure is implemented through a piece of logic which may reside in the memory sub-system  40 . The verify logic can be programmed to verify a specific region of memory such as the contents of a single memory cartridge  42   a-e  or to verify the validity of the entire memory. The verify procedure relies on the normal ECC and error logging mechanisms to validate the health of the memory sub-system  40 . The verify routine may be exercised by an operator instruction, as part of a sequence of memory operations (such as a hot-plug event), or based on a predetermined schedule. Simply put, the verify logic will read a defined memory region. If errors are detected they may be recorded and corrected, as further discussed below with reference to FIG.  6 . Verify may then be executed again to validate that the correction mechanism in fact corrected the errors that were reported. The verify logic may reside in each memory controlled device  48   a-e  or on the memory system board  50 . 
     FIG. 6 is a block diagram illustrating one embodiment of the verify technique which incorporates the RAID memory architecture. As previously described, a computer system includes a memory sub-system  40  comprising memory cartridges  42   a-e . As described with reference to FIG. 5, each memory cartridge  42   a-e  may include a memory control device  48   a-e  (shown in FIG.  5 ). Thus, to access the memory devices  46  (shown in FIG. 5) in memory cartridge  42   a , a READ command is issued and data is passed through the memory control device  48   a , and so forth. 
     Each memory control device  48   a-e  may comprise ECC fault tolerance capability. As data is passed from the memory sub-system  40  to the host controller  58  via a memory network bus  60 , each data word being produced by a memory cartridge  42   a-e  is checked for single bit memory errors in each respective memory control device  48   a-e  (residing on each respective memory cartridge  42   a-e ) by typical ECC methods. If no errors are detected, the data is simply passed to the host controller  58  and eventually to a requesting device via an OUTPUT  68 . If a single-bit error is detected by a memory control device  48   a-e , the data is corrected by the memory control device  48   a-e . When the corrected data is sent to the host controller  58  via the memory network bus  60 , error detection and correction devices  62   a-e , which reside in the first controller  58  and may be identical to the ECC devices in the memory control devices  48   a-e , will not detect any erroneous data words since the single-bit errors have been corrected by the memory control devices  48   a-e  in the memory sub-system  40 . Therefore, if an error is detected and corrected by the memory control devices  48   a-e , a message is sent from the memory control devices  48  are to the host controller  58  indicating that a memory error has been detected and corrected and that the corresponding memory cartridge  42   a-e  should be over-written with corrected data, as discussed in more detail below. 
     In an alternate embodiment, the error detection capabilities in the memory control devices  48   a-e  may be turned off or eliminated. Because the host controller  58  also includes error detection and correction devices  62   a-e , any single bit errors can still be corrected using the standard ECC methods available in the host controller  58 . Further, it is possible that errors may be injected while the data is on the memory network bus  60 . In this instance, even if the error detection capabilities are turned on in the memory control devices  48   a-e , the memory control devices  48   a-e  will not detect an error since the error is injected after the data has passed from the memory sub-system  40 . Advantageously, since the host controller  58  includes similar or even identical error detection and correction devices  62   a-e , the errors can be detected and corrected in the host controller  58 . 
     If a multi-bit error is detected in one of the memory control devices  48   a-e , the memory control device  48   a-e , with standard ECC capabilities, can detect the errors but will not be able to correct the data error. Therefore, the erroneous data is passed to the error detection and correction devices  62   a-e . Like the memory control devices  48   a-e , the error detection and correction devices  62   a-e , which also have typical ECC detection, can only detect but not correct the multi-bit errors. The erroneous data words may be passed to the RAID memory engine  64  via some READ/WRITE control logic  66 , for correction. 
     In a typical memory READ operation, the host controller  58  will issue a READ command on the memory network bus  60 , the READ command originating from an external device such as a disk drive. The memory control devices  48   a-e  receive the request and retrieve the data from the corresponding memory cartridge  42   a-e . The data is then passed from the memory sub-system  40  to the host controller  58 . As described above, single-bit errors may either be corrected in the memory control devices  48   a-e  or the detection and correction devices  62   a-e . The RAID memory engine  64  will correct the multi-bit errors, as described above. The corrected data will be delivered from the host controller  58  to the requesting controller or I/O device via an OUTPUT  68 . 
