Abstract:
One embodiment of the present invention sets forth a memory module that includes at least one memory chip, and an intelligent chip coupled to the at least one memory chip and a memory controller, where the intelligent chip is configured to implement at least a part of a RAS feature. The disclosed architecture allows one or more RAS features to be implemented locally to the memory module using one or more intelligent register chips, one or more intelligent buffer chips, or some combination thereof. Such an approach not only increases the effectiveness of certain RAS features that were available in prior art systems, but also enables the implementation of certain RAS features that were not available in prior art systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The current application is a continuation-in-part of U.S. patent application Ser. No. 11/474,076, filed on Jun. 23, 2006, which claims priority to U.S. Provisional Patent Application No. 60/693,631, filed on Jun. 24, 2005. The current application is a continuation-in-part of U.S. patent application Ser. No. 11/515,223, filed on Sep. 1, 2006, which claims priority to U.S. Provisional Patent Application No. 60/713,815, filed on Sep. 2, 2005. The current application also claims the priority benefit of U.S. Provisional Patent Application No. 60/814,234, filed on Jun. 16, 2006 and titled, “Memory Systems and Memory Modules.” The subject matter of the above-listed related applications is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This application relates generally to memory systems and more specifically to memory modules that include reliability and serviceability features and functionalities. 
     2. Description of the Related Art 
     Memory systems in computers are typically built from memory modules. An example of a common type of memory module is a Dual Inline Memory Module or DIMM. It is very important for many computers, particularly in classes of computers known as servers, that the memory system be as reliable as possible. It is also very important that the computer and memory system are built in such a way that the computer can be serviced as quickly, easily and as inexpensively as possible. Together these important features are often known as Reliability and Serviceability (RAS) features. 
     SUMMARY OF THE INVENTION 
     Computers are often required to have a very low probability of failure. In order to reduce the probability of failure it is often required to add reliability features to the memory system and to the memory modules in a computer memory system. 
     The embodiments described herein are a series of RAS features that may be used in memory systems. One embodiment of the present invention sets forth a memory module that includes at least one memory chip, and an intelligent chip coupled to the at least one memory chip and a memory controller, where the intelligent chip is configured to implement at least a part of a RAS feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts connectivity in an embodiment that includes an intelligent register and multiple buffer chips. 
         FIG. 1B  depicts a generalized layout of components on a DIMM, including LEDs. 
         FIG. 2A  depicts a memory subsystem with a memory controller in communication with multiple DIMMs. 
         FIG. 2B  depicts a side view of a stack of memory including an intelligent buffer chip. 
         FIG. 3  depicts steps for performing a sparing substitution. 
         FIG. 4  depicts a memory subsystem where a portion of the memory on a DIMM is spared. 
         FIG. 5  depicts a selection of functions optionally implemented in an intelligent register chip or an intelligent buffer chip. 
         FIG. 6A  depicts a memory stack in one embodiment with eight memory chips and one intelligent buffer. 
         FIG. 6B  depicts a memory stack in one embodiment with nine memory chips and one intelligent buffer. 
         FIG. 7A  depicts an embodiment of a DIMM implementing checkpointing. 
         FIG. 7B  depicts an depicts an exploded view of an embodiment of a DIMM implementing checkpointing. 
         FIG. 8A  depicts adding a memory chip to a memory stack. 
         FIG. 8B  depicts adding a memory stack to a DIMM. 
         FIG. 8C  depicts adding a DIMM to another DIMM. 
         FIG. 9A  depicts a memory subsystem that uses redundant signal paths. 
         FIG. 9B  a generalized bit field for communicating data. 
         FIG. 9C  depicts the bit field layout of a multi-cycle packet. 
         FIG. 9D  depicts examples of bit fields for communicating data. 
     
    
    
     DETAILED DESCRIPTION 
     In order to build cost-effective memory modules it can be advantageous to build register and buffer chips that do have the ability to perform logical operations on data, dynamic storage of information, manipulation of data, sensing and reporting or other intelligent functions. Such chips are referred to in this specification as intelligent register chips and intelligent buffer chips. The generic term, “intelligent chip,” is used herein to refer to either of these chips. Intelligent register chips in this specification are generally connected between the memory controller and the intelligent buffer chips. The intelligent buffer chips in this specification are generally connected between the intelligent register chips and one or more memory chips. One or more RAS features may be implemented locally to the memory module using one or more intelligent register chips, one or more intelligent buffer chips, or some combination thereof. 
     In the arrangement shown in  FIG. 1A , one or more intelligent register chips  102  are in direct communication with the host system  104  via the address, control, clock and data signals to/from the host system. One or more intelligent buffer chips  107 A- 107 D are disposed between the intelligent register chips and the memory chips  106 A- 106 D. The signals  110 ,  111 ,  112 ,  113 ,  118  and  119  between an intelligent register chip and one or more intelligent buffer chips may be shared by the one or more intelligent buffer chips. In the embodiment depicted, the signals from the plural intelligent register chips to the intelligent buffer chips and, by connectivity, to the plural memory chips, may be independently controllable by separate instances of intelligent register chips. In another arrangement the intelligent buffer chips are connected to a stack of memory chips. 
     The intelligent buffer chips may buffer data signals and/or address signals, and/or control signals. The buffer chips  107 A- 107 D may be separate chips or integrated into a single chip. The intelligent register chip may or may not buffer the data signals as is shown in  FIG. 1A . 
     The embodiments described here are a series of RAS features that may be used in memory systems. The embodiments are particularly applicable to memory systems and memory modules that use intelligent register and buffer chips. 
     Indication of Failed Memory 
     As shown in  FIG. 1B , light-emitting diodes (LEDs)  108 ,  109  can be mounted on a memory module  100 . The CPU or host or memory controller, or an intelligent register can recognize or determine if a memory chip  106 A- 106 J on a memory module has failed and illuminate one or more of the LEDs  108 ,  109 . If the memory module contains one or more intelligent buffer chips  107 A,  107 H or intelligent register chips  102 , these chips may be used to control the LEDs directly. As an alternative to the LEDs and in combination with the intelligent buffer and/or register chips, the standard non-volatile memory that is normally included on memory modules to record memory parameters may be used to store information on whether the memory module has failed. 
     In  FIG. 1B , the data signals are not buffered (by an intelligent register chip or by an intelligent buffer chip). Although the intelligent buffer chips  107 A- 107 H are shown in  FIG. 1B  as connected directly to the intelligent register chip and act to buffer signals from the intelligent register chip, the same or other intelligent buffer chips may also be connected to buffer the data signals. 
