Abstract:
An integrated circuit buffer device comprises a first receiver circuit to receive control information and address information from a controller device. A first interface includes a first interface portion to provide a first address to a first memory device. A second interface portion provides a first control signal to the first memory device. The first control signal specifies a read operation such that the first memory device provides a first data, accessed from a memory location based on the first address, to the integrated circuit buffer device in response to the first control signal specifying the read operation. A third interface portion provides a first clock signal to the first memory device. The first clock signal synchronizes communication of the first control signal from the integrated circuit buffer device to the first memory device. A fourth interface portion receives the first data. A second interface includes a first interface portion to provide a second address to a second memory device. A second interface portion provides a second control signal to the second memory device. A third interface portion provides a second clock signal to the second memory device. A fourth interface portion receives the second data. A first transmitter circuit transmits the first read data and the second read data to the controller device.

Description:
This is a continuation of U.S. patent application Ser. No. 11/078,244 filed on Mar. 11, 2005 now U.S. Pat. No. 7,003,618, which is a continuation of U.S. patent application Ser. No. 11/054,797 filed on Feb. 10, 2005 now U.S. Pat. No. 7,000,062, which is a continuation of U.S. patent application Ser. No. 10/952,667 filed on Sep. 28, 2004 (still pending), which is a continuation of U.S. patent application Ser. No. 10/625,276 filed on Jul. 23, 2003 (allowed), which is a continuation of U.S. patent application Ser. No. 10/272,024 filed on Oct. 15, 2002 (allowed), which is a continuation of application Ser. No. 09/479,375 filed on Jan. 5, 2000 (now U.S. Pat. No. 6,502,161). 

   BACKGROUND OF THE INVENTION 
   This invention relates to memory systems, memory subsystems, memory modules or a system having memory devices. More specifically, this invention is directed toward memory system architectures which may include integrated circuit devices such as one or more controllers and a plurality of memory devices. 
   Some contemporary memory system architectures may demonstrate tradeoffs between cost, performance and the ability to upgrade, for example; the total memory capacity of the system. Memory capacity is commonly upgraded via memory modules or cards featuring a connector/socket interface. Often these memory modules are connected to a bus disposed on a backplane to utilize system resources efficiently. System resources include integrated circuit die area, package pins, signal line traces, connectors, backplane board area, just to name a few. In addition to upgradeability, many of these contemporary memory systems also require high throughput for bandwidth intensive applications, such as graphics. 
   With reference to  FIG. 1 , a representational block diagram of a conventional memory system employing memory modules is illustrated. Memory system  100  includes memory controller  110  and modules  120   a – 120   c . Memory controller  110  is coupled to modules  120   a – 120   c  via control/address bus  130 , data bus  140 , and corresponding module control lines  150   a – 150   c . Control/address bus  130  typically comprises a plurality of address lines and control signals (e.g., RAS, CAS and WE). 
   The address lines and control signals of control/address bus  130  are bussed and “shared” between each of modules  120   a – 120   c  to provide row/column addressing and read/write, precharge, refresh commands, etc., to memory devices on a selected one of modules  120   a – 120   c . Individual module control lines  150   a – 150   c  are typically dedicated to a corresponding one of modules  120   a – 120   c  to select which of modules  120   a – 120   c  may utilize the control/address bus  130  and data bus  140  in a memory operation. 
   Here and in the detailed description to follow, “bus” denotes a plurality of signal lines, each having more than two connection points for “transceiving” (i.e., transmitting or receiving). Each connection point electrically connects or couples to a transceiver (i.e., a transmitter-receiver) or one of a single transmitter or receiver circuit 
   With further reference to  FIG. 1 , memory system  100  may provide an upgrade path through the usage of modules  120   a – 120   c . A socket and connector interface may be employed which allows each module to be removed and replaced by a memory module that is faster or includes a higher capacity. Memory system  100  may be configured with unpopulated sockets or less than a full capacity of modules (i.e., empty sockets/connectors) and provided for increased capacity at a later time with memory expansion modules. Since providing a separate group of signals (e.g., address lines and data lines) to each module is avoided using the bussed approach, system resources in memory system  100  are efficiently utilized. 
   U.S. Pat. No. 5,513,135 discloses a contemporary dual inline memory module (DIMM) having one or more discrete buffer devices. In this patent, the discrete buffer devices are employed to buffer or register signals between memory devices disposed on the module and external bussing (such as control/address bus  130  in memory system  100 ). The discrete buffer devices buffer or register incoming control signals such as RAS, and CAS, etc., and address signals. Local control/address lines are disposed on the contemporary memory module to locally distribute the buffered or registered control and address signals to each memory device on the module. By way of note, the discrete buffer devices buffer a subset of all of the signals on the memory module since data path signals (e.g., data bus  140  in  FIG. 1 ) of each memory device are connected directly to the external bus. 