     It should be evident from the discussion above, that performing error detection and correction on data residing in the memory sub-system  40  by relying on READ operations sent from peripheral devices will only result in detection of errors on those devices from which data is read. By relying on the READ command from a peripheral device, certain areas of memory may sit idle for extended periods thereby allowing data errors to accumulate undetected. To address this issue, an additional piece of logic may reside in the memory sub-system  40 . The verify logic  70  initiates a routine based on an operator instruction, a pre-determined periodic instruction, or some sequence of events such as a hot-plug event, for example. The verify logic  70  initiates a check of the specified memory location in the memory sub-system  40  without depending on normal READ accesses by external devices. 
     The verify logic  70  initiates a verify procedure through an arbiter  72  in the host controller  58 . The arbiter  72  is generally responsible for prioritizing accesses to the memory sub-system  40 . A queue comprises a plurality of requests such as memory READ, memory WRITE, memory verify, and memory scrubs (discussed further below), for example. The arbiter  72  prioritizes the requests and otherwise manages the queue. The verify logic  70  essentially initiates its own internal READ command to check specified regions of the memory sub-system  40 . Once the verify logic  70  initiates a request to the arbiter  72 , the verify procedure is scheduled in the queue. The request will pass through the READ/WRITE control logic  66  and to the memory sub-system  40 . The specified memory locations in the memory sub-system  40  will be read and any errors will be detected and/or corrected by the means described above with reference to the READ command issued by a peripheral device. The verify procedure implemented by the verify logic  70  can be initiated in a variety of ways. For instance, a user may be able to check specified memory locations by pulling up a window on an operating system. The window may allow a user to specify what locations in memory the user would like checked. By providing a user with the ability to check specified memory locations, the verify procedure provides user confidence in the validity of data stored in the memory sub-system  40 . 
     Alternately, the verify procedure may be a periodically scheduled event. In this instance, the verify logic  70  may include a timer and a buffer for storing a list of each address location in the memory sub-system  40 . At programmed or specified time intervals, the verify logic  70  may initiate READ commands to the arbiter  72  to verify the data stored in the corresponding address locations in the memory sub-system  40 . The verify logic  70  may initiate READ commands through successive addresses in the memory sub-system  40  such that every memory address is eventually checked. The verify logic  70  thus may insure that all address locations in the memory sub-system  40  or a specified set of address locations are periodically checked for validity. Furthermore, the READ command issued by the verify logic  70  may be scheduled as a low priority thread in the arbiter  72  to minimize system impact. In this way, the verify procedure may only be run during periods of low system activity (e.g. when the queue in the arbiter  72  does not include READ/WRITE requests from external devices). 
     Yet another implementation of the verify logic  70  includes a verify operation to validate a memory cartridge when the memory sub-system  40  is switching from a non-redundant mode of operation to a redundant mode of operation (i.e. during a hot-plug event). For example, referring back to FIG. 5, the memory cartridges  42   b-e  are currently connected to the memory system board  50 . Assuming that the memory system board  50  is operably coupled to a host system including a host controller  58  (as illustrated in FIG.  6 ), the memory sub-system  40  is operating in a non-redundant mode since there is no additional memory cartridge  42   a  to be used for parity. If a memory cartridge  42   a  is installed into the memory sub-system  40 , it may be advantageous to verify the memory devices  46  residing on the memory cartridge  42   a . The verify logic  70  can be implemented to check each address location on the memory devices  46  on the memory cartridge  42   a  before the system transitions to a redundant mode of operation. 
     First, the verify logic  70  initializes the memory cartridge  42   a  by writing zeros to each address location in the memory cartridge  42   a . The verify logic  70  schedules the initialization WRITEs through the arbiter  70 . Next, the verify logic  70  rebuilds the memory cartridge  42   a  by using the techniques described in FIGS. 3 and 4 to recreate the parity data that should be stored in the memory cartridge  42   a . As previously described, each cache line of data from the memory cartridges  42   b-e  are used to recreate the parity cache line by using the XOR module in the RAID memory engine  64 . Each recreated cache line is then written to the corresponding location in the memory cartridge  42   a . Finally, once the data in the memory cartridge  42   a  is rebuilt, the verify logic  70  may initiate a READ to insure that the data that should have been written to the memory cartridge  42   a  was in fact stored there. This procedure can be performed by again using the data stored in the memory cartridges  42   b-e  to again recreate the data that should be stored in the memory cartridge  42   a , and then by comparing those values to the values that were stored in the memory cartridge  42   a  during the rebuild procedure. If the data does not match an error message may be provided to a user indicating that a DIMM on the memory cartridge  42   a  may be bad. If there are no errors found in the new memory cartridge  42   a , the system may switch from a non-redundant mode of operation to a redundant mode of operation. 