     Currently indication of a failed memory module is done indirectly if it is done at all. One method is to display information on the failed memory module on a computer screen. Often only the failing logical memory location is shown on a screen, perhaps just the logical address of the failing memory cell in a DRAM, which means it is very difficult for the computer operator or repair technician to quickly and easily determine which physical memory module to replace. Often the computer screen is also remote from the physical location of the memory module and this also means it is difficult for an operator to quickly and easily find the memory module that has failed. Another current method uses a complicated and expensive combination of buttons, panels, switches and LEDs on the motherboard to indicate that a component on or attached to the motherboard has failed. None of these methods place the LED directly on the failing memory module allowing the operator to easily and quickly identify the memory module to be replaced. This embodiment adds just one low-cost part to the memory module. 
     This embodiment is part of the memory module and thus can be used in any computer. The memory module can be moved between computers of different types and manufacturer. 
     Further, the intelligent register chip  102  and/or buffer chip  107 A- 107 J on a memory module can self-test the memory and indicate failure by illuminating an LED. Such a self-test may use writing and reading of a simple pattern or more complicated patterns such as, for example, “walking-1&#39;s” or “checkerboard” patterns that are known to exercise the memory more thoroughly. Thus the failure of a memory module can be indicated via the memory module LED even if the operating system or control mechanism of the computer is incapable of working. 
     Further, the intelligent buffer chip and/or register chip on a memory module can self-test the memory and indicate correct operation via illumination of a second LED  109 . Thus a failed memory module can be easily identified using the first LED  108  that indicates failure and switched by the operator with a replacement. The first LED might be red for example to indicate failure. The memory module then performs a self-test and illuminates the second LED  109 . The second LED might be green for example to indicate successful self-test. In this manner the operator or service technician can not only quickly and easily identify a failing memory module, even if the operating system is not working, but can effect a replacement and check the replacement, all without the intervention of an operating system. 
     Memory Sparing 
     One memory reliability feature is known as memory sparing. 
     Under one definition, the failure of a memory module occurs when the number of correctable errors caused by a memory module reaches a fixed or programmable threshold. If a memory module or part of a memory module fails in such a manner in a memory system that supports memory sparing, another memory module can be assigned to take the place of the failed memory module. 
     In the normal mode of operation, the computer reads and writes data to active memory modules. In some cases, the computer may also contain spare memory modules that are not active. In the normal mode of operation the computer does not read or write data to the spare memory module or modules, and generally the spare memory module or modules do not store data before memory sparing begins. The memory sparing function moves data from the memory module that is showing errors to the spare memory modules if the correctable error count exceeds the threshold value. After moving the data, the system inactivates the failed memory module and may report or record the event. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful memory sparing capabilities may be implemented. 
     For example, and as illustrated in  FIG. 2A  the intelligent register chip  242  that is connected indirectly or directly to all DRAM chips  243  on a memory module  250  may monitor temperature of the DIMM, the buffer chips and DRAM, the frequency of use of the DRAM and other parameters that may affect failure. The intelligent register chip can also gather data about all DRAM chip failures on the memory module and can make intelligent decisions about sparing memory within the memory module instead of having to spare an entire memory module. 
     Further, as shown in  FIG. 2A  and  FIG. 2B , an intelligent buffer chip  247  that may be connected to one or more DRAMs  245  in a stack  200  is able to monitor each DRAM  245  in the stack and if necessary spare a DRAM  246  in the stack. In the exemplary embodiment, the spared DRAM  246  is shown as an inner component of the stack. In other possible embodiments the spared DRAM may be any one of the components of the stack including either or both of the top and bottom DRAMs. 
     Although the intelligent buffer chips  247  are shown in  FIG. 2B  as connected directly to the intelligent register chip  242  and to buffer signals from the intelligent register chip, the same or other intelligent buffer chips may also be connected to buffer the data signals. Thus, by including intelligent register and buffer chips in a memory module, it is possible to build memory modules that can implement memory sparing at the level of being able to use a spare individual memory, a spare stack of memory, or a spare memory module. 
     In some embodiments, and as shown in  FIG. 3 , a sparing method  380  may be implemented in conjunction with a sparing strategy. In such a case, the intelligent buffer chip may calculate replacement possibilities  382 , optimize the replacement based on the system  384  or a given strategy and known characteristics of the system, advise the host system of the sparing operation to be performed  386 , and perform the sparing substitution or replacement  388 . 
     Memory Mirroring 
     Another memory reliability feature is known as memory mirroring. 
     In normal operation of a memory mirroring mode, the computer writes data to two memory modules at the same time: a primary memory module (the mirrored memory module) and the mirror memory module. 
     If the computer detects an uncorrectable error in a memory module, the computer will re-read data from the mirror memory module. If the computer still detects an uncorrectable error, the computer system may attempt other means of recovery beyond the scope of simple memory mirroring. If the computer does not detect an error, or detects a correctable error, from the mirror module, the computer will accept that data as the correct data. The system may then report or record this event and proceed in a number of ways (including returning to check the original failure, for example). 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful memory mirroring capabilities may be implemented. 
     For example, as shown in  FIG. 4 , the intelligent register chip  442  allows a memory module to perform the function of both mirrored and mirror modules by dividing the DRAM on the module into two sections  460  and  470 . The intelligent buffer chips may allow DRAM stacks to perform both mirror and mirrored functions. In the embodiment shown in  FIG. 4 , the computer or the memory controller  400  on the computer motherboard may still be in control of performing the mirror functions by reading and writing data to as if there were two memory modules. 
     In another embodiment, a memory module with intelligent register chips  442  and/or intelligent buffer chips  447  that can perform mirroring functions may be made to look like a normal memory module to the memory controller. Thus, in the embodiment of  FIG. 4 , the computer is unaware that the module is itself performing memory mirroring. In this case, the computer may perform memory sparing. In this manner both memory sparing and memory mirroring may be performed on a computer that is normally not capable of providing mirroring and sparing at the same time. 
     Other combinations are possible. For example a memory module with intelligent buffer and/or control chips can be made to perform sparing with or without the knowledge and/or support of the computer. Thus the computer may, for example, perform mirroring operations while the memory module simultaneously provides sparing function. 
     Although the intelligent buffer chips  447  are shown in  FIG. 4  as connected directly to the intelligent register chip  442  and to buffer signals from the intelligent register chip, the same or other intelligent buffer chips may also be connected to buffer the data signals. 