   In addition to the discrete buffer device(s), a phase locked Loop (PLL) device may be disposed on the contemporary DIMM described above. The PLL device receives an external clock and generates a local phase adjusted clock for each memory device as well as the discrete buffer devices. 
   Modules such as the DIMM example disclosed in U.S. Pat. No. 5,513,135 feature routed connections between input/outputs (I/Os) of each memory device and connector pads disposed at the edge of the module substrate. These routed connections introduce long stub lines between the signal lines of the bus located off of the module (e.g., control address bus  130  and data bus  140 ), and memory device I/Os. A stub line is commonly known as a routed connection that deviates from the primary path of a signal line. Stub lines commonly introduce impedance discontinuities to the signal line. Impedance discontinuities may produce undesirable voltage reflections manifested as signal noise that may ultimately limit system operating frequency. 
   Examples of contemporary memory systems employing buffered modules are illustrated in  FIGS. 2A and 2B .  FIG. 2A  illustrates a memory system  200  based on a Rambus™ channel architecture and  FIG. 2B  illustrates a memory system  210  based on a Synchronous Link architecture. Both of these systems feature memory modules having buffer devices  250  disposed along multiple transmit/receive connection points of bus  260 . In both of these examples, the lengths of stubs are significantly shortened in an attempt to minimize signal reflections and enable higher bandwidth characteristics. Ultimately however, memory configurations such as the ones portrayed by memory systems  100 ,  200  and  210  may be significantly bandwidth limited by the electrical characteristics inherent in the bussed approach as described below. 
   In the bussed approach exemplified in  FIGS. 1 ,  2 A and  2 B, the signal lines of the bussed signals become loaded with a (load) capacitance associated with each bus connection point. These load capacitances are normally attributed to components of input/output (I/O) structures disposed on an integrated circuit (IC) device, such as a memory device or buffer device. For example, bond pads, electrostatic discharge devices, input buffer transistor capacitance, and output driver transistor parasitic and interconnect capacitances relative to the IC device substrate all contribute to the memory device load capacitance. 
   The load capacitances connected to multiple points along the length of the signal line may degrade signaling performance. As more load capacitances are introduced along the signal line of the bus, signal settling time correspondingly increases, reducing the bandwidth of the memory system. In addition, impedance along the signal line may become harder to control or match as more load capacitances are present along the signal line. Mismatched impedance may introduce voltage reflections that cause signal detection errors. Thus, for at least these reasons, increasing the number of loads along the bus imposes a compromise to the bandwidth of the memory system. 
   In an upgradeable memory system, such as conventional memory system  100 , different memory capacity configurations become possible. Each different memory capacity configuration may present different electrical characteristics to the control/address bus  130 . For example, load capacitance along each signal line of the control/address bus  130  may change with two different module capacity configurations. 
   As memory systems incorporate an increasing number of memory module configurations, the verification and validation of the number of permutations that these systems make possible may become increasingly more time consuming. Verification involves the confirmation of operation, logical and/or physical functionality of an IC by running tests on models of the memory, associated devices and/or bus prior to manufacturing the device. Validation involves testing the assembled system or components thereof (e.g., a memory module). Validation typically must account for a majority of the combinations or permutations of system conditions and possibilities which different memory configurations (e.g., 256 Mbyte, 1 Gbyte . . . ) present including signaling electrical characteristics (e.g., impedance, capacitance, and inductance variations), temperature effects, different operating frequencies, different vendor interfaces, etc, to name a few. Thus, as the number of possible memory configurations increase, the test and verification time required also increases. More time required to test a system often increases the cost of bringing the system to market or delays a product introduction beyond an acceptable window of time to achieve competitiveness. 
   There is a need for memory system architectures or interconnect topologies that provide cost effective upgrade capabilities without compromising bandwidth. Using conventional signaling schemes, the bussed approaches lend efficiency towards resource utilization of a system and permits module interfacing for upgradeability. However, the bussed approach may suffer from bandwidth limitations that stem from the electrical characteristics inherent in the bus topology. In addition, impedance along a signal line may be increasingly more difficult to control with increased connection points along a signal line, introducing impedance mismatch and signal reflections. Utilizing the bussed approach in implementing an upgradeable memory system introduces many possible electrical permutations and combinations with each unique module configuration. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward memory system architectures (i.e., interconnect topologies) which include a controller communicating to at least one memory subsystem (e.g., a buffered memory module). An independent point-to-point link may be utilized between the controller and each memory subsystem to eliminate physical inter-dependence between memory subsystems. According to an embodiment, the memory system may be upgraded by coupling additional memory module(s), each via a dedicated point-to-point link to the controller. Bandwidth may scale upwards as the memory system is upgraded by the additional memory module(s). 