     To this point, error detection via peripheral READ commands and READ commands implemented by the verify logic  70  have been discussed. The memory control devices  48   a-e , the error detection and correction devices  62   a-e  and the RAID memory engine  64  can be used to correct the data before it is written to the output  68 . However, at this point the data residing in the memory sub-system  40  may still be corrupted. To rectify this problem, the data in the memory sub-system  40  may be overwritten or “scrubbed.” For every data word in which a single bit error is detected and flagged by the memory control devices  48   a-e  or the error detection and correction devices  62   a-e , a request is sent to the scrubbing control logic  74  indicating that the corresponding memory location should be scrubbed during a subsequent WRITE operation initiated by the scrubbing control logic  74 . Similarly, if a multi-bit error is detected by the error detection and correction devices  62   a-e , the data is corrected through the RAID memory engine  64 , and the scrubbing control logic  74  is notified by the corresponding error detection and correction device  62   a-e  that the corresponding memory location in the memory sub-system  40  should be scrubbed. If a single-bit error is detected in one of the memory control devices  48   a-e , or a multi-bit error is detected in one of the error detection and correction devices  62   a-e  a message is sent to the scrubbing control logic  74  indicating that an erroneous data word has been detected. At this time, the corrected data word and corresponding address location are sent from the RAID memory engine  64  to a buffer  76  which is associated with the scrubbing process. The buffer  76  is used to store the corrected data and corresponding address location temporarily until such time that the scrubbing process can be implemented. Once the scrubbing control logic  74  receives an indicator that a corrupted data word has been detected and should be corrected in the memory sub-system  40 , a request is sent to the arbiter  72  which schedules and facilitates all accesses to the memory sub-system  40 . To insure proper timing and data control, each time a data word is rewritten back to the memory sub-system  40 , an entire cache line may be rewritten into each of the corresponding memory cartridges  42   a-e  in the subsystem  40  rather than just rewriting the erroneous data word. The scrubbing logic can be used to rewrite the locations in the memory sub-system  40  when errors are found during a typical READ operation or a verify procedure initiated by the verify logic  70 . 
     Further, the host controller  58  may include a content addressable memory (CAM) controller  78 . The CAM controller  78  provides a means of insuring that memory WRITEs are only performed when necessary. Because many READ and WRITE requests are active at any given time on the memory network bus  60  and because a scrubbing operation to correct corrupted data may be scheduled after a WRITE to the same memory location, the CAM controller  78  will compare all outstanding WRITE requests to subsequent memory scrub requests which are currently scheduled in the queue. It is possible that a corrupted memory location in the memory sub-system  40  which has a data scrub request waiting in the queue may be overwritten with new data prior to the scrubbing operation to correct the old data previously present in the memory sub-system  40 . In this case, the CAM controller  78  will recognize that new data has been written to the address location in the memory sub-system  40  by implementing a simple compare function between the addresses and will cancel the scheduled scrubbing operation. The CAM controller  78  will insure that the old corrected data does not over-write new data which has been stored in the corresponding address location in the memory sub-system  40 . 
     It should be noted that the error detection and scrubbing technique described herein may not distinguish between soft and hard errors. While corrected data may still be distributed through the output of the host controller  58 , if the errors are hard errors, the scrubbing operation to correct the erroneous data words in the memory sub-system  40  will be unsuccessful. To solve this problem, software in the host controller  58 , indicated in FIG. 6 by reference numeral  80 , may track the number of data errors associated with a particular data word or memory location. After some pre-determined number of repeated errors are detected in the same data word or memory location, the host controller  58  may send an error message to a user or illuminate an LED corresponding to the device in which the repeat error is detected. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.