     Memory RAID 
     Another memory reliability feature is known as memory RAID. 
     To improve the reliability of a computer disk system it is usual to provide a degree of redundancy using spare disks or parts of disks in a disk system known as Redundant Array of Inexpensive Disks (RAID). There are different levels of RAID that are well-known and correspond to different ways of using redundant disks or parts of disks. In many cases, redundant data, often parity data, is written to portions of a disk to allow data recovery in case of failure. Memory RAID improves the reliability of a memory system in the same way that disk RAID improves the reliability of a disk system. Memory mirroring is equivalent to memory RAID level 1, which is equivalent to disk RAID level 1. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful memory RAID capabilities may be implemented. 
     For example, as shown in  FIG. 4 , the intelligent register chip  442  on a memory module allows portions of the memory module to be allocated for RAID operations. The intelligent register chip may also include the computation necessary to read and write the redundant RAID data to a DRAM or DRAM stack allocated for that purpose. Often the parity data is calculated using a simple exclusive-OR (XOR) function that may simply be inserted into the logic of an intelligent register or buffer chip without compromising performance of the memory module or memory system. 
     In some embodiments, portions  460  and  470  of the total memory on a memory module  450  are allocated for RAID operations. In other embodiments, the portion of the total memory on the memory module that is allocated for RAID operations may be a memory device on a DIMM  243  or a memory device in a stack  245 . 
     In some embodiments, physically separate memory modules  451 , and  452  of the total memory in a memory subsystem are allocated for RAID operations. 
     Memory Defect Re-Mapping 
     One of the most common failure mechanisms for a memory system is for a DRAM on a memory module to fail. The most common DRAM failure mechanism is for one or more individual memory cells in a DRAM to fail or degrade. A typical mechanism for this type of failure is for a defect to be introduced during the semiconductor manufacturing process. Such a defect may not prevent the memory cell from working but renders it subject to premature failure or marginal operation. Such memory cells are often called weak memory cells. Typically this type of failure may be limited to only a few memory cells in array of a million (in a 1 Mb DRAM) or more memory cells on a single DRAM. Currently the only way to prevent or protect against this failure mechanism is to stop using an entire memory module, which may consist of dozens of DRAM chips and contain a billion (in a 1 Gb DIMM) or more individual memory cells. Obviously the current state of the art is wasteful and inefficient in protecting against memory module failure. 
     In a memory module that uses intelligent buffer or intelligent register chips, it is possible to locate and/or store the locations of weak memory cells. A weak memory cell will often manifest its presence by consistently producing read errors. Such read errors can be detected by the memory controller, for example using a well-known Error Correction Code (ECC). 
     In computers that have sophisticated memory controllers, certain types of read errors can be detected and some of them can be corrected. In detecting such an error the memory controller may be designed to notify the DIMM of both the fact that a failure has occurred and/or the location of the weak memory cell. One method to perform this notification, for example, would be for the memory controller to write information to the non-volatile memory or SPD on a memory module. This information can then be passed to the intelligent register and/or buffer chips on the memory module for further analysis and action. For example, the intelligent register chip can decode the weak cell location information and pass the correct weak cell information to the correct intelligent buffer chip attached to a DRAM stack. 
     Alternatively the intelligent buffer and/or register chips on the memory module can test the DRAM and detect weak cells in an autonomous fashion. The location of the weak cells can then be stored in the intelligent buffer chip connected to the DRAM. 
     Using any of the methods that provide information on weak cell location, it is possible to check to see if the desired address is a weak memory cell by using the address location provided to the intelligent buffer and/or register chips. The logical implementation of this type of look-up function using a tabular method is well-known and the table used is often called a Table Lookaside Buffer (TLB), Translation Lookaside Buffer or just Lookaside Buffer. If the address is found to correspond to a weak memory cell location, the address can be re-mapped using a TLB to a different known good memory cell. In this fashion the TLB has been used to map-out or re-map the weak memory cell in a DRAM. In practice it may be more effective or efficient to map out a row or column of memory cells in a DRAM, or in general a region of memory cells that include the weak cell. In another embodiment, memory cells in the intelligent chip can be distributed for the weak cells in the DRAM. 
       FIG. 5  shows an embodiment of an intelligent buffer chip or intelligent register chip which contains a TLB  560  and a store  580  for a mapping from weak cells to known good memory cells. 
     Memory Status and Information Reporting 
     There are many mechanisms that computers can use to increase their own reliability if they are aware of status and can gather information about the operation and performance of their constituent components. As an example, many computer disk drives have Self Monitoring Analysis and Reporting Technology (SMART) capability. This SMART capability gathers information about the disk drive and reports it back to the computer. The information gathered often indicates to the computer when a failure is about to occur, for example by monitoring the number of errors that occur when reading a particular area of the disk. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful self-monitoring and reporting capabilities may be implemented. 
     Information such as errors, number and location of weak memory cells, and results from analysis of the nature of the errors can be stored in a store  580  and can be analyzed by an analysis function  590  and/or reported to the computer. In various embodiments, the store  580  and the analysis function  590  can be in the intelligent buffer and/or register chips. Such information can be used either by the intelligent buffer and/or register chips, by an action function  570  included in the intelligent buffer chip, or by the computer itself to take action such as to modify the memory system configuration (e.g. sparing) or alert the operator or to use any other mechanism that improves the reliability or serviceability of a computer once it is known that a part of the memory system is failing or likely to fail. 
     Memory Temperature Monitoring and Thermal Control 
     Current memory system trends are towards increased physical density and increased power dissipation per unit volume. Such density and power increases place a stress on the thermal design of computers. Memory systems can cause a computer to become too hot to operate reliably. If the computer becomes too hot, parts of the computer may be regulated or performance throttled to reduce power dissipation. 
     In some cases a computer may be designed with the ability to monitor the temperature of the processor or CPU and in some cases the temperature of a chip on-board a DIMM. In one example, a Fully-Buffered DIMM or FB-DIMM, may contain a chip called an Advanced Memory Buffer or AMB that has the capability to report the AMB temperature to the memory controller. Based on the temperature of the AMB the computer may decide to throttle the memory system to regulate temperature. The computer attempts to regulate the temperature of the memory system by reducing memory activity or reducing the number of memory reads and/or writes performed per unit time. Of course by measuring the temperature of just one chip, the AMB, on a memory module the computer is regulating the temperature of the AMB not the memory module or DRAM itself. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, more powerful temperature monitoring and thermal control capabilities may be implemented. 
