Abstract:
A memory subsystem system including a rectangular printed circuit card having a first side and a second side, a length of between 149.5 and 153.5 millimeters, and first and second ends having a width smaller than the length. The memory system also includes a first plurality of pins on the first side extending along a first edge of the card that extends the length of the card, and a second plurality of pins on the second side extending on the first edge of the card. The memory system further includes a positioning key having it center positioned on the first edge of the card and located between 84.5 and 88.5 millimeters from the first end of the card and located between 62.5 and 66.5 millimeters from the second end of the card.

Description:
BACKGROUND 
   The invention relates to computer memory subsystems and, more particularly, to a buffered memory module having enhanced memory system interconnect and features. 
   Contemporary high performance computing main memory systems are generally composed of one or more memory devices, which are connected to one or more memory controllers and/or processors via one or more memory interface elements such as buffers, hubs, bus-to-bus converters, etc. The memory devices are generally located on a memory subsystem such as a memory card or memory module and are often connected via a pluggable interconnection system (e.g. one or more connectors) to a system board (e.g. a PC motherboard). 
   Overall computer system performance is affected by each of the key elements of the computer structure, including the performance/structure of the processor(s), any memory cache(s), the input/output (I/O) subsystem(s), the efficiency of the memory control function(s), the performance of the main memory devices(s) and any associated memory interface elements, and the type and structure of the memory interconnect interface(s). 
   Extensive research and development efforts are invested by the industry, on an ongoing basis, to create improved and/or innovative solutions to maximizing overall system performance and density by improving the memory system/subsystem design and/or structure. High-availability systems present further challenges as related to overall system reliability due to customer expectations that new computer systems will markedly surpass existing systems in regard to mean-time-between-failure (MTBF), in addition to offering additional functions, increased performance, increased storage, lower operating costs, etc. Other frequent customer requirements further exacerbate the memory system design challenges, and include such items as ease of upgrade and reduced system environmental impact (such as space, power and cooling). In addition, customers are requiring the ability to access an increasing number of higher density memory devices (e.g. DDR2 and DDR3 SDRAMs) at faster and faster access speeds. 
   SUMMARY 
   An exemplary embodiment is a memory subsystem system including a rectangular printed circuit card having a first side and a second side, a length of between 149.5 and 153.5 millimeters, and first and second ends having a width smaller than the length. The memory system also includes a first plurality of pins on the first side extending along a first edge of the card that extends the length of the card, and a second plurality of pins on the second side extending on the first edge of the card. The memory system further includes a positioning key having it center positioned on the first edge of the card and located between 84.5 and 88.5 millimeters from the first end of the card and located between 62.5 and 66.5 millimeters from the second end of the card. 
   Another exemplary embodiment is a memory subsystem including a card having a length of between 149.5 and 153.5 millimeters. The card has a first side including a first end, a second end, and a first edge that extends the length of the card between the first end and the second end. The memory subsystem also includes a positioning key having its center positioned on the first edge and located between 84.5 and 88.5 millimeters from the first end of the card and located between 62.5 and 66.5 millimeters from the second end of the card when viewed from the first side. The memory subsystem further includes a plurality of primary upstream bus interface pins arranged on the card for communicating with a high-speed upstream bus, such that the primary upstream bus interface pins associated with the high-speed upstream bus are located between greater than 13.5 millimeters to the left of the positioning key and the first end when viewed from the first side. 
   A further exemplary embodiment is a memory subsystem including a card having a length of between 149.5 and 153.5 millimeters. The card has a first side and a second side, the first side including a first end, a second end, and a first edge that extends the length of the card between the first end and the second end. The memory subsystem also includes a positioning key having its center positioned on the first edge and located between 84.5 and 88.5 millimeters from the first end of the card and located between 62.5 and 66.5 millimeters from the second end of the card when viewed from the first side. The memory subsystem also includes a plurality of paired adjacent differential high-speed signal interface pins for communicating with differential high-speed signals, such that each pair of adjacent differential high-speed signal pair is separated by at least one additional pin or by a distance between the first and the second side. 
   A further exemplary embodiment is a design structure tangibly embodied in a machine-readable medium for designing, manufacturing, or testing an integrated circuit. The design structure includes a memory subsystem that includes a rectangular printed circuit card having a first side and a second side, a length of between 149.5 and 153.5 millimeters, and first and second ends having a width smaller than the length. The memory subsystem also includes a first plurality of pins on the first side extending along a first edge of the card that extends the length of the card, and a second plurality of pins on the second side extending on the first edge of the card. The memory subsystem further includes a positioning key having it center positioned on the first edge of the card and located between 84.5 and 88.5 millimeters from the first end of the card and located between 62.5 and 66.5 millimeters from the second end of the card. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
       FIG. 1  depicts a cascade interconnect memory system in which a memory controller is shown to have a single memory channel with unidirectional buses that may be implemented by an exemplary embodiment; 
       FIG. 2  depicts a front view of a buffered memory module that may be implemented by an exemplary embodiment; 
       FIG. 3  is a block diagram of the high-level logic flow of a hub device that may be implemented by an exemplary embodiment; 
       FIG. 4  is made up of  FIGS. 4   a ,  4   b  and  4   c , and depicts a table illustrating a functional pin assignment for a 276-pin DIMM that may be implemented by an exemplary embodiment; 
       FIG. 5  depicts a memory system, comprising a single DIMM, that may be implemented by an exemplary embodiment; 
       FIG. 6  depicts a dual-ported buffer on a memory module that may be implemented by an exemplary embodiment; 
       FIG. 7  depicts a memory system including a memory module with two hub devices that may be implemented by an exemplary embodiment; 
       FIG. 8  depicts a memory system including a memory module with two hub devices that may be implemented by a second exemplary embodiment; 
       FIG. 9  depicts a memory module with eight ranks of memory that may be implemented by an exemplary embodiment; 
       FIG. 10  depicts a memory module with an interface test capability that may be implemented by an exemplary embodiment; and 
       FIG. 11  is a flow diagram of a design process used in semiconductor design, manufacture and/or test. 
   

   DETAILED DESCRIPTION 
   An exemplary embodiment of the present invention provides for an improvement in memory system/subsystem reliability and fault tolerance. As computer systems evolve, the need for enhanced performance as well as improved reliability/fault tolerance becomes more important. The memory channels (interface) between a memory controller and one or more memory subsystems (e.g. memory module(s)), are required to operate at increasing data rates. The wide range of memory requirements such as increased memory capacity, reduced latency, and power management create the need for a very flexible set of memory configurations. 
   An exemplary embodiment implements one or more high speed cascade interconnect communication interfaces (e.g. one or more cascade interconnect memory channels) between a memory controller and one or more memory hub device(s) using two unidirectional links each with full differential signaling. The downstream link from the memory controller to the hub device includes sixteen differential pairs made up of thirteen active logical signals, two spare lanes, and a bus clock. The upstream link from the hub device to the memory controller includes twenty-three differential pairs made up of twenty active logical signals, two spare lanes, and a bus clock. Each hub device has the capability to support two full memory ports (e.g. having distinct and separate address, command, control and data connections), with each memory port communicating with one to eight ranks of memory devices and each rank including eight bytes of data as well as eight bits of error code correction (ECC). In an exemplary embodiment, these memory ports can be operated simultaneously and independently of each other. 
   In an exemplary memory system embodiment, numerous memory module configurations are supported, as dictated by various system requirements. Example memory module configurations include, but are not limited to: a buffered memory module designed for use in a point-to-point interconnect structure either directly or cascade-interconnected with a memory controller; a buffered memory module having two hub devices, configured to permit communication with a memory controller via two memory channels (thereby appearing as two parallel memory modules and/or otherwise increasing the effective data bus width to the memory controller); and a buffered memory module having two hub devices internally cascaded-interconnected on the module. 