   In one aspect, the present invention is a memory system comprising a memory controller having an interface and at least a first memory subsystem. The interface includes a plurality of memory subsystem ports including a first memory subsystem port. The first memory subsystem includes a buffer device having a first port and a second port, and a plurality of memory devices coupled to the buffer device via the second port. A plurality of point-to-point links include a first point-to-point link. Each point-to-point link has a connection to a respective memory subsystem port of the plurality of memory subsystem ports. The first point-to-point link connecting the first port to a first memory subsystem port to transfer data between the plurality of memory devices and the memory controller. 
   In another aspect, the present invention is a memory system comprising a controller device and first and second buffer devices, each having a first interface and a second interface. A first point-to-point link includes a first connection to the controller device and a second connection to the first interface of the first buffer device. A first channel is connected to the second interface of the first buffer device, and a first plurality of memory devices are electrically coupled to the first channel. A second point-to-point link includes a first connection to the controller device and a second connection to the first interface of the second buffer. A second channel is connected to the second interface of the second buffer device, and a second plurality of memory devices are electrically coupled to the second channel. 
   In yet another aspect, the present invention comprises a controller device, and a first and second plurality of buffer devices, each buffer device having a first interface connected to a plurality of memory devices. First and second point-to-point links each include a first end connected to the controller device and a second end connected to a repeater device. A plurality of repeater links couple the first and second repeater devices to respective first and second pluralities of buffer devices. 
   In another aspect the present invention is a memory system comprising a controller device; a first, second and third connectors; and first second and third point-to-point links. Each of the respective first, second point-to-point links includes a first connection to the interface and a second connection to the respective first, second and third connectors. In this aspect the present invention also includes a first memory subsystem having a buffer device and a plurality of memory devices. The buffer device includes a first interface connected to the first connector, and a second interface connected to the plurality of memory devices. The second and third connectors may support coupling to respective second and third memory subsystems. 
   The present invention is described in the detailed description, including the embodiments to follow. The detailed description and embodiments are given by way of illustration only. The scope of the invention is defined by the attached claims. Various modifications to the embodiments of the present invention remain within the scope defined by the attached claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the course of the detailed description to follow, reference will be made to the attached drawings, in which: 
       FIG. 1  illustrates a representational block diagram of a conventional memory system employing memory modules; 
       FIGS. 2A and 2B  illustrate contemporary memory systems employing buffered modules; 
       FIGS. 3A and 3B  illustrate a block diagram representing memory systems according to embodiments of the present invention; 
       FIGS. 4A ,  4 B, and  4 C illustrate buffered memory modules according to embodiments of the present invention; 
       FIG. 5  illustrates a block diagram of a buffer device according to another embodiment of the present invention; 
       FIGS. 6A and 6B  illustrate block diagrams of a memory system according to other embodiments of the present invention; 
       FIG. 7  illustrates a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention; 
       FIG. 8A  illustrates a block diagram of a large capacity memory system according to another embodiment of the present invention; and 
       FIGS. 8B and 8C  illustrate another approach utilize to expand the memory capacity of a memory system in accordance to yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention relates to a memory system which includes a plurality of point-to-point links connected to a master. At least one point-to-point link connects at least one memory subsystem to the master, (e.g., a processor or controller). The memory system may be upgraded by coupling memory subsystems to the master via respective dedicated point-to-point links. Each memory subsystem includes a buffer device that communicates to a plurality of memory devices. The master communicates with each buffer device via each point-to-point link. The buffer device may be disposed on a memory module along with the plurality of memory devices and connected to the point-to-point link via a connector. Alternatively, the buffer device may be disposed on a common printed circuit board or backplane link along with the corresponding point-to-point link and master. 