     For example if a temperature monitoring device  595  is included into an intelligent buffer or intelligent register chip, measured temperature can be reported. This temperature information provides the intelligent register chips and/or the intelligent buffer chips and the computer much more detailed and accurate thermal information than is possible in absence of such a temperature monitoring capability. With more detailed and accurate thermal information, the computer is able to make better decisions about how to regulate power or throttle performance, and this translates to better and improved overall memory system performance for a fixed power budget. 
     As in the example of  FIG. 6A , the intelligent buffer chip  610  may be placed at the bottom of a stack of DRAM chips  630 A. By placing the intelligent buffer chip in close physical proximity and also close thermal proximity to the DRAM chip or chips, the temperature of the intelligent buffer chip will accurately reflect the temperature of the DRAM chip or chips. It is the temperature of the DRAM that is the most important temperature data that the computer needs to make better decisions about how to throttle memory performance. Thus, the use of a temperature sensor in an intelligent buffer chip greatly improves the memory system performance for a fixed power budget 
     Further the intelligent buffer chip or chips may also report thermal data to an intelligent register chip on the memory module. The intelligent buffer chip is able to make its own thermal decisions and steer, throttle, re-direct data or otherwise regulate memory behavior on the memory module at a finer level of control than is possible by using the memory controller alone. 
     Memory Failure Reporting 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful memory failure reporting may be implemented. 
     For example, memory failure can be reported, even in computers that use memory controllers that do not support such a mechanism, by using the Error Correction Coding (ECC) signaling as described in this specification. 
     ECC signaling may be implemented by deliberately altering one or more data bits such that the ECC check in the memory controller fails. 
     Memory Access Pattern Reporting and Performance Control 
     The patterns of operations that occur in a memory system, such as reads, writes and so forth, their frequency distribution with time, the distribution of operations across memory modules, and the memory locations that are addressed, are known as memory system access patterns. In the current state of the art, it is usual for a computer designer to perform experiments across a broad range of applications to determine memory system access patterns and then design the memory controller of a computer in such a way as to optimize memory system performance. Typically, a few parameters that are empirically found to most affect the behavior and performance of the memory controller may be left as programmable so that the user may choose to alter these parameters to optimize the computer performance when using a particular computer application. In general, there is a very wide range of memory access patterns generated by different applications, and, thus, a very wide range of performance points across which the memory controller and memory system performance must be optimized. It is therefore impossible to optimize performance for all applications. The result is that the performance of the memory controller and the memory system may be far from optimum when using any particular application. There is currently no easy way to discover this fact, no way to easily collect detailed memory access patterns while running an application, no way to measure or infer memory system performance, and no way to alter, tune or in any way modify those aspects of the memory controller or memory system configuration that are programmable. 
     Typically a memory system that comprises one or more memory modules is further subdivided into ranks (typically a rank is thought of as a set of DRAM that are selected by a single chip select or CS signal), the DRAM themselves, and DRAM banks (typically a bank is a sub-array of memory cells inside a DRAM). The memory access patterns determine how the memory modules, ranks, DRAM chips and DRAM banks are accessed for reading and writing, for example. Access to the ranks, DRAM chips and DRAM banks involves turning on and off either one or more DRAM chips or portions of DRAM chips, which in turn dissipates power. This dissipation of power caused by accessing DRAM chips and portions of DRAM chips largely determines the total power dissipation in a memory system. Power dissipation depends on the number of times a DRAM chip has to be turned on or off or the number of times a portion of a DRAM chip has to be accessed followed by another portion of the same DRAM chip or another DRAM chip. The memory access patterns also affect and determine performance. In addition, access to the ranks, DRAM chips and DRAM banks involves turning on and off either whole DRAM chips or portions of DRAM chips, which consumes time that cannot be used to read or write data, thereby negatively impacting performance. 
     In the compute platforms used in many current embodiments, the memory controller is largely ignorant of the effect on power dissipation or performance for any given memory access or pattern of access. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, however, powerful memory access pattern reporting and performance control capabilities may be implemented. 
     For example an intelligent buffer chip with an analysis block  590  that is connected directly to an array of DRAMs is able to collect and analyze information on DRAM address access patterns, the ratio of reads to writes, the access patterns to the ranks, DRAM chips and DRAM banks. This information may be used to control temperature as well as performance. Temperature and performance may be controlled by altering timing, power-down modes of the DRAM, and access to the different ranks and banks of the DRAM. Of course, the memory system or memory module may be sub-divided in other ways. 
     Check Coding at the Byte Level 
     Typically, data protection and checking is provided by adding redundant information to a data word in a number of ways. In one well-known method, called parity protection, a simple code is created by adding one or more extra bits, known as parity bits, to the data word. This simple parity code is capable of detecting a single bit error. In another well-known method, called ECC protection, a more complex code is created by adding ECC bits to the data word. ECC protection is typically capable of detecting and correcting single-bit errors and detecting, but not correcting, double-bit errors. In another well-known method called ChipKill, it is possible to use ECC methods to correctly read a data word even if an entire chip is defective. Typically, these correction mechanisms apply across the entire data word, usually 64 or 128 bits (if ECC is included, for example, the data word may be 72 or 144 bits, respectively). 
     DRAM chips are commonly organized into one of a very few configurations or organizations. Typically, DRAMs are organized as x4, x8, or x16; thus, four, eight, or 16 bits are read and written simultaneously to a single DRAM chip. 
     In the current state of the art, it is difficult to provide protection against defective chips for all configurations or organizations of DRAM. 
     In a memory module that includes intelligent register and/or intelligent buffer, chips powerful check coding capabilities may be implemented. 
     For example, as shown in  FIG. 6B , using an intelligent buffer chip  610  connected to a stack of x8 DRAMs  630 B checking may be performed at the byte level (across 8 bits), rather than at the data word level. One possibility, for example, is to include a ninth DRAM  620 , rather than eight DRAMs, in a stack and use the ninth DRAM for check coding purposes. 
     Other schemes can be used that give great flexibility to the type and form of the error checking. Error checking may not be limited to simple parity and ECC schemes, other more effective schemes may be used and implemented on the intelligent register and/or intelligent buffer chips of the memory module. Such effective schemes may include block and convolutional encoding or other well-known data coding schemes. Errors that are found using these integrated coding schemes may be reported by a number of techniques that are described elsewhere in this specification. Examples include the use of ECC Signaling. 