   In addition, for exemplary memory modules that support higher memory rank counts (e.g., four or eight ranks), discrete registering drivers (e.g. discrete registers and/or register/clock devices) may be included between the hub device and the memory devices to re-drive one or more of the command, address, and control signals, with some devices also and/or independently operable to buffer and/or re-sync (e.g. via a PLL or DLL) the memory interface clock(s). To support improved test capability, additional memory interface signals may be provided on the memory subsystem connector (e.g. on a DIMM and the DIMM connector) to provide the reference voltage levels supplied to the memory devices (VREFCA_TST and VREFDQ_TST). 
   With increased memory performance and memory capacity comes the additional adverse effect of increased overall memory subsystem power. One mechanism to partially offset the additional power is to use wider I/O memory devices on the memory subsystem (e.g. DIMM), such as x8 devices rather than x4 devices. The use of wider memory devices generally results in a negative impact in regard to overall memory subsystem reliability, often due in part to the limited coverage of the error detection and correction codes often utilized in such memory systems (e.g. 72/80 codes). As a result of the increased data I/O width, in conjunction with the total data bits available to the memory controller (generally 72 bits for a 8 byte memory subsystem), when an uncorrectable error or “chip kill” event occurs on the wide I/O memory device (e.g. a “x8” device), the ability of the memory subsystem to correctly identify, correct and/or otherwise recover from the event is reduced. 
     FIG. 1  depicts a memory system with cascade-interconnected buffered memory modules  103  and unidirectional buses  106  that may be implemented by an exemplary embodiment. Although only a single channel is shown on the memory controller, most controllers will include additional memory channel(s), with those channels also connected or not connected to one or more buffered memory modules in a given memory system/configuration. One of the functions provided by the hub devices  104  on the memory modules  103  in the cascade structure is a cascade-interconnect (e.g. re-drive and/or re-sync and re-drive) function which, in exemplary embodiments, is programmable to send or not send signals on the unidirectional buses  106  to other memory modules  103  or to the memory controller  110 . The final module in the cascade interconnect communication system does not include unidirectional buses  106  for connection to further buffered memory modules, although in the exemplary embodiment, such buses are present but powered-down to reduce system power consumption.  FIG. 1  includes the memory controller  110  and four memory modules  103 , each module connected to one or more memory buses  106  (with each bus  106  comprising a downstream memory bus and an upstream memory bus), connected to the memory controller  110  in either a direct or cascaded manner. The memory module  103  next to the memory controller  110  is connected to the memory controller  110  in a direct manner. The other memory modules  103  are connected to the memory controller  110  in a cascaded manner (e.g. via a hub or buffer device). Each memory module  103  may include one or more ranks of memory devices  109 . 
   In an exemplary embodiment, the speed of the memory buses  106  is a multiple of the speed of the memory module data rate (e.g. the bus(es) that communicate such information as address, control and data between the hub, buffer and/or register device(s) and the memory devices), operating at a higher speed than the memory device interface(s). Although not shown, the memory devices may include additional signals (such as reset and error reporting) which may communicate with the buffer device  104  and/or with other devices on the module or external to the module by way of one or more other signals and/or bus(es). For example, the high-speed memory data bus(es)  106  may transfer information at a rate that is four times faster than the memory module data rate (e.g. the data rate of the interface between the buffer device and the memory devices). In addition, in exemplary embodiments the signals received on the memory buses  106  are in a packetized format and it may require several transfers (e.g., four, five, six or eight) to receive a packet. For packets intended for a given memory module, the signals received in the packetized memory interface format are converted into a memory module interface format by the hub devices  104  once sufficient information transfers are received to permit communication with the memory device(s). In exemplary embodiments, a packet may comprise one or more memory device operations and less than a full packet may be needed to initiate a first memory operation. In an exemplary embodiment, the memory module interface format is a serialized format. In addition, the signals received in the packetized memory interface format are re-driven on the high speed memory buses (e.g., after error checking and re-routing of any differential signal pairs (e.g. by use of bitlane sparing between any two devices in the cascade interconnect bus) has been completed. 
   In the exemplary embodiment, the unidirectional buses  106  include an upstream bus and a downstream bus each with full differential signaling. As described previously, the downstream bus from the memory controller  110  to the hub device  104  includes sixteen differential signal pairs made up of thirteen active logical signals, two spare lanes, and a bus clock. The upstream bus from the hub device  104  to the memory controller  110  includes twenty-three differential signal pairs made up of twenty active logical signals, two spare lanes, and a bus clock. 
     FIG. 2  depicts a front view of a buffered memory module  206  (e.g. a dual in-line memory module or “DIMM”) that may be implemented by an exemplary embodiment. In an exemplary embodiment such as that shown in  FIG. 2 , each memory module  206  includes a raw card (e.g. a printed circuit board) having dimensions of nominally 151.35 millimeters long and 54.6 millimeters tall, a plurality of DRAM positions, a hub device  202  and numerous small components as known in the art that are not shown and/or identified but are included in exemplary embodiments, such as capacitors, resistors and EEPROM(s). It is understood that the stated dimensions are nominal and that they may vary (e.g., plus or minus three millimeters) in various implementations (e.g. pinned or pluggable modules optimized for varying memory systems). In an exemplary embodiment of the present invention, the hub device  202  is located in the center region of the front side of the memory module  206 , as depicted in  FIG. 2 . The memory devices  204  are located on either side of the hub device  202 , as well as on the backside of the memory module  206 , which is not shown. In the exemplary embodiment, the configuration is utilized to facilitate high speed wiring between the hub device  202  and memory devices  204  as well to facilitate high speed wiring from the hub device  202  to the module pins. 
   In further exemplary embodiments, sixteen, eighteen, thirty two, thirty six, seventy two or other memory device positions may be included on the memory module  206 , and the card height may be increased (e.g., to 96 millimeters or other dimensions) or decreased (e.g., to 30.48 millimeters or other dimensions) to commensurate with such factors as the dimensional requirements of the memory devices  204 , the dimensional requirements of the hub device  202 , the system dimensional requirements, the space required for the interconnect wiring on the module as well as the area required for support devices (e.g. capacitors, EEPROM(s) and/or resistors). 
   As is also shown in  FIG. 2 , the location of a positioning key  210  (e.g. a “notch”) is specifically shifted from the midpoint of the length, 1, of the card  208  (with respect to prior generation modules) in order to ensure that the module cannot be fully inserted into a connector intended for a different module type. In addition, the positioning key  210  location also prevents reverse insertion of the DIMM and provides a visual aid to the end-user regarding proper DIMM insertion. In the exemplary embodiment shown in  FIG. 2 , the positioning key  210  is located between pins  84 / 222  and  85 / 223  (front pin number/back pin number, read from left to right when facing the DIMM with the pins facing downward, as shown in the figure). As such, the distance d 1  along the length, 1, of the card  208  is larger than the distance d 2 .  FIG. 2  also depicts a midpoint  212  with respect to the length of the card. As such, the distance on either side of the midpoint  212  is represented as “½ l” in  FIG. 2 . In an exemplary embodiment, such as the one depicted in  FIG. 2 , the center of the positioning key is located a nominal distance of 86.675 millimeters from one end of the card (the end on the left in  FIG. 2 ) and a nominal distance of 64.675 millimeters from the other end of the card (the end on the right in  FIG. 2 ). It is understood that the stated dimensions are nominal and that they may vary (e.g., plus or minus three millimeters) in various implementations. 
   In an exemplary embodiment, the card  208  depicted in  FIG. 2  includes a first plurality of pins located on the first side (the front side depicted in  FIG. 2 ) and a second plurality of pins located on the second side (the back side not depicted in  FIG. 2 ). 