   “Memory devices” are a common class of integrated circuit devices that have an array of memory cells, such as, dynamic random access memory (DRAM), static random access memory (SRAM), etc. A “memory subsystem” is a plurality of memory devices interconnected with an integrated circuit device (e.g., a buffer device) providing access between the memory devices and an overall system, for example, a computer system. It should be noted that a memory system is distinct from a memory subsystem in that a memory system may include one or more memory subsystems. A “memory module” or simply just “module” denotes a substrate having a plurality of memory devices employed with a connector interface. It follows from these definitions that a memory module having a buffer device isolating data, control, and address signals of the memory devices from the connector interface is a memory subsystem. With reference to  FIGS. 3A and 3B , block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory systems  300  and  305  include a controller  310 , a plurality of point-to-point links  320   a – 320   n , and a plurality of memory subsystems  330   a – 330   n . For simplicity, a more detailed embodiment of memory subsystem  330   a  is illustrated as memory subsystem  340 . Buffer device  350  and a plurality of memory devices  360  are disposed on memory subsystem  340 . Buffer device  350  is coupled to the plurality of memory devices  360  via channels  370 . Interface  375  disposed on controller  310  includes a plurality of memory subsystem ports  378   a – 378   n . A “port” is a portion of an interface that serves a congruent I/O functionality. One of memory subsystem ports  378   a – 378   n  includes I/Os, for sending and receiving data, addressing and control information over one of point-to-point links  320   a – 320   n.    
   According to an embodiment of the present invention, at least one memory subsystem is connected to one memory subsystem port via one point-to-point link. The memory subsystem port is disposed on the memory controller interface which includes a plurality of memory subsystem ports, each having a connection to a point-to-point link. 
   In  FIG. 3A , point-to-point links  320   a – 320   n , memory subsystems  330   a – 330   c , and controller  310 , are incorporated on a common substrate (not shown) such as a wafer or a printed circuit board (PCB) in memory system  300 . In an alternate embodiment, memory subsystems are incorporated onto individual substrates (e.g., PCBs) that are incorporated fixedly attached to a single substrate that incorporates point-to-point links  320   a – 320   n  and controller  310 . In another alternate embodiment illustrated in  FIG. 3B , memory subsystems  330   a – 330   c  are incorporated onto individual substrates which include connectors  390   a – 390   c  to support upgradeability in memory system  305 . Corresponding mating connectors  380   a – 380   n  are connected to a connection point of each point-to-point link  320   a – 320   n . Each of mating connectors  380   a – 380   n  interface with connectors  390   a – 390   c  to allow removal/inclusion of memory subsystems  330   a – 330   c  in memory system  305 . In one embodiment, mating connectors  380   a – 380   n  are sockets and connectors  390   a – 390   c  are edge connectors disposed on an edge of each substrate  330   a – 330   c . Mating connectors  380   a – 380   n , are attached to a common substrate shared with point-to-point connections  320   a – 320   n  and controller  310 . 
   With further reference to  FIGS. 3A and 3B , buffer device  350  transceives and provides isolation between signals interfacing to controller  310  and signals interfacing to the plurality of memory devices  360 . In a normal memory read operation, buffer device  350  receives control, and address information from controller  310  via point-to-point link  320   a  and in response, transmits corresponding signals to one or more, or all of memory devices  360  via channel  370 . One or more of memory devices  360  may respond by transmitting data to Buffer device  350  which receives the data via one or more of channels  370  and in response, transmits corresponding signals to controller  310  via point-to-point link  320   a . Controller  310  receives the signals corresponding to the data at corresponding port  378   a – 378   n . In this embodiment, memory subsystems  330   a – 330   n  are buffered modules. By way of comparison, buffers disposed on the conventional DIMM module in U.S. Pat. No. 5,513,135 are employed to buffer or register control signals such as RAS, and CAS, etc., and address signals. Data I/Os of the memory devices disposed on the DIMM are connected directly to the DIMM connector (and ultimately to data lines on an external bus when the DIMM is employed in memory system  100 ). 
   Buffer device  350  provides a high degree of system flexibility. New generations of memory devices may be phased in with controller  310  or into memory system  300  by modifying buffer device  350 . Backward compatibility with existing generations of memory devices (i.e., memory devices  360 ) may also be preserved. Similarly, new generations of controllers may be phased in which exploit features of new generations of memory devices while retaining backward compatibility with existing generations of memory devices. 
   Buffer device  350  effectively reduces the number of loading permutations on the corresponding point-to-point link to one, thus simplifying test procedures. For example, characterization of a point to point link may involve aspects such as transmitters and receivers at opposite ends, few to no impedance discontinuities, and relatively short interconnects. By way of contrast, characterisaion of control/address bus  130  (see  FIG. 1 ) may involve aspects such as multiple transmit and receive points, long stub lines, and multiple load configurations, to name a few. Thus, the increased number of electrical permutations tend to add more complexity to the design, test, verification and validation of memory system  100 . 