     Checkpointing 
     In High-Performance Computing (HPC), it is typical to connect large numbers of computers in a network, also sometimes referred to as a cluster, and run applications continuously for a very long time using all of the computers (possibly days or weeks) to solve very large numerical problems. It is therefore a disaster if even a single computer fails during computation. 
     One solution to this problem is to stop the computation periodically and save the contents of memory to disk. If a computer fails, the computation can resume from the last saved point in time. Such a procedure is known as checkpointing. One problem with checkpointing is the long period of time that it takes to transfer the entire memory contents of a large computer cluster to disk. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful checkpointing capabilities may be implemented. 
     For example, an intelligent buffer chip attached to stack of DRAM can incorporate flash or other non-volatile memory. The intelligent register and/or buffer chip can under external or autonomous command instigate and control the checkpointing of the DRAM stack to flash memory. Alternatively, one or more of the chips in the stack may be flash chips and the intelligent register and/or buffer chips can instigate and control checkpointing one or more DRAMs in the stack to one or more flash chips in the stack. 
     In the embodiment shown in the views of  FIG. 7A  and  FIG. 7B , the DIMM PCB  710  is populated with a stacks of DRAM S 0 -S 8  on one side and stacks of flash S 9 -S 17 , on the other side, where each flash memory in a flash stack corresponds with one of the DRAM in the opposing DRAM stack. Under normal operation, the DIMM uses only the DRAM circuits—the flash devices may be unused, simply in a ready state. However, upon a checkpoint event, memory contents from the DRAMs are copied by the intelligent register and/or buffer chips to their corresponding Flash memories. In other implementations, the flash chips do not have to be in a stack orientation. 
     Read Retry Detection 
     In high reliability computers, the memory controller may supports error detection and error correction capabilities. The memory controller may be capable of correcting single-bit errors and detecting, but typically not correcting, double-bit errors in data read from the memory system. When such a memory controller detects a read data error, it may also be programmed to retry the read to see if an error still occurs. If the read data error does occur again, there is likely to be a permanent fault, in which case a prescribed path for either service or amelioration of the problem can be followed. If the error does not occur again, the fault may be transient and an alternative path may be taken, which might consist solely of logging the error and proceeding as normal. More sophisticated retry mechanisms can be used if memory mirroring is enabled, but the principles described here remain the same. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful read retry detection capabilities may be implemented. Such a memory module is also able to provide read retry detection capabilities for any computer, not just those that have a special-purpose and expensive memory controllers. 
     For example, the intelligent register and/or buffer chips can be programmed to look for successive reads to memory locations without an intervening write to that same location. In systems with a cache between the processor and memory system, this is an indication that the memory controller is retrying the reads as a result of seeing an error. In this fashion, the intelligent buffer and/or register chips can monitor the errors occurring in the memory module to a specific memory location, to a specific region of a DRAM chip, to a specific bank of a DRAM or any such subdivision of the memory module. With this information, the intelligent buffer and/or register chip can make autonomous decisions to improve reliability (such as making use of spares) or report the details of the error information back to the computer, which can also make decisions to improve reliability and serviceability of the memory system. 
     In some embodiments, a form of retry mechanism may be employed in a data communication channel. Such a retry mechanism is used to catch errors that occur in transmission and ask for an incomplete or incorrect transmission to be retried. The intelligent buffer and/or register chip may use this retry mechanism to signal and communicate to the host computer. 
     Hot-Swap and Hot-Plug 
     In computers used as servers, it is often desired to be able to add or remove memory while the computer is still operating. Such is the case if the computer is being used to run an application, such as a web server, that must be continuously operational. The ability to add or remove memory in this fashion is called memory hot-plug or hot-swap. Computers that provide the ability to hot-plug or hot-swap memory use very expensive and complicated memory controllers and ancillary hardware, such as latches, programmable control circuits, microcontrollers, as well as additional components such as latches, indicators, switches, and relays. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful hot-swap and hot plug capabilities may be implemented. 
     For example, using intelligent buffer and/or register chips on a memory module, it is possible to incorporate some or all of the control circuits that enable memory hot-swap in these chips. 
     In conventional memory systems, hot-swap is possible by adding additional memory modules. Using modules with intelligent buffer and/or intelligent register chips, hot-swap may be achieved by adding DRAM to the memory module directly without the use of expensive chips and circuits on the motherboard. In the embodiment shown in  FIG. 8A , it is possible to implement hot-swap by adding further DRAMs to the memory stack. In another implementation as shown in  FIG. 8B , hot-swap can be implemented by providing sockets on the memory module that can accept DRAM chips or stacks of DRAM chips (with or without intelligent buffer chips). In still another implementation as shown in  FIG. 8C , hot-swap can be implemented by providing a socket on the memory module that can accept another memory module, thus allowing the memory module to be expanded in a hot-swap manner. 
     Redundant Paths 
     In computers that are used as servers, it is essential that all components have high reliability. Increased reliability may be achieved by a number of methods. One method to increase reliability is to use redundancy. If a failure occurs, a redundant component, path or function can take the place of a failure. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, extensive datapath redundancy capabilities may be implemented. 
     For example, intelligent register and/or intelligent buffer chips can contain multiple paths that act as redundant paths in the face of failure. An intelligent buffer or register chip can perform a logical function that improves some metric of performance or implements some RAS feature on a memory module, for example. Examples of such features would include the Intelligent Scrubbing or Autonomous Refresh features, described elsewhere in this specification. If the logic on the intelligent register and/or intelligent buffer chips that implements these features should fail, an alternative or bypass path may be switched in that replaces the failed logic. 
     Autonomous Refresh 
     Most computers use DRAM as the memory technology in their memory system. The memory cells used in DRAM are volatile. A volatile memory cell will lose the data that it stores unless it is periodically refreshed. This periodic refresh is typically performed through the command of an external memory controller. If the computer fails in such a way that the memory controller cannot or does not institute refresh commands, then data will be lost. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful autonomous refresh capabilities may be implemented. 
     For example, the intelligent buffer chip attached to a stack of DRAM chips can detect that a required refresh operation has not been performed within a certain time due to the failure of the memory controller or for other reasons. The time intervals in which refresh should be performed are known and specific to each type of DRAM. In this event, the intelligent buffer chip can take over the refresh function. The memory module is thus capable of performing autonomous refresh. 