   In another exemplary embodiment, the DIMM is a “winged” DIMM. For exemplary “winged” DIMMs, some distance above the edge containing the pins (with the distance typically selected such that the DIMM extensions (“wings”) are above the connector latching mechanisms) the DIMM raw card includes an extension to the left and/or right of the normal exemplary DIMM length. The extension(s) permit one or more of the use of mechanical stiffeners and/or brackets (e.g. placed on the system board, memory cover, etc) providing additional means to further stabilize the DIMM beyond that provided by the connector, allow the placement of additional components (e.g. active and/or passive circuitry), etc. while permitting the existing connector to be utilized. For tall DIMMs, such as exemplary DIMMs being 90 to 100 mm in height, the use of mechanical stiffeners permitted by the raw card extensions (“wings”) results in less connector contact wear due to vibration and will reduce the probability of intermittent signal transmission across the connector/DIMM interface. In an exemplary embodiment, the DIMM raw card extends both 5 millimeters to the right and to the left of as compared to DIMM raw cards having no extensions. In another exemplary embodiment, the DIMM raw card extends more or less distance from the edge(s) of the DIMM being retained by the connector, based on the additional retention means and/or additional components placed in the additional raw card area provided by the extensions to the right and/or to the left. 
     FIG. 3  is a block diagram of the high-level logic flow of a hub device that may be implemented by an exemplary embodiment. The blocks in the lower left and right portions of the drawing ( 324 ,  330 ,  328 ,  334 ) are associated with receiving or driving a high-speed bus including the high speed unidirectional upstream and downstream buses previously referenced. “Upstream” refers to the one or more bus(es) passing information to and/or from the hub device in the direction of the memory controller, and “downstream” refers to the bus(es) passing information from the hub device to and/or from the modules and/or buffers located further away from the memory controller. 
   Referring to  FIG. 3 , data, command, address, error detection information (e.g. ECC bits and/or CRC bits), and clock signals from an upstream memory module or memory controller are received (in the exemplary embodiment, in the form of data packets), from the high speed cascade interconnect downstream (e.g. the “primary downstream” (PDS)) memory bus into a receiver functional block  324 . The receiver functional block  324  includes receiver and re-sync circuitry and other support logic to enable the capturing of information from an upstream device, as well as bitlane sparing circuitry to enable the replacement of one or more defective data and/or clock differential pairs between the receiving device and the sending device (e.g. one or more segments comprising a part of the cascade interconnect channel). In the exemplary embodiment, said sparing circuitry exists in each of the driver and receiver functional blocks ( 324 ,  330 ,  328  and  334 ), as well as in the memory controller, and permits any one or more of the differential pairs between any two devices on the memory bus to be replaced by an unused an/or underutilized (“spare”) differential signal pair—retaining full function and failure coverage on the affected bus—thereby resulting in improved product long-term memory system reliability and usability given the ability to continue normal operation of the memory system independent of one or more faults resident in one or more segments of the one or more memory system bus(es) which further operate together to comprise the memory controller channel(s). 
   An exemplary embodiment of the downstream memory bus further enables operation of a sixteen bit, high-speed fully differential, slave receiver bus further including one or more (e.g.) differential) spare bitlane(s). In an exemplary embodiment, the receiver functional block  324  transmits the received signals to a memory controller (MC) protocol functional block  312  which both passes the received signals to driver functional block  328  (e.g. secondary downstream (SDS) driver functional block) for re-driving the received and/or re-synchronized signals (e.g. data packet(s)) to a downstream memory buffer and/or buffered memory module, as well as captures the received packet(s) and forwards the data included in the packet(s), in a memory module format and at a memory module data rate to command state machine functional block  314  (e.g. address, command and control information) and read/write data buffers  316  (e.g. data to be written to the memory device(s), generally also including memory date ECC “check” bits). In an exemplary embodiment, MC protocol functional block  312  further includes circuitry to validate the received data prior to and/or after re-driving the received data, e.g. using ECC and/or CRC bits included in the memory packet, enabling the hub to identify and report faults present in the received data since being sent by the transmitting device. 
   In other exemplary embodiments the hub device circuitry (e.g. one or more of blocks  312  and  314 ) may first determine if the information is intended solely for use by that hub and/or module, negating the need to pass the information to the driver functional block  328  with the accompanying power utilization by that block. The driver functional block  328  provides circuitry (often embodied in the form of logic macros) macros and support logic for the downstream memory bus. As described above, the MC protocol functional block  312  may perform numerous functions, including, but not limited to: segment level sparing to replace one or more defective segments (e.g., differential wires between two memory modules or between a memory module and a memory controller) with one of the spare segments; error detection circuitry; error reporting circuitry; packet capture and data extraction from the packet; and the merging of local data onto the downstream and/or upstream cascade interconnect bus(es), etc. 
   In an exemplary embodiment, the command state machine functional block  314  determines if the signals (which will generally include one or more of data, command, control and address signals) are directed to and should be processed by the current memory module where the hub device is located. If the signals are directed to the current memory module, then the command state machine functional block  314  determines what actions to take (e.g. by decoding the one or more commands which may be included in the packet) and may initiate memory device actions, write buffer actions, read buffer actions, internal hub actions (e.g. MCBIST) or a combination thereof. In the exemplary embodiment, depending on the type of memory module, the command state machine functional block  314  selects the appropriate drive characteristics, timings and timing relationships based on register settings established during initialization and/or periodic operational evaluation of the memory device interface. In an exemplary embodiment, the MC protocol functional block  312  provides the conversion between signals received via the high speed bus in a packetized memory interface format into a memory module data rate, currently a non-packetized memory module interface format although the memory devices may operate using a device-specific packetized interface in future embodiments, necessitating conversion to that interface. The read/write data buffers  316  transmit the data (e.g. the information to be written to the memory device(s) to a memory data interface block  306  and the command state machine functional block  314  transmits the associated addresses, control and command signals to a memory command interface block  308 , with the signals consistent with the memory device specification in the exemplary embodiment. The memory command interface functional block  308  transmits the associated addresses, control and command signals  304  to a memory device via an address/command bus (which includes one or more of address, control, command information and error information, as indicated in the exemplary list comprising  304 ). The memory data interface functional block  306  reads from and writes memory data  342  to a memory device via a data bus, and in the exemplary embodiment, further includes data strobes (e.g. “DQS” signals) to facilitate the identification and capture of data at the receiving device (the buffer and/or the memory device(s). With ever-higher speed address, control, command and data bus operation, other methods in addition to or instead of strobes will be adopted to enable the identification and capture of information at the receiving device(s). 
   As shown in the exemplary memory device interface located between the memory data interface functional block  306  and the signals listed in  342 , two copies of the memory interface information (such as address, control, command, data, error detection bits, etc) required to enable independent operation of a memory port are included on the hub device. In memory data interface functional block  306 ,  144  data signals are shown, with 72 data signals utilized for each read and/or write port. In addition, 36 DQS (strobe) differential signals are shown, with 18 intended for communication with the 72 data signals (e.g. data bits) comprising each of the two read/write data ports in this embodiment. Similarly, separate copies of such information as address, control and command data are included in the interface(s) connected to the memory command interface block  308 . In this exemplary embodiment, two memory ports are supported by the hub device, implemented using additional circuitry (such as drivers and/or receivers) for each of the ports, thereby enabling simultaneous and/or independent operation of the two memory ports and the memory devices attached to these ports. In the exemplary embodiment the simultaneous and/or independent operation of the ports is determined by one or more of the initialization of the hub device and/or one or more of the control and/or command information and the address information received by the hub device. The two ports, as implemented in the exemplary hub circuitry shown in  FIG. 3 , enable modules and/or other memory subsystems using the hub device to selectively operate as one or two memory subsystems. 