   Buffered modules added to upgrade memory system  300  (e.g., increase memory capacity) are accommodated by independent point-to-point links. Relative to a bussed approach, system level design, verification and validation considerations are reduced, due to the deceased amount of module inter-dependence provided by the independent point-to-point links. Additionally, the implementation, verification and validation of buffered modules may be performed with less reliance on system level environment factors. 
   Several embodiments of point-to-point links  320   a – 320   n  include a plurality of link architectures, signaling options, clocking options and interconnect types. Embodiments having different link architectures include simultaneous bi-directional links, time-multiplexed bi-sectional links and multiple unidirectional links. Voltage or current mode signaling may be employed in any of these link architectures. Clocking methods include any of globally synchronous clocking; source synchronous clocking (i.e., where data is transported alongside the clock) and encoding the data and the clock together. In one embodiment, differential signaling is employed and is transported over differential pair lines. In alternate embodiments, one or more common voltage or current references are employed with respective one or more current/voltage mode level signaling. In yet other embodiments, multi-level signaling-where information is transferred using symbols formed from multiple signal (i.e., voltage/current) levels is employed. 
   Signaling over point-to-point links  320   a – 320   n  may incorporate different modulation methods such as non-return to zero (NRZ, multi-level pulse amplitude modulation (PAM), phase shift keying, delay or time modulation, quadrature amplitude modulation (QAM) and Trellis coding. Other signaling methods and apparatus may be employed in point-to-point links  320   a – 320   n , for example, optical fiber based apparatus and methods. 
   The term “point-to-point link” denotes one or a plurality of signal lines, each signal line having only two transceiver connection points, each transceiver connection point coupled to transmitter circuitry, receiver circuitry or transceiver circuitry. For example, a point-to-point link may include a transmitter coupled at or near one end and a receiver coupled at or near the other end. The point-to-point link may be synonymous and interchangeable with a point-to-point connection or a point-to-point coupling. 
   In keeping with the above description, the number of transceiver points along a signal line distinguishes between a point-to-point link and a bus. According to the above, the point-to-point link consists of two transceiver connection points while a bus consists of more than two transceiver points. 
   One or more terminators (e.g., a resistive element) may terminate each signal line in point-to-point links  320   a – 320   n . In several embodiments of the present invention, the terminators are connected to the point-to-point link and situated on buffer device  350 , on a memory module substrate and optionally on controller  310  at memory subsystem ports  378   a – 378   n . The terminator(s) connect to a termination voltage, such as ground or a reference voltage. The terminator may be matched to the impedance of each transmission line in point-to-point links  320   a – 320   n , to help reduce voltage reflections. 
   In an embodiment of the present invention employing multi-level PAM signaling, the data rate may be increased without increasing either the system clock frequency or the number of signal lines by employing multiple voltage levels to encode unique sets of consecutive digital values or symbols. That is, each unique combination of consecutive digital symbols may be assigned to a unique voltage level, or pattern of voltage levels. For example, a 4-level PAM scheme may employ four distinct voltage ranges to distinguish between a pair of consecutive digital values or symbols such as 00, 01, 10 and 11. Here, each voltage range would correspond to one of the unique pairs of consecutive symbols. 
   With reference to  FIGS. 4A ,  4 B and  4 C, buffered memory modules according to embodiments of the present invention are shown. Modules  400  and  405  include buffer device  405  and a plurality of memory devices  410   a – 410   h  communicating over a pair of channels  415   a  and  415   b . In these embodiments channel  415   a  communicates to memory devices  410   a – 410   d  and channel  415   b  communicates to memory devices  401   e – 410   h.    
   In an embodiment, channels  415   a  and  415   b  consist of a plurality of signal lines in a relatively short multi-drop bus implementation. The plurality of signal lines may be controlled impedance transmission lines that are terminated using respective termination elements  420   a  and  420   b . Channels  415   a  and  415   b  are relatively short (i.e., are coupled to relatively few memory devices relative to a conventional memory system, for example see  FIGS. 2A and 2B ) and connect to an I/O interface (not shown) of each memory device via a short stub. Signal lines of channels  415   a  and  415   b  include control lines (RQ), data lines (DQ) and clock lines (CFM, CTM). The varieties of interconnect topologies, interconnect types, clocking methods, signaling references, signaling methods, and signaling apparatus described above in reference to point-to-point links  320   a – 320   n  may equally apply to channels  415   a  and  415   b.    
   In accordance with an embodiment of the present invention, control lines (RQ) transport control (e.g., read, write, precharge . . . ) information and address (e.g., row and column) information contained in packets. By bundling control and address information in packets, protocols required to communicate to memory devices  410   a – 410   h  are independent of the physical control/address interface implementation. 