     Intelligent Scrubbing 
     In computers used as servers, the memory controller may have the ability to scrub the memory system to improve reliability. Such a memory controller includes a scrub engine that performs reads, traversing across the memory system deliberately seeking out errors. This process is called “patrol scrubbing” or just “scrubbing.” In the case of a single-bit correctable error, this scrub engine detects, logs, and corrects the data. For any uncorrectable errors detected, the scrub engine logs the failure, and the computer may take further actions. Both types of errors are reported using mechanisms that are under configuration control. The scrub engine can also perform writes known as “demand scrub” writes or “demand scrubbing” when correctable errors are found during normal operation. Enabling demand scrubbing allows the memory controller to write back the corrected data after a memory read, if a correctable memory error is detected. Otherwise, if a subsequent read to the same memory location were performed without demand scrubbing, the memory controller would continue to detect the same correctable error. Depending on how the computer tracks errors in the memory system, this might result in the computer believing that the memory module is failing or has failed. For transient errors, demand scrubbing will thus prevent any subsequent correctable errors after the first error. Demand scrubbing provides protection against and permits detection of the deterioration of memory errors from correctable to uncorrectable. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, more powerful and more intelligent scrubbing capabilities may be implemented. 
     For example, an intelligent register chip or intelligent buffer chip may perform patrol scrubbing and demand scrubbing autonomously without the help, support or direction of an external memory controller. The functions that control scrubbing may be integrated into intelligent register and/or buffer chips on the memory module. The computer can control and configure such autonomous scrubbing operations on a memory module either through inline or out-of-band communications that are described elsewhere in this specification. 
     Parity Protected Paths 
     In computers used as servers, it is often required to increase the reliability of the memory system by providing data protection throughout the memory system. Typically, data protection is provided by adding redundant information to a data word in a number of ways. As previously described herein, in one well-known method, called parity protection, a simple code is created by adding one or more extra bits, known as parity bits, to the data word. This simple parity code is capable of detecting a single bit error. In another well-known method, called ECC protection, a more complex code is created by adding ECC bits to the data word. ECC protection is typically capable of detecting and correcting single-bit errors and detecting, but not correcting, double-bit errors. 
     These protection schemes may be applied to computation data. Computation data is data that is being written to and read from the memory system. The protection schemes may also be applied to the control information, memory addresses for example, that are used to control the behavior of the memory system. 
     In some computers, parity or ECC protection is used for computation data. In some computers, parity protection is also used to protect control information as it flows between the memory controller and the memory module. The parity protection on the control information only extends as far as the bus between the memory controller and the memory module, however, as current register and buffer chips are not intelligent enough to extend the protection any further. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, advanced parity protection coverage may be implemented. 
     For example, as shown in  FIG. 9A , a memory module that includes intelligent buffer and/or register chips, the control paths (those paths that involve control information, such as memory address, clocks and control signals and so forth) may be protected using additional parity signals to ECC protect any group of control path signals in part or in its entirety. Address parity signals  915  computed from the signals of the address bus  916 , for example, may be carried all the way through the combination of any intelligent register  902  and/or intelligent buffer chips  907 A- 907 D, including any logic functions or manipulations that are applied to the address or other control information. 
     Although the intelligent buffer chips  907 A- 907 D are shown in  FIG. 9A  as connected directly to the intelligent register chip  902  and to buffer signals from the intelligent register chip, the same or other intelligent buffer chips may also be connected to buffer the data signals. The data signals may or may not be buffered by the intelligent register chip. 
     ECC Signaling 
     The vast majority of computers currently use an electrical bus to communicate with their memory system. This bus typically uses one of a very few standard protocols. For example, currently computers use either Double-Data Rate (DDR) or Double-Date Rate 2 (DDR2) protocols to communicate between the computer&#39;s memory controller and the DRAM on the memory modules that comprise the computer&#39;s memory system. Common memory bus protocols, such as DDR, have limited signaling capabilities. The main purpose of these protocols is to communicate or transfer data between computer and the memory system. The protocols are not designed to provide and are not capable of providing a path for other information, such as information on different types of errors that may occur in the memory module, to flow between memory system and the computer. 
     It is common in computers used as servers to provide a memory controller that is capable of detecting and correcting certain types of errors. The most common type of detection and correction uses a well-known type of Error Correcting Code (ECC). The most common type of ECC allows a single bit error to be detected and corrected and a double-bit error to be detected, but not corrected. Again, the ECC adds a certain number of extra bits, the ECC bits, to a data word when it is written to the memory system. By examining these extra bits when the data word is read, the memory controller can determine if an error has occurred. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, a flexible error signaling capability may be implemented. 
     For example, as shown in  FIG. 9 , if an error occurs in the memory module, an intelligent register and/or buffer chip may deliberately create an ECC error on the data parity signals  917  in order to signal this event to the computer. This deliberate ECC error may be created by using a known fixed, hard-wired or stored bad data word plus ECC bits, or a bad data word plus ECC bits can be constructed by the intelligent register and/or buffer chip. Carrying this concept to a memory subsystem that includes one or more intelligent register chips and or one or more intelligent buffer chips, the parity signals  909 ,  911 , and  913  are shown implemented for signals  908 ,  910 , and  912 . Such parity signals can be implemented optionally for all or some, or none of the signals of a memory module. 
     This signaling scheme using deliberate ECC errors can be used for other purposes. It is very often required to have the ability to request a pause in a bus protocol scheme. The DDR and other common memory bus protocols used today do not contain such a desirable mechanism. If the intelligent buffer chips and/or register chips wish to instruct the memory controller to wait or pause, then an ECC error can be deliberately generated. This will cause the computer to pause and then typically retry the failing read. If the memory module is then able to proceed, the retried read can be allowed to proceed normally and the computer will then, in turn, resume normal operation. 
     Sideband and Inline Signaling 
     Also, as shown in  FIG. 9 , a memory module that includes intelligent buffer and/or register chips, may communicate with an optional Serial Presence Detect (SPD)  920 . The SPD may be in communication with the host through the SPD interface  922  and may be connected to any combination of any intelligent register  902  and/or any intelligent buffer chips  907 A- 907 D. The aforementioned combination implements one or more data sources that can program and/or read the SPD in addition to the host. Such connectivity with the SPD provides the mechanism to perform communication between the host and memory module in order to transfer information about memory module errors (to improve Reliability and Serviceability features, for example). Another use of the SPD is to program the intelligent features of the buffer and/or register chips, such as latency, timing or other emulation features. One advantage of using the SPD as an intermediary to perform communication between intelligent buffer and/or register chips with the host is that a standard mechanism already exists to use the SPD and host to exchange information about standard memory module timing parameters. 