   Data signals to be transmitted to the memory controller may be temporarily stored in the read/write data buffers  316  after a command, such as a read command, has been executed by the memory module, consistent with the memory device ‘read’ timings. The read/write data buffers  316  selectively transfer the read data into the upstream and/or downstream bus(es) via the MC protocol functional block  312  and upstream and/or downstream bus driver functional block(s)  330  and  328 . The driver functional blocks  330  and  328  provide macros and support logic for the upstream (and/or downstream) memory bus(es). While most operations completed in the memory module are expected to be read and/or write operations initiated by the memory controller, in the exemplary embodiment the memory controller built in self-test (MCBIST) functional block  310  may also initiate read and/or write operations to the memory device(s) attached to the present hub device and/or one or more upstream and downstream hub device(s), using the one or more memory ports on the hub device(s). Data read from the memory device(s) may be locally processed within the hub device (e.g. by the MCBIST circuitry) and/or sent upstream and/or downstream to other device(s) on the cascade interconnect bus(es) as shown in the communication paths defined by the arrows in the hub device shown in  FIG. 3 . 
   Signals such as data, ECC, CRC, clock error, and other information from the high speed upstream memory bus are received by the hub device receiver functional block  334 . In the exemplary embodiment, these signals are passed upstream to the next memory module or to the memory controller, although other embodiments may first decode the information to determine if the information is intended solely for use by that hub and/or module, negating the need to pass the information upstream with the accompanying power utilization. This determination may be made in relation to any upstream or downstream packet(s), thereby either preventing the re-driving of the received information (e.g. packet(s) and or allowing such re-drive, depending on the information (e.g. command and/or control) information received by the hub. In the exemplary embodiment shown in  FIG. 3 , such information as data, ECC, CRC, error and clock signals from a downstream memory module are received on the upstream memory bus (e.g. secondary upstream bus (SUS)) into receiver functional block  334 . The receiver functional block  334  provides macros and support logic to enable the capture of information received from the upstream memory bus and in exemplary embodiments, also includes segment sparing logic as previously described. The receiver functional block  334  passes the received signals, through the MC protocol functional block  312 , to the upstream memory bus via the driver functional block  330 . In exemplary embodiments, the received information is also passed to the command state machine and may further pass to the MCBIST and/or R/W data buffers, depending on the contents of the information received on the bus. This operability such modes as the testing of local memory devices by an MCBIST engine in the memory system, without the direct involvement of the memory controller—thus permitting the memory controller to delegate diagnostic, test, characterization and/or other operations to the one or more hub devices on a memory channel. 
   As described earlier, the MCBIST functional block  310  provides built in self-test functions which act upon one or more of the local buffer, the local memory device(s) attached to the hub device, upstream memory hub device(s), upstream memory device(s), downstream hub device(s) and downstream memory device(s). In response to the built-in self test (BIST) circuitry initiating the test functions, the resulting data, error and/or other information derived from the test is analyzed by the local hub and/or the hub device initiating the test, which may be located upstream or downstream from the memory module and/or interconnections being tested. The test and pervasive functional block  302  communicates with one or more of FSI, 12C, JTAG or alternate bus types to which it is connected, providing an alternate means of communication to the memory controller and/or a service processor (the latter not shown in the figures but otherwise known in the art). The information sent and received by this block is used in exemplary embodiments for such operations as the initialization of the high speed bus(es), initialization of the hub device(s), initialization of the memory device(s) attached to the hub(s), error reporting, error recovery, diagnostic initialization, the reset of attached device(s), the programming of registers, drivers, and/or other circuitry related to the described operations, etc. 
   The block diagram in  FIG. 3  is one implementation of a hub device that may be utilized by exemplary embodiments of the present invention. Other implementations and/or functional assignment for these and other functional blocks are possible without departing from the scope of the present invention. 
     FIG. 4  is made up of  FIGS. 4   a  and  4   b , and depicts a table illustrating a functional pin assignment and dimensional positioning of such pins on a  276 -pin DIMM that may be implemented by an exemplary embodiment. In addition to the layout and approximate distance (shown in millimeters) from the positioning key to each pin.  FIG. 4  also provides a high level functional description of each of the pins, including those used for special control functions. As indicated previously, designated pins  1 - 138  run from left to right on the front side (also referred to as the first side) of the DIMM when viewed from the front, with pins  139 - 276  located behind pins  1 - 138  when viewing the front side of the DIMM. Although not shown, other pin counts may be utilized in the exemplary embodiment, with pins added or removed, while maintaining the overall dimensional attributes and functions of the exemplary memory module. 
   The pins depicted in  FIG. 4  include primary upstream bus interface pins labeled PUS and /PUS (e.g., PUS10 and /PUS10, located 53.5 millimeters and 52.5 millimeters respectively from the notch on the top of the module) and secondary upstream bus interface pins labeled SUS and /SUS (e.g., PDSCK and /PDSCK, located 53.5 mm and 52.5 mm respectively from the notch on the bottom or backside of the module) arranged on the card for communicating with one or more differential upstream buses. In the exemplary embodiment, the upstream pins are located greater than or equal to approximately 13.5 mm to the left of the positioning key, when viewed from the front of the module. In addition, the differential pin pairs are separated from each other by one or more pins assigned as power or ground pins, the thickness of the raw card or other signals which do not significantly detract from the high speed operation of the upstream bus(es). In addition, the pins depicted in  FIG. 4  include primary downstream bus interface pins (labeled PDS and /PDS, located approximately 15.5 mm and 16.5 mm respectively from the notch on the top of the module) and secondary downstream bus interface pins (labeled SDS and /SDS, located approximately 15.5 mm and 16.5 mm respectively from the notch on the bottom or backside of the module) for communication with one or more differential downstream buses. In the exemplary embodiment, the downstream pins are located less than or equal to approximately 13.5 mm to the left of the positioning key (continuing to the right of the key), when viewed from the front of the module. In addition, the differential pin pairs are separated from each other by one or more pins assigned as power or ground pins, the thickness of the raw card or other signals which do not significantly detract from the high speed operation of the downstream bus(es). 
   The pins depicted in  FIG. 4  also include downstream clock interface pins (labeled PDSCK and /PDSCK) comprise a differential clock signal traveling with the high speed data to facilitate capture of that data on the bus(es) by the receiving device. In addition, upstream clock interface pins (labeled PUSCK and /PUSCK) comprise a differential clock signal traveling with the high speed data to facilitate capture of that data on the bus(es) by the receiving device. In the exemplary embodiment shown, the clock(s) are implemented as a “forwarded clock”, traveling with the data being transferred by way of the high speed memory bus(es). Other clocking methods may be used in alternate embodiments to facilitate accurate capturing of the data by the receiving device(s). 
   Pins labeled GND are ground pins and pins labeled 3.3V, 1.5V or 1.0V are voltage pins. In alternate exemplary embodiments, one or more of the defined voltage pins will be set to different voltage levels, dependent on the receiving technology and/or communication methodology utilized, without departing from the teachings herein. The exemplary embodiment further includes a redundant set of “reset” pins (/RESET and /RESETr located one behind the other on the front and back of the card, approximately 50.5 mm to the right of the notch when viewing the module from the front. These redundant pins provide a reliable means of externally receiving a synchronous and/or asynchronous “reset” signal—which is wired to the memory devices and well as to any of the other devices on the module (e.g. the buffer, registers, EEPROMs, etc) which may utilize the signal. In the exemplary embodiment, the reset function permits an external device (e.g. the memory controller, the processor, the service processor, a test device, etc) to return the receiving devices to a “known” state, facilitating power-up and initialization, permitting a rapid recovery from a fault and/or lock-up condition without the need to power-down and re-power the memory module and/or devices, etc. The use of redundant pins are intended to prevent the unintentional activation of the signal due to noise, coupling, vibration, corrosion (e.g. module and/or socket contacts) or other means, thereby dramatically increasing the fault-tolerance of the system relative to what would be a significant failure of the memory system if activated unintentionally. 