   In alternate embodiments, control lines (RQ) may comprise individual control lines, for example, row address strobe, column address strobe, etc., and address lines. Individual point-to-point control and address lines increase the number of parallel signal connection paths, thereby increasing system layout resource requirements with respect to a narrow “packet protocol” approach. In one alternate embodiment illustrated in  FIG. 6A , individual device select lines  633   a  and  633   b  are employed to perform device selection. Individual device select lines  633   a  and  633   b  decrease some latency consumed by decoding device identification which normally is utilized when multiple devices share the same channel and incorporate individual device identification values. 
   Clock lines of channels  415   a  and  415   b  include a terminated clock-to-master (CTM) (i.e., clock to buffer) and clock-from-master (CFM) (i.e., clock from buffer) line. In a source synchronous clocking method, CTM may be transition or edge aligned with control and/or data communicated to buffer device  405  from one or more of memory devices  410   a – 410   d  in, for example, a read operation. CFM may be aligned with or used to synchronize control and/or data from the buffer to memory in, for example, a write operation. 
   Although two channels  415   a  and  415   b  are shown in  FIG. 4A , a single channel is also feasible. In other embodiments, more than two channels may be incorporated onto module  400 . It is conceivable that if each channel and memory device interface is made narrow enough, then a dedicated channel between each memory device and the buffer device may be implemented on the module. The width of the channel refers to the number of parallel signal paths included in each channel.  FIG. 4B  illustrates a quad-channel module  450  having channels  415   a – 4 l 5   d . In this embodiment, channels  415   c  and  415   d  are routed in parallel with channels  415   a  and  415   b  to support more memory devices (e.g., 32 memory devices). By incorporating more channels and additional memory devices, module  400  ( FIG. 4B ) may be implemented in memory systems that require large memory capacity, for example, in server or workstation class systems. 
   In alternate embodiments, channels  415   a  and  415   b  may operate simultaneously with channels  415   c  and  415   d  to realize greater bandwidth. By operating a plurality of channels in parallel, the bandwidth of the module may be increased independently of the memory capacity. The advantages of greater bandwidth may be realized in conjunction with larger capacity as more modules incorporated the memory system  305  (see  FIG. 3B ) increase the system memory capacity. In other alternate embodiments, the modules are double sided and channels along with corresponding pluralities of memory devices are implemented on both sides. Using both sides of the module increases capacity or increases bandwidth without impacting module height. Both capacity and bandwidth may increase using this approach. Indeed, these techniques may increase capacity and bandwidth singly or in combination. 
   Other features may also be incorporated to enhance module  400  in high capacity memory systems, for example, additional memory devices and interface signals for error correction code storage and transport (ECC). Referring to  FIG. 4C , memory devices  410   i  and  410   r  intended for ECC are disposed on module  470 . 
   In one embodiment, memory devices  410   a – 410   h  are Rambus Dynamic Random access Memory (RDRAM) devices operating at a data rate of 1066 Mbits/sec. Other memory devices may be implemented on module  400 , for example, Double Data Rate (DDR) DRAM devices and Synchronous DRAM (SDRAM) devices. Utilizing buffer device  405  between the memory devices and controller in accordance with the present invention (e.g., see  FIG. 3 ) may feasibly render the type of memory device transparent to the system. Different types of memory devices may be included on different modules within a memory system, by employing buffer device  405  to translate protocols employed by controller  310  to the protocol utilized in a particular memory device implementation. 
   With reference to  FIG. 5 , a block diagram of a buffer device according to an embodiment of the present invention is illustrated. Buffer device  405  includes interface  510 , interface  520   a  and  520   b , multiplexing  530   a  and  530   b , control logic  540 , write buffer  550 , optional cache  560 , computation block  565 , clock circuitry  570  and operations circuitry  572 . 
   In an embodiment, interface  510  couples to external point-to-point link  320  (e.g. point-to-point links  320   a – 320   n  in  FIGS. 3A and 3B ). Interface  510  includes a port having transceiver  575  (i.e. transmit and receive circuitry) that connects to a point-to-point link. Point-to-point link  320  comprises one or a plurality of signal lines, each signal line having no more than two transceiver connection points. One of the two transceiver connection points is included on interface  510 . Buffer device  405  may include additional ports to couple additional point-to-point links between buffer device  405  and other buffer devices on other memory modules. These additional ports may be employed to expand memory capacity as is described in more detail below. In the embodiment shown in  FIG. 5 , buffer device  405  may function as a transceiver between point-to-point link  320  and other point-to-point links. 