     The SPD is a small, typically 256-byte, 8-pin EEPROM chip mounted on a memory module. The SPD typically contains information on the speed, size, addressing mode and various timing parameters of the memory module and its component DRAMs. The SPD information is used by the computer&#39;s memory controller to access the memory module. 
     The SPD is divided into locked and unlocked areas. The memory controller (or other chips connected to the SPD) can write SPD data only on unlocked (write-enabled) DIMM EEPROMs. The SPD can be locked via software (using a BIOS write protect) or using hardware write protection. The SPD can thus also be used as a form of sideband signaling mechanism between the memory module and the memory controller. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, extensive sideband as well as in-band or inline signaling capabilities may be implemented and used for various RAS functions, for example. 
     More specifically, the memory controller can write into the unlocked area of the SPD and the intelligent buffer and/or register chips on the memory module can read this information. It is also possible for the intelligent buffer and/or register chips on the memory module to write into the SPD and the memory controller can read this information. In a similar fashion, the intelligent buffer and/or register chips on the memory module can use the SPD to read and write between themselves. The information may be data on weak or failed memory cells, error, status information, temperature or other information. 
     An exemplary use of a communication channel (or sideband bus) between buffers or between buffers and register chips is to communicate information from one (or more) intelligent register chip(s) to one (or more) intelligent buffer chip(s). 
     In exemplary embodiments, control information communicated using the sideband bus  908  between intelligent register  902  and intelligent buffer chip(s)  907 A- 907 D may include information such as the direction of data flow (to or from the buffer chips), and the configuration of the on-die termination resistance value (set by a mode register write command). As shown in the generalized example  900  of  FIG. 9B , the data flow direction on the intelligent buffer chip(s) may be set by a “select port N, byte lane Z” command sent by the intelligent register via the sideband bus, where select  950  indicates the direction of data flow (for a read or a write), N  951  is the Port ID for one of the multiple data ports belonging to the intelligent buffer chip(s), and Z  952  would be either 0 or 1 for a buffer chip with two byte lanes per port. The bit field  953  is generalized for illustration only, and any of the fields  950 ,  951 ,  952  may be used to carry different information, and may be shorter or longer as required by the characteristics of the data. 
     The intelligent register chip(s) use(s) the sideband signal to propagate control information to the multiple intelligent buffer chip(s). However, there may be a limited numbers of pins and encodings used to deliver the needed control information. In this case, the sideband control signals may be transmitted by intelligent register(s) to intelligent buffer chip(s) in the form of a fixed-format command packet. Such a command packet be may two cycles long, for example. In the first cycle, a command type  960  may be transmitted. In the second cycle, the value  961  associated with the specific command may be transmitted. In one embodiment, the sideband command types and encodings to direct data flow or to direct Mode Register Write settings to multiple intelligent buffer chip(s) can be defined as follows (as an example, the command encoding for the command type  960  for presentation on the sideband bus in the first cycle is shown in parenthesis): 
     Null operation, NOP (000) 
     Read byte-lane 0 (001) 
     Write byte-lane 0 (010) 
     Update Mode Register Zero MR0 (011) 
     Write to both byte lanes 0 and 1 (100) 
     Read byte-lane 1 (101) 
     Write byte-lane 1 (110) 
     Update Extended Mode Register One EMR1 (111) 
     The second cycle contains values associated with the command in the first cycle. 
     There may be many uses for such signaling. Thus, for example, as shown in  FIG. 9D  if the bi-directional multiplexer/de-multiplexer on intelligent buffer chip(s) is a four-port-to-one-port structure, the Port IDs would range from 0 to 3 to indicate the path of data flow for read operations or write operations. The Port IDs may be encoded as binary values on the sideband bus as Cmd[1:0]  962  in the second cycle of the sideband bus protocol (for read and write commands). 
     Other uses of these signals may perform additional features. Thus, for example, a look-aside buffer (or LAB) may used to allow the substitution of data from known-good memory bits in the buffer chips for data from known-bad memory cells in the DRAM. In this case the intelligent buffer chip may have to be informed to substitute data from a LAB. This action may be performed using a command and data on the sideband bus as follows. The highest order bit of the sideband bus Cmd[2] 963 may used to indicate a LAB. In the case that the sideband bus Cmd[2] may indicate a LAB hit on a read command, Intelligent buffer chip(s) may then take data from a LAB and drive it back to the memory controller. In the case that the sideband bus Cmd[2] indicates a LAB hit on a write command, Intelligent buffer chip(s) may take the data from the memory controller and write it into the LAB. In the case that the sideband bus Cmd[2] does not indicate a LAB hit, reads and writes may be performed to DRAM devices on the indicated Port IDs. 
     Still another use as depicted in  FIG. 9D  of the sideband signal may be to transfer Mode Register commands sent by the memory controller to the proper destination, possibly with (programmable) modifications. In the above example command set, two commands have been set aside to update Mode Registers. 
     One example of such a register mode command is to propagate an MR0 command, such as burst ordering, to the intelligent buffer chip(s). For example, Mode Register MR0 bit A[3] 964 sets the Burst Type. In this case the intelligent register(s) may use the sideband bus to instruct the intelligent buffer chip(s) to pass the burst type (through the signal group  906 ) to the DRAM as specified by the memory controller. As another example, Mode Register MR0 bit A[2:0] sets the Burst Length  965 . In this case, in one configuration of memory module, the intelligent register(s) may use the sideband bus to instruct the intelligent buffer chip(s) to always write &#39;010 (corresponding to a setting of burst length equal to four or BL4) to the DRAM. In another configuration of memory module, if the memory controller had asserted &#39;011, then the intelligent register(s) must emulate the BL8 column access with two BL4 column accesses. 
     In yet another example of this type sideband bus use, the sideband bus may be used to modify (possibly under programmable control) the values to be written to Mode Registers. For example, one Extended Mode Register EMR1 command controls termination resistor values. This command sets the Rtt (termination resistor) values for ODT (on-die termination), and in one embodiment the intelligent register chip(s) may override existing values in the A[6] A[2] bits in EMR1 with &#39;00 to disable ODT on the DRAM devices, and propagate the expected ODT value to the intelligent buffer chip(s) via the sideband bus. 
     In another example, the sideband signal may be used to modify the behavior of the intelligent buffer chip(s). For example, the sideband signal may be used to reduce the power consumption of the intelligent buffer chip(s) in certain modes of operation. For example, another Extended Mode Register EMR1 command controls the behavior of the DRAM output buffers using the Qoff command. In one embodiment, the intelligent register chip(s) may respect the Qoff request meaning the DRAM output buffers should be disabled. The intelligent register chip(s) may then pass through this EMR1 Qoff request to the DRAM devices and may also send a sideband bus signal to one or more of the intelligent buffer chip(s) to turn off their output buffers also—in order to enable IDD measurement or to reduce power for example. When the Qoff bit it set, the intelligent register chip(s) may also disable all intelligent buffer chip(s) in the system. 