   Reference voltage pins are also included, for address, control, command and data pins (identified as VREFCA and VREFDQ respectively) and are used for one or more of the receipt and distribution of reference voltages to devices on the module receiving such information and the innovative “in-situ” voltage margin testing and/or adjustment (e.g. optimization) of the receiving devices while the module is installed in a test, system or other environment. 
     FIG. 5  depicts a single point-to-point interconnect memory system that may be implemented by an exemplary embodiment. The configuration in  FIG. 5  includes a single buffered memory module  502  directly connected to a memory controller  504  via a high-speed memory bus. Although not shown, exemplary embodiments would include one, two or more memory controller channels, enabling connection to the one memory module shown or to two or more memory modules  502 . 
     FIG. 6  depicts a memory module  608  with two full memory ports  612  (shown as memory port A and memory port B) that may be implemented by an exemplary embodiment. As depicted in  FIG. 6 , the memory module  608  includes a memory hub device  602  with two full memory ports  612  which enable communication (e.g. read, write, refresh, reset, memory device configuration, etc) of the one to eight ranks of memory devices connected to each of the two memory ports A and B. As previously described, the memory buffer (or hub) memory ports communicate with the attached memory devices at a device data rate, and comprise and/or permit communication of such signals as addresses, commands, controls, data, reset and other memory device signals and/or functions. As further described, memory port A connects to a first group of one to eight ranks of memory devices  606  attached to one of the memory ports  612  and memory port B connects to a second group of one to eight ranks of memory devices  604  attached to a second one of the memory ports  612 . The exemplary embodiment of memory module  608  also includes two thermal sensors  610 , placed in different areas of the memory module (e.g. to the left or right of memory buffer  602  on one side of the module, on the front and backside of the module and or in other separate regions of the module to better measure local module/device temperatures. Through the use of two or more independent (thermal (e.g. temperature) sensors  610 , shown as independent devices but which may also be incorporated in one or more of the EEPROM(s), buffer/hub device(s), memory device(s), etc, the device monitoring the temperature (such as the buffer, the memory controller, the processor, the service processor or other system element) can more accurately determine the local module temperature of each memory rank (which will often be operated independently) and/or more accurately reflect module temperature(s) when cooling air is blown across an exemplary module from the left to the right or from the right to the left in different applications. Thermal sensors may also be used as an early indicator of a potential card failure, such as resulting from a memory device and/or buffer device overheating due to excessive current drains (e.g. due to a semiconductor resistive fault such as due to a dielectric faults/breakdown), support device failures (such as capacitor low resistance shorts due to dielectric faults/breakdowns) and a raw card failure (such as due to low resistance shorts between raw card layers. In an exemplary embodiment, each memory port  612  can support one to eight ranks of memory devices with each rank having eight bytes of data and eight bits of ECC (e.g. supporting a 64/72 bit data/ECC structure). Memory module  608  is also shown to include one or more connections to upstream bus(es)  618  and downstream bus(es)  616  to permit high speed communication to a high speed cascade interconnection bus further enabling communication with the memory controller and any additional modules attached to said bus(es). Exemplary embodiments will often include additional connections to memory module  608  for such purposes as the communication of module characteristics, module faults, the completion of initialization, module and/or device reset, etc. 
     FIG. 7  depicts a memory system including a memory module with two buffer (e.g. hub) devices that can be implemented by an exemplary embodiment. The memory system depicted in  FIG. 7  can be referred to as a dual point-to-point memory system, comprising a single memory module  702  and a memory controller  704 . This configuration allows for up to four ports on a single DIMM  702 , providing for significant system performance, reliability and/or other advantages as compared to contemporary buffered modules having only a single unique memory port and a single memory buffer device, especially when only a single module is installed in the system. Performance advantages could include one or more of reduced memory latency and higher memory data rate (both reads and writes), whereas higher reliability could be obtained via the use of a wider memory bus (e.g. using an ECC code having improved detection and/or correction, especially for wider (e.g. x8) devices), the mirroring of data on the DIMM, etc. The reduced memory latency could be obtained by several methods including the high number of ranks (and therefore the large amount of memory storage) possible on the module (e.g. up to 8 memory ranks per port and up to four ports per module), with all ranks located at the same approximate physical and/or electrical (time) distance from the memory controller (in the exemplary embodiment, the distance from the buffer device(s) to the memory controller would be closely matched), the ability for multiple, concurrent memory operations to take place (e.g. a read, write or refresh operation to one memory buffer port  612  concurrent with a read, write or refresh operation to a second buffer port  612 , independent to and/or parallel to similar operation(s) taking place as controlled by the second memory channel and the second memory buffer, etc. As detailed above, improved reliability could also be obtained in addition to and/or separate from improved performance, by such means as operating both memory channels in parallel to obtain a wide data path (e.g. 128 data bits and 16 ECC bits in an exemplary embodiment), thereby increasing ECC coverage on data written to and read from the memory controller through the use of 128/144 bit ECC codes. Memory mirroring could also be utilized, with one method including the use of the exemplary memory module  702  wherein the memory controller writes the same data to the memory device(s) attached to each of the two high speed memory channels connected to memory module  702 , and wherein a subsequent read from the memory module by way of the two high speed memory channels results in two sets of data. In one embodiment of memory mirroring, the data from access to the second memory channel would be used when the data from the access to the first channel results in bad data (e.g. as indicated by the ECC check bits retrieved, CRC bits retrieved or some other error detection method utilized for the writing and reading of data to and from the memory module. Mirroring would also provide a means of recovery given the failure of one or more other non-correctable elements in one of the two memory buffer/memory device subsystems, such as a memory buffer fault (e.g. as indicated by a parity or other error indication during an operation), a wire fault (as determined from a current or previous memory operation), etc. To reduce noise-induced or other time-related fails (due to periodic and/or random events), the mirroring and/or other described functions could be implemented such that the two memory controller channels transfer information with a time offset from each other, such that a momentary fault (mechanical, electrical, thermal, etc) affects only the information transfers on one of the two channels. As depicted in  FIG. 7 , each buffer device can (selectively) operate independently of the other buffer device and each hub device is connected to the memory controller  704  via a separate set of high speed memory buses, with the module operation completed in response to the information provided by the memory controller, which may vary over time and be based on the system requirements at that time. 
     FIG. 7  depicts a memory system wherein an exemplary memory module includes connection means for two memory channels, which may be operated simultaneously or independently at any given time, generally as determined by the memory controller. As the exemplary module pin assignments shown in  FIG. 4  permit connection to two upstream and two downstream buses, the memory system shown in  FIG. 7  would be therefore be limited to a single DIMM in point-to-point structures, with no cascade interconnection to a second module from the module shown. By adding additional pins to the DIMM and/or the adoption of an alternate pin placement structure (e.g. by including two rows of pins on each side of the DIMM), the module could be extended to include connection to additional memory channels and/or include one or more cascade interconnection buses to memory module(s) located downstream (e.g. farther away from the memory controller) of the module shown. In addition, the memory controller may include additional memory channels beyond that of the two channels shown, permitting additional module(s) to be connected to the memory controller. 
     FIG. 8  depicts a memory system including a memory module with two hub devices that may be implemented by another exemplary embodiment. The memory system depicted in  FIG. 8  can be referred to as a dual point-to-point cascaded interconnect memory system, comprised of a single memory module  802  and a memory controller  804 . This configuration also allows for four ports on a single DIMM  802 , although as depicted in  FIG. 8 , the hub devices are cascade connected to each other and are connected to the memory controller  804  via one high speed memory channel (comprised of one or more high speed memory buses). This structure can be utilized to obtain many of the same performance and reliability benefits as those described in  FIG. 7 , although in many cases to a lesser extent due to the addition of a second cascaded high speed bus between the two memory buffers shown and the use of only a single memory channel to the memory controller. Additional performance would be achieved, as compared to two independent, cascade interconnected modules, due to the very short and tightly controlled high speed bus(es) between the two memory buffers (e.g. having short wires, no connectors between buffers, etc), increased module density would be possible by permitting up to 32 ranks of memory devices on the module (e.g. by having one to 8 ranks for each of the two ports on the two memory buffers shown), increased reliability would be possible with the use of mirroring (e.g. between two ports on a single memory buffer or between two memory buffers) and/or increased data width (e.g. operating both ports of the buffer near simultaneously to double the module data width), etc.  FIG. 8  also enables the interconnection of the two memory buffers with a low number of signals, due to the use of the high speed packetized cascade interconnection bus(es), and, although not shown, retains the second set of upstream and downstream module pins to enable cascade interconnection to one or more additional modules located further away from the memory controller than the module  802  shown. 
     FIG. 9  depicts a memory module with eight ranks of memory that may be implemented by an exemplary embodiment. The eight rank configuration depicted in  FIG. 9  includes memory module  902  with a memory buffer device  904  having two full memory ports “A” and “B” and including four registering clock driver devices  906  (implemented in the exemplary embodiment as two registering clock driver devices for each port on the memory buffer. The discrete registering clock driver devices  906  are included between the hub device  904  (ports A and B) and the memory devices to re-drive the memory device buses, including one or more of the device address, control, command, data, reset and/or other device pins sourced from the memory buffer. The memory devices  908  connect to port “A” of the memory buffer, wherein the memory devices  910  connect to port “B” of the memory buffer. Although four memory devices are shown in each memory device position of the exemplary embodiment (the four devices produced as memory device stacks within one package, memory device packages stacked together and/or a combination of the two), memory modules could be produced having more or less than four memory devices in each memory device position. Although two registering clock drivers are shown for each memory port, a single higher density and/or higher performance registering clock driver (or a larger number of registering clock drivers) could be connected between the memory buffer port and the memory devices attached to that port, depending on the performance and reliability targets as well as the signal/clock counts for a given module configuration. 
   The use of registering clock drivers (or related re-drive and/or re-synchronization circuitry, often having 2 sets of outputs for at least a portion of the signal inputs) external to the memory buffer present an innovative solution for increasing memory module density and/or performance, with minimal if any increase in pins on the memory buffer device, which is often pin or power-limited. In further exemplary embodiments, point-to-point interconnections can be used between the memory buffer and registering clock driver for critical nets, the registering clock driver can increase module performance by distributing the load presented by the memory devices across multiple driver circuits, driver circuits can be separately configured to optimize signal performance and quality for memory devices located near to or far from the memory buffer, the memory buffer can communicate with the registering clock driver(s) using a narrower, higher speed bus and/or a different voltage interface than that required for communication with the memory devices to reduce memory buffer pincount and/or allow the memory buffer to be memory device technology independent. Although described as a “registering clock driver”, the re-drive device(s) may be comprised solely of re-drive circuitry and/or include clock re-syncing circuitry such as PLL and DLL function(s). 
   Although not shown in  FIG. 9 , the “quad stacked”/“quad die” DDR3 memory devices used on the exemplary memory module shown are interconnected in an innovative manner to minimize the signals needed on the “quad” DRAMs and the memory buffer, as well as the pin counts of these devices, while meeting performance and power consumption targets. In the exemplary embodiment of a Quad “x4” package, the clock enable pins (CKE 0  and CKE 1 ) on the four memory devices are interconnected such that the first and third devices in the stack connect to CKE 0  and the second and fourth devices to CK 1 . The ODT (on die termination) signals of the two memory devices connected to CKE 1  are tied to VDD (internal or external to the memory devices/package), whereas the ODT signal on one of the remaining devices (connected to CKE 0 ) is wired to ODT 0  and the ODT signal on the last of the four memory devices (also connected to CKE 0 ) is wired to ODT 1 . The ODT signals are sourced by the memory buffer, and in the embodiment shown in  FIG. 9 , are first re-driven by the registering clock driver circuits. In the exemplary embodiment, all other utilized signal, power and ground pins (or pads) on the four memory devices are interconnected to corresponding signal, power and ground pins. 
   The exemplary module set also includes a two port and two rank memory module constructed with x4 devices packaged in a two-high device stack (although planar (1-high) memory devices and other packaging methods may also be used in this and/or alternate compatible versions). The exemplary embodiment further includes a memory buffer device and two DDR3 registering clock driver devices, which re-drive and/or re-sync the signals from the buffer device to the memory devices, similar to that shown in  FIG. 9 . Other exemplary embodiments may be manufactured, using x1, x2, x4, x8, x16 and/or other memory device data widths, with or without external re-drive circuitry depending on the memory device availability, module density requirement, performance requirement, reliability requirement, power requirement and/or other application needs. The nominal operating performance range of the exemplary buffered memory module embodiments for data I/Os range from 800 MT (million transfers) per second per pin to 1600 MT/s between the memory devices and the memory buffer, with specific designs operating at transfer rates consistent with system requirements. In the exemplary embodiment, the corresponding transfer rate for the high speed interface is four times that of the memory device data interface, or 3200 MT/s to 6400 MT/s. Other slower transfer rates are supported for very high density modules (e.g. 600 MT/s at the device and 2400 MT/s for the module respectively), with other (even slower) transfer rates supported (e.g. 200 MT/s at the buffer to memory device interface) by at least the memory module(s) and buffer device(s) for one or more of debug and bring-up purposes. 
     FIG. 10  depicts a memory module with a memory device interface test capability that may be implemented by an exemplary memory module and memory system embodiment. In this exemplary embodiment, the hub device  1004  outputs a programmable VREFDQ (data I/O reference voltage)  1006  for each port of the hub device  1004 . During normal operation, the VREF reference voltage is nominally set by the hub device  1004  to half of the data I/O voltage (e.g. VDD) supply (e.g. ½ of 1.5V, or 0.75V), with valid 1&#39;s and 0&#39;s detected by the hub device  10014  and memory device(s) by comparing the received data bits relative to the reference voltage, consistent with the valid “1” and “0” levels specified in the memory device specifications as voltage values relative to VREF. During initialization, diagnostic or related operations, the reference voltage is adjusted by the hub device  1004  to be more or less than half of the data I/O voltage supply, thereby permitting the hub device  1004  (e.g. using BIST circuitry), the memory controller, a service processor or some other controlling device and/or circuitry to determine the reference voltage at which 1&#39;s and 0&#39;s are accurately detected. Once the optimal voltage reference for the data I/O interface is determined (e.g. by finding the voltage reference value at which 1&#39;s and 0&#39;s are accurately received relative to the voltage reference value(s) during a read operation and determining the midpoint between the two data I/O reference voltage values), the hub device  1004  can set the local voltage reference to that value for the period of time until the data I/O reference voltage is re-characterized. In addition to maximizing the voltage margin for data transferred between the hub device  1004  and the memory devices, this innovative solution further permits voltage margin testing of the hub device  1004  and/or memory devices to be completed in-situ (e.g. in the actual environment), further allowing for marginal memory devices and/or memory modules to be detected as a means of improving overall system reliability and serviceability. An external device (such as a memory controller, processor, service processor, etc) may monitor the data I/O reference voltage by attaching to the VREFDQA_TST and VREFDQV_TST pins located on the module (e.g., a memory module connector  1002 ). Although the data I/O reference voltage is locally determined and/or generated on the module by the hub device  1004  in exemplary embodiments, further embodiments may have the I/O voltage generated by other means (e.g. by a voltage regulation device, the memory controller or some other means), with the VREFDQA_TST and VREFDQB_TST pins then used in a manner such as to characterize and set the reference voltage at the hub device  1004  and memory devices. The exemplary embodiment offers a dramatic increase in flexibility and reliability over previous module solutions, relative to data communication/capture integrity. 
     FIG. 10  also includes an exemplary VREFCA_TST pin which connects to a voltage divider which nominally sets a VREFCA voltage of half of VDD, with this reference voltage further connected to each of the memory devices. With the exemplary VREFCA pin, a device external to the memory module can override, as well as measure, the reference voltage set by the voltage divider, resulting in the same benefits as described previously for the VREFDQ test pins, except now related to the command, control and address pins of the memory devices. Although not shown in  FIG. 10 , alternate exemplary embodiments could include the VREFCA_TST voltage setting determined and/or adjusted by the memory hub device  1004 , such as under the control of the BIST circuitry during on-module margin testing of the address, control and command inputs. A VREFCA_TST voltage pin on the memory hub device  1004  would be connected to the VREFCA_TST wire shown, to enable the hub device  1004  to generate, monitor, increase and/or decrease the VREFCA voltage to the DRAMs. 
     FIG. 11  shows a block diagram of an exemplary design flow  1100  used for example, in semiconductor IC logic design, simulation, test, layout, and manufacture. Design flow  1100  includes processes and mechanisms for processing design structures or devices to generate logically or otherwise functionally equivalent representations of the design structures and/or devices described above and shown in  FIG. 3 . The design structures processed and/or generated by design flow  1100  may be encoded on machine readable transmission or storage media to include data and/or instructions that when executed or otherwise processed on a data processing system generate a logically, structurally, mechanically, or otherwise functionally equivalent representation of hardware components, circuits, devices, or systems. Design flow  1100  may vary depending on the type of representation being designed. For example, a design flow  1100  for building an application specific IC (ASIC) may differ from a design flow  1100  for designing a standard component or from a design flow  1100  for instantiating the design into a programmable array, for example a programmable gate array (PGA) or a field programmable gate array (FPGA) offered by Altera® Inc. or Xilinx® Inc. 
     FIG. 11  illustrates multiple such design structures including an input design structure  1120  that is preferably processed by a design process  1110 . Design structure  1120  may be a logical simulation design structure generated and processed by design process  1110  to produce a logically equivalent functional representation of a hardware device. Design structure  1120  may also or alternatively comprise data and/or program instructions that when processed by design process  1110 , generate a functional representation of the physical structure of a hardware device. Whether representing functional and/or structural design features, design structure  1120  may be generated using electronic computer-aided design (ECAD) such as implemented by a core developer/designer. When encoded on a machine-readable data transmission, gate array, or storage medium, design structure  1120  may be accessed and processed by one or more hardware and/or software modules within design process  1110  to simulate or otherwise functionally represent an electronic component, circuit, electronic or logic module, apparatus, device, or system such as those shown in  FIG. 3 . As such, design structure  1120  may comprise files or other data structures including human and/or machine-readable source code, compiled structures, and computer-executable code structures that when processed by a design or simulation data processing system, functionally simulate or otherwise represent circuits or other levels of hardware logic design. Such data structures may include hardware-description language (HDL) design entities or other data structures conforming to and/or compatible with lower-level HDL design languages such as Verilog and VHDL, and/or higher level design languages such as C or C++. 
   Design process  1110  preferably employs and incorporates hardware and/or software modules for synthesizing, translating, or otherwise processing a design/simulation functional equivalent of the components, circuits, devices, or logic structures shown in  FIG. 3  to generate a netlist  1180  which may contain design structures such as design structure  1120 . Netlist  1180  may comprise, for example, compiled or otherwise processed data structures representing a list of wires, discrete components, logic gates, control circuits, I/O devices, models, etc. that describes the connections to other elements and circuits in an integrated circuit design. Netlist  1180  may be synthesized using an iterative process in which netlist  1180  is resynthesized one or more times depending on design specifications and parameters for the device. As with other design structure types described herein, netlist  1180  may be recorded on a machine-readable data storage medium or programmed into a programmable gate array. The medium may be a non-volatile storage medium such as a magnetic or optical disk drive, a programmable gate array, a compact flash, or other flash memory. Additionally, or in the alternative, the medium may be a system or cache memory, buffer space, or electrically or optically conductive devices and materials on which data packets may be transmitted and intermediately stored via the Internet, or other networking suitable means. 
   Design process  1110  may include hardware and software modules for processing a variety of input data structure types including netlist  1180 . Such data structure types may reside, for example, within library elements  1130  and include a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.). The data structure types may further include design specifications  1140 , characterization data  1150 , verification data  1160 , design rules  1170 , and test data files  1185  which may include input test patterns, output test results, and other testing information. Design process  1110  may further include, for example, standard mechanical design processes such as stress analysis, thermal analysis, mechanical event simulation, process simulation for operations such as casting, molding, and die press forming, etc. One of ordinary skill in the art of mechanical design can appreciate the extent of possible mechanical design tools and applications used in design process  1110  without deviating from the scope and spirit of the invention. Design process  1110  may also include modules for performing standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. 
   Design process  1110  employs and incorporates logic and physical design tools such as HDL compilers and simulation model build tools to process design structure  1120  together with some or all of the depicted supporting data structures along with any additional mechanical design or data (if applicable), to generate a second design structure  1190 . Design structure  1190  resides on a storage medium or programmable gate array in a data format used for the exchange of data of mechanical devices and structures (e.g. information stored in a IGES, DXF, Parasolid XT, JT, DRG, or any other suitable format for storing or rendering such mechanical design structures). Similar to design structure  1120 , design structure  1190  preferably comprises one or more files, data structures, or other computer-encoded data or instructions that reside on transmission or data storage media and that when processed by an ECAD system generate a logically or otherwise functionally equivalent form of one or more of the embodiments of the invention shown in  FIG. 3 . In one embodiment, design structure  1190  may comprise a compiled, executable HDL simulation model that functionally simulates the devices shown in  FIG. 3 . 
   Design structure  1190  may also employ a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g. information stored in a GDSII (GDS2), GL1, OASIS, map files, or any other suitable format for storing such design data structures). Design structure  1190  may comprise information such as, for example, symbolic data, map files, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a manufacturer or other designer/developer to produce a device or structure as described above and shown in  FIG. 3 . Design structure  1190  may then proceed to a stage  1195  where, for example, design structure  1190 : proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
   In an exemplary embodiment, hub devices may be connected to the memory controller through a multi-drop or point-to-point bus structure (which may further include a cascade connection to one or more additional hub devices). Memory access requests are transmitted by the memory controller through the bus structure (e.g., the memory bus) to the selected hub(s). In response to receiving the memory access requests, the hub device translates the memory access requests to control the memory devices to store write data from the hub device or to provide read data to the hub device. Read data is encoded into one or more communication packet(s) and transmitted through the memory bus(es) to the memory controller. 
   In alternate exemplary embodiments, the memory controller(s) may be integrated together with one or more processor chips and supporting logic, packaged in a discrete chip (commonly called a “northbridge” chip), included in a multi-chip carrier with the one or more processors and/or supporting logic, or packaged in various alternative forms that best match the application/environment. Any of these solutions may or may not employ one or more narrow/high speed links to connect to one or more hub chips and/or memory devices. 
   The memory modules may be implemented by a variety of technology including a DIMM, a single in-line memory module (SIMM) and/or other memory module or card structures. In general, a DIMM refers to a small circuit board which is comprised primarily of random access memory (RAM) integrated circuits or die on one or both sides with signal and/or power pins on both sides of the board. This can be contrasted to a SIMM which is a small circuit board or substrate composed primarily of RAM integrated circuits or die on one or both sides and single row of pins along one long edge. DIMMs have been constructed with pincounts ranging from 100 pins to over 300 pins. In exemplary embodiments described herein, memory modules may include two or more hub devices. 
   In exemplary embodiments, the memory bus is constructed using multi-drop connections to hub devices on the memory modules and/or using point-to-point connections. The downstream portion of the controller interface (or memory bus), referred to as the downstream bus, may include command, address, data and other operational, initialization or status information being sent to the hub devices on the memory modules. Each hub device may simply forward the information to the subsequent hub device(s) via bypass circuitry; receive, interpret and re-drive the information if it is determined to be targeting a downstream hub device; re-drive some or all of the information without first interpreting the information to determine the intended recipient; or perform a subset or combination of these options. 
   The upstream portion of the memory bus, referred to as the upstream bus, returns requested read data and/or error, status or other operational information, and this information may be forwarded to the subsequent hub devices via bypass circuitry; be received, interpreted and re-driven if it is determined to be targeting an upstream hub device and/or memory controller in the processor complex; be re-driven in part or in total without first interpreting the information to determine the intended recipient; or perform a subset or combination of these options. 
   In alternate exemplary embodiments, the point-to-point bus includes a switch or bypass mechanism which results in the bus information being directed to one of two or more possible hub devices during downstream communication (communication passing from the memory controller to a hub device on a memory module), as well as directing upstream information (communication from a hub device on a memory module to the memory controller), often by way of one or more upstream hub devices. Further embodiments include the use of continuity modules, such as those recognized in the art, which, for example, can be placed between the memory controller and a first populated hub device (i.e., a hub device that is in communication with one or more memory devices), in a cascade interconnect memory system, such that any intermediate hub device positions between the memory controller and the first populated hub device include a means by which information passing between the memory controller and the first populated hub device can be received even if the one or more intermediate hub device position(s) do not include a hub device. The continuity module(s) may be installed in any module position(s), subject to any bus restrictions, including the first position (closest to the main memory controller, the last position (prior to any included termination) or any intermediate position(s). The use of continuity modules may be especially beneficial in a multi-module cascade interconnect bus structure, where an intermediate hub device on a memory module is removed and replaced by a continuity module, such that the system continues to operate after the removal of the intermediate hub device. In more common embodiments, the continuity module(s) would include either interconnect wires to transfer all required signals from the input(s) to the corresponding output(s), or be re-driven through a repeater device. The continuity module(s) might further include a non-volatile storage device (such as an EEPROM), but would not include main memory storage devices. 
   In exemplary embodiments, the memory system includes one or more hub devices on one or more memory modules connected to the memory controller via a cascade interconnect memory bus, however other memory structures may be implemented such as a point-to-point bus, a multi-drop memory bus or a shared bus. Depending on the signaling methods used, the target operating frequencies, space, power, cost, and other constraints, various alternate bus structures may be considered. A point-to-point bus may provide the optimal performance in systems produced with electrical interconnections, due to the reduced signal degradation that may occur as compared to bus structures having branched signal lines, switch devices, or stubs. However, when used in systems requiring communication with multiple devices or subsystems, this method will often result in significant added component cost and increased system power, and may reduce the potential memory density due to the need for intermediate buffering and/or re-drive. 
   Although not shown in the Figures, the memory modules or hub devices may also include a separate bus, such as a ‘presence detect’ bus, an 12C bus and/or an SMBus which is used for one or more purposes including the determination of the hub device an/or memory module attributes (generally after power-up), the reporting of fault or status information to the system, the configuration of the hub device(s) and/or memory subsystem(s) after power-up or during normal operation or other purposes. Depending on the bus characteristics, this bus might also provide a means by which the valid completion of operations could be reported by the hub devices and/or memory module(s) to the memory controller(s), or the identification of failures occurring during the execution of the main memory controller requests. 
   Performances similar to those obtained from point-to-point bus structures can be obtained by adding switch devices. These and other solutions offer increased memory packaging density at lower power, while retaining many of the characteristics of a point-to-point bus. Multi-drop buses provide an alternate solution, albeit often limited to a lower operating frequency, but at a cost/performance point that may be advantageous for many applications. Optical bus solutions permit significantly increased frequency and bandwidth potential, either in point-to-point or multi-drop applications, but may incur cost and space impacts. 
   As used herein the term “buffer” or “buffer device” refers to a temporary storage unit (as in a computer), especially one that accepts information at one rate and delivers it another. In exemplary embodiments, a buffer is an electronic device that provides compatibility between two signals (e.g., changing voltage levels or current capability). The term “hub” is sometimes used interchangeably with the term “buffer.” A hub is a device containing multiple ports that is connected to several other devices. A port is a portion of an interface that serves a congruent I/O functionality (e.g., a port may be utilized for sending and receiving data, address, and control information over one of the point-to-point links, or buses). A hub may be a central device that connects several systems, subsystems, or networks together. A passive hub may simply forward messages, while an active hub, or repeater, amplifies and refreshes the stream of data which otherwise would deteriorate over a distance. The term hub device, as used herein, refers to a hub chip that includes logic (hardware and/or software) for performing memory functions. 
   Also as used herein, the term “bus” refers to one of the sets of conductors (e.g., wires, and printed circuit board traces or connections in an integrated circuit) connecting two or more functional units in a computer. The data bus, address bus and control signals, despite their names, constitute a single bus since each are often useless without the others. A bus may include a plurality of signal lines, each signal line having two or more connection points, that form a main transmission path that electrically connects two or more transceivers, transmitters and/or receivers. The term “bus” is contrasted with the term “channel” which is often used to describe the function of a “port” as related to a memory controller in a memory system, and which may include one or more buses or sets of buses. The term “channel” as used herein refers to a port on a memory controller. Note that this term is often used in conjunction with I/O or other peripheral equipment, however the term channel has been adopted by some to describe the interface between a processor or memory controller and one of one or more memory subsystem(s). 
   Further, as used herein, the term “daisy chain” refers to a bus wiring structure in which, for example, device A is wired to device B, device B is wired to device C, etc. The last device is typically wired to a resistor or terminator. All devices may receive identical signals or, in contrast to a simple bus, each device may modify one or more signals before passing them on. A “cascade” or cascade interconnect’ as used herein refers to a succession of stages or units or a collection of interconnected networking devices, typically hubs, in which the hubs operate as a logical repeater, further permitting merging data to be concentrated into the existing data stream. Also as used herein, the term “point-to-point” bus and/or link refers to one or a plurality of signal lines that may each include one or more terminators. In a point-to-point bus and/or link, each signal line has two transceiver connection points, with each transceiver connection point coupled to transmitter circuitry, receiver circuitry or transceiver circuitry. A signal line refers to one or more electrical conductors or optical carriers, generally configured as a single carrier or as two or more carriers, in a twisted, parallel, or concentric arrangement, used to transport at least one logical signal. 
   Memory devices are generally defined as integrated circuits that are composed primarily of memory (storage) cells, such as DRAMs (Dynamic Random Access Memories), SRAMs (Static Random Access Memories), FeRAMs (Ferro-Electric RAMs), MRAMs (Magnetic Random Access Memories), Flash Memory and other forms of random access and related memories that store information in the form of electrical, optical, magnetic, biological or other means. Dynamic memory device types may include asynchronous memory devices such as FPM DRAMs (Fast Page Mode Dynamic Random Access Memories), EDO (Extended Data Out) DRAMs, BEDO (Burst EDO) DRAMs, SDR (Single Data Rate) Synchronous DRAMs, DDR (Double Data Rate) Synchronous DRAMs or any of the expected follow-on devices such as DDR2, DDR3, DDR4 and related technologies such as Graphics RAMs, Video RAMs, LP RAM (Low Power DRAMs) which are often based on the fundamental functions, features and/or interfaces found on related DRAMs. 
   Memory devices may be utilized in the form of chips (die) and/or single or multi-chip packages of various types and configurations. In multi-chip packages, the memory devices may be packaged with other device types such as other memory devices, logic chips, analog devices and programmable devices, and may also include passive devices such as resistors, capacitors and inductors. These packages may include an integrated heat sink or other cooling enhancements, which may be further attached to the immediate carrier or another nearby carrier or heat removal system. 
   Module support devices (such as buffers, hubs, hub logic chips, registers, PLL&