   In one embodiment, termination  580  is disposed on buffer device  405  and is connected to transceiver  575  and point-to-point link  320 . In this embodiment, transceiver  575  includes an output driver and a receiver. Termination  580  may dissipate signal energy reflected (i.e., a voltage reflection) from transceiver  575 . Termination  580  may be a resistor or capacitor or inductor, singly or a series/parallel combination thereof. In alternate embodiments, termination  580  may be external to buffer device  405 . For example, termination  580  may be disposed on a module substrate or on a memory system substrate. 
   In another approach, signal energy reflected from transceiver  575  may be utilized in a constructive manner according to an embodiment. By correctly placing a receive point spaced by a distance from the end of point-to-point link  320 , a reflected waveform is summed with an incident waveform to achieve a greater signal amplitude. In this approach, layout space may be saved by eliminating termination  580 . System power may also be saved using this approach since smaller incident voltage amplitude waveforms may be employed. This approach may be equally applicable to the transceiver end of the point-to-point link, or to channels  415   a  and  415   b  (see  FIGS. 4A to 4C ). 
   With further reference to  FIG. 5 , interfaces  520   a  and  520   b  receive and transit to memory devices disposed on the module (e.g., see  FIGS. 4A ,  4 B and  4 C) via channels. Ports included on interface  520   a  and  520   b  connect to each channel. In alternate embodiments of the present invention, interfaces  520   a  and  520   b  include any number of channels e.g., two, four, eight or more channels. 
   According to an embodiment of the present invention, multiplexers  530   a  and  530   b  perform bandwidth-concentrating operations, between interface  510  and interfaces  520   a  and  520   b . The concept of bandwidth concentration involves combining the (smaller) bandwidth of each channel in a multiple channel embodiment to match the (higher) overall bandwidth utilized in a smaller group of channels. This approach typically utilizes multiplexing and demultiplexing of throughput between the multiple channels and smaller group of channels. In an embodiment, buffer device  405  utilizes the combined bandwidth of interfaces  520   a  and  520   b  to match the bandwidth of interface  510 . Bandwidth concentration is described in more detail below. 
   Cache  560  is one performance enhancing feature that may be incorporated onto buffer device  405 . Employing a cache  560  may improve memory access time by providing storage of most frequently referenced data and associated tag addresses with lower access latency characteristics than those of the memory devices. Computation block  565  may include a processor or controller unit a compression/decompression engine, etc, to further enhance the performance and/or functionality of the buffer device. In an embodiment, write buffer  550  may improve interfacing efficiency by utilizing available data transport windows over point-to-point link  320  to receive write data and optional address/mask information. Once received, this information is temporarily stored in write buffer  550  until it is ready to be transferred to at least one memory device over interfaces  520   a  and  520   b.    
   A serial interface  574  may be employed to couple signals utilized in initialization of module or memory device identification values, test function, set/reset, access latency values, vendor specific functions or calibration. Operations circuitry  572  may include registers or a read-only memory (ROM) to store special information (e.g., vendor or configuration information) that may be used by the controller. Operations circuitry may reduce costs by eliminating the need for separate devices on the module conventionally provided to perform these features (e.g., serial presence detect (SPD) employed in some conventional DIMM modules). 
   According to an embodiment of the present invention, sideband signals are employed to handle special functions such as reset, initialization and power management functions. Sideband signals are connected via serial interface  574  and are independent from point-to-point link  320  for handling the special functions. In other embodiments sideband signals are independently coupled to memory devices  410   a – 410   h  to directly promote initialization, reset, power-up or other functionality independently of buffer device  405 . Other interconnect topologies of sideband signals are possible. For example, sideband signals may be daisy chained between buffer devices and coupled to the memory controller or daisy chained between all memory devices to the memory controller. Alternatively, dedicated sideband signals may be employed throughout. 
   Clock circuitry  570  may include clock generator circuitry (e.g., Direct Rambus Clock Generator) which may be incorporated onto buffer device  405  and thus may eliminate the need for a separate clock generating device. Here, module or system costs may be decreased since the need for a unique clock generator device on the module or in the system may be eliminated. Since reliability to provide adequate clocking on an external device is eliminated, complexity is reduced since the clock may be generated on the buffer device  570 . By way of comparison, some of the conventional DIMM modules require a phase lock loop (PLL) generator device to generate phase aligned clock signals for each memory device disposed on the module. 
   According to an embodiment of the present invention, clocking circuitry  570  includes one or more clock alignment circuits for phase or delay adjusting internal clock signals with respect to an external clock (not shown). Clock alignment circuitry may utilize an external clock from an existing clock generator, or an internal clock generator to provide an internal clock, to generate internal synchronizing clock signals having a predetermined temporal relationship. 
   With reference to  FIGS. 6A , and  6 B, block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory system  660  includes modules  400   a  and  400   b , controller  610 , and populated primary point-to-point links  620   a  and  620   b . Unpopulated primary point-to-point links  630  are populated by coupling additional modules (not shown) thereto. The additional modules may be provided to upgrade memory system  600 . Connectors may be disposed at an end of each primary point-to-point link to allow insertion or removal of the additional modules. Modules  400   a  and  400   b  may also be provided with a connector or may be fixedly disposed (i.e., soldered) in memory system  600 . Although only two populated primary point-to-point links are shown in  FIG. 6A , any number of primary point-to-point links may be disposed in memory system  600 , for example, tree primary point-to-point links  400   a – 400   c , as shown in  FIG. 6B . 
   With reference to  FIG. 7  and  FIG. 4B , a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention is illustrated. Memory systems  700  incorporate quad-channel modules  450   a – 450   d , each coupled via point-to-point links  620   a – 620   d  respectively. 
   Referring to  FIG. 4B , buffer device  405  may operate in a bandwidth concentrator approach. By employing quad channels  415   a – 415   d  on each of modules  50   a – 450   d , bandwidth in each module may be concentrated from all quad channels  415   a – 415   d  on each module to corresponding point-to-point links  620   a – 620   d . In this embodiment throughput on each of point-to-point links  620   a – 620   d  is concentrated to four times the throughput achieved on each of quad channels  415   a – 451   d . Here, each of channels  415   a – 415   d  transfers information between one or more respective memory devices on each channel and buffer device  405  simultaneously. 
   Any number of channels  4 l 5   a – 4 l 5   d , for example; two channels  415   c  and  415   d  may transfer information simultaneously and the memory devices on the other two channels  415   a  and  415   b  remain in a ready or standby state until called upon to perform memory access operations. Different applications may have different processing throughput requirements. In addition, the throughput requirements of a particular application may dynamically change during processing. Typically, more power is consumed as throughput is increased as power consumption relates in proportion to operation frequency. The amount of throughput in a system may be implemented on a dynamic throughput requirement basis to save on power consumption. In this embodiment, memory system  700  may concentrate bandwidth as it is required while in operation. For example, memory system  700  may employ only one of channels  415   a – 415   d  and match throughput to the corresponding point-to-point link. As bandwidth requirements increase, memory system  700  may dynamically activate more of channels  415   a – 415   d  and increase the throughput on the point-to-point link along with the number of channels accordingly to meet the bandwidth requirements for a given operation. 
   With reference to  FIG. 8A , a block diagram of a large capacity memory system according to an embodiment of the present invention is illustrated. Memory system  900  includes modules  470   a – 470   p , coupled to controller  610  via repeaters  910   a – 910   d , primary links  920   a – 920   d , and repeater links  930   a – 930   p . Primary links  920   a – 920   d  provide a point to point link between controller  610  and a respective repeater  910   a – 910   d . In an embodiment of the present invention, each of repeaters  910   a – 910   d  decode packets transmitted from controller  610  which are then directed over one or more, or none of repeater links  930   a–d , depending the type of access required. Each repeater link  930   a – 930   p  may utilize a point-to-point link configuration. By incorporating, repeated links  930   a – 930   p  and repeaters  910   a – 910   d , a larger number of modules may be accessed and a larger capacity memory system may be realized. Such a large capacity may be suited in a computer server system. 
     FIG. 8B  illustrates another approach utilized to expand the memory capacity of a memory system in accordance to yet another embodiment. Here, a plurality of buffered modules  950   a – 950   d  are “daisy chained” via a plurality of point-to-point links  960   a – 960   d  to increase the overall memory capacity. Connection points of each point-to-point link are connected to two adjacent buffered modules. Each of buffered modules  950   a – 950   c  transceive signals between adjacent point-to-point links  960   a – 960   d . Point-to-point link  960   a  may be coupled to a controller or another buffered module. Additional point-to-point links may be coupled to a buffer device in a tree configuration approach. For example, three point-to-point links  970   a – 970   c  each having a single end connected to one buffer device may be employed as shown in  FIG. 8C . 
   While this invention has been described in conjunction with what is presently considered the most practical embodiments, the invention is not limited to the disclosed embodiments. In the contrary, the embodiments disclosed cover various modifications that are within the scope of the invention as set forth in the following claims.