     Additional uses envisioned for the communication between intelligent registers and intelligent buffers through side-band or inline signaling include:
         a. All conceivable translation and mapping functions performed on the Data coming into the Intelligent Register  902 . A ‘function’ in this case should go beyond merely repeating input signals at the outputs.   b. All conceivable translation and mapping functions performed on the Address and Control signals coming into the Intelligent Register  902 . A ‘function’ in this case should go beyond merely repeating input signals at the outputs.   c. Uses of any and every signal originating from the DRAM going to the Intelligent Register or intelligent buffer.   d. Use of any first signal that is the result of the combination of a second signal and any data stored in non-volatile storage (e.g. SPD) where such first signal is communicated to one or more intelligent buffers  907 .   e. Clock and delay circuits inside the Intelligent Register or intelligent buffer. For example, one or more intelligent buffers can be used to de-skew data output from the DRAM.       

     Still more uses envisioned for the communication between intelligent registers and intelligent buffers through sideband or inline signaling include using the sideband as a time-domain multiplexed address bus. That is, rather than routing multiple physical address busses from the intelligent register to each of the DRAMs (through an intelligent buffer), a single physical sideband shared between a group of intelligent buffers can be implemented. Using a multi-cycle command &amp; value technique or other intelligent register to intelligent buffer communication techniques described elsewhere in this specification, a different address can be communicated to each intelligent buffer, and then temporally aligned by the intelligent buffer such that the data resulting from (or presented to) the DRAMs is temporally aligned as a group. 
     Bypass and Data Recovery 
     In a computer that contains a memory system, information that is currently being used for computation is stored in the memory modules that comprise a memory system. If there is a failure anywhere in the computer, the data stored in the memory system is at risk to be lost. In particular, if there is a failure in the memory controller, the connections between memory controller and the memory modules, or in any chips that are between the memory controller and the DRAM chips on the memory modules, it may be impossible to retain and retrieve data in the memory system. This mode of failure occurs because there is no redundancy or failover in the datapath between the memory controller and DRAM. A particularly weak point of failure in a typical DIMM lies in the register and buffer chips that pass information to and from the DRAM chips. For example, in an FB-DIMM, there is an AMB chip. If the AMB chip on an FB-DIMM fails, it is not possible to retrieve data from the DRAM on that FB-DIMM. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, more powerful memory buffer bypass and data recovery capabilities may be implemented. 
     As an example, in a memory module that uses an intelligent buffer or intelligent register chip, it is possible to provide an alternative memory datapath or read mechanism that will allow the computer to recover data despite a failure. For example, the alternative datapath can be provided using the SMBus or I2C bus that is typically used to read and write to the SPD on the memory module. In this case the SMBus or I2C bus is also connected to the intelligent buffer and/or register chips that are connected to the DRAM on the memory module. Such an alternative datapath is slower than the normal memory datapath, but is more robust and provides a mechanism to retrieve data in an emergency should a failure occur. 
     In addition, if the memory module is also capable of autonomous refresh, which is described elsewhere in this specification, the data may still be retrieved from a failed or failing memory module or entire memory system, even under conditions where the computer has essentially ceased to function, due to perhaps multiple failures. Provided that power is still being applied to the memory module (possibly by an emergency supply in the event of several failures in the computer), the autonomous refresh will keep the data in each memory module. If the normal memory datapath has also failed, the alternative memory datapath through the intelligent register and/or buffer chips can still be used to retrieve data. Even if the computer has failed to the extent that the computer cannot or is not capable of reading the data, an external device can be connect to a shared bus such as the SMBus or I2C bus used as the alternative memory datapath. 
     Control at Sub-DIMM Level 
     In a memory module that includes intelligent register and/or intelligent buffer chips, powerful temperature monitoring and control capabilities may be implemented, as described elsewhere in this specification. In addition, in a memory module that includes intelligent register and/or intelligent buffer chips, extensive control capabilities, including thermal and power control at the sub-DIMM level, that improve reliability, for example, may be implemented. 
     As an example, one particular DRAM on a memory module may be subjected to increased access relative to all the other DRAM components on the memory module. This increased access may lead to excessive thermal dissipation in the DRAM and require access to be reduced by throttling performance. In a memory module that includes intelligent register and/or intelligent buffer chips, this increased access pattern may be detected and the throttling performed at a finer level of granularity. Using the intelligent register and/or intelligent buffer chips, throttling at the level of the DIMM, a rank, a stack of DRAMs, or even an individual DRAM may be performed. 
     In addition, by using intelligent buffer and/or register chips, the throttling or thermal control or regulation may be performed. For example the intelligent buffer and/or register chips can use the Chip Select, Clock Enable, or other control signals to regulate and control the operation of the DIMM, a rank, a stack of DRAMs, or individual DRAM chips. Self-Test 
     Memory modules used in a memory system may form the most expensive component of the computer. The largest current size of memory module is 4 GB (a GB or gigabyte is 1 billion bytes or 8 billion bits) and such a memory module costs several thousands of dollars. In a computer that uses several of these memory modules (it is not uncommon to have 64 GB of memory in a computer), the total cost of the memory may far exceed the cost of the computer. 
     In memory systems, it is thus exceedingly important to be able to thoroughly test the memory modules and not discard memory modules because of failures that can be circumvented or repaired. 
     In a memory module that includes intelligent register and/or intelligent buffer chips, extensive DRAM advanced self-test capabilities may be implemented. 
     For example, an intelligent register chip on a memory module may perform self-test functions by reading and writing to the DRAM chips on the memory module, either directly or through attached intelligent buffer chips. The self-test functions can include writing and reading fixed patterns, as is commonly done using an external memory controller. As a result of the self-test, the intelligent register chip may indicate success or failure using an LED, as described elsewhere in this specification. As a result of the self-test, the intelligent register or intelligent buffer chips may store information about the failures. This stored information may then be used to re-map or map out the defective memory cells, as described elsewhere in this specification. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. For example, persons skilled in the art will appreciate that the features and functionalities described herein may be implemented using either an intelligent register chip, an intelligent buffer chip, both intelligent chips, or any combination thereof. The scope of the present invention is thereof determined by the claims that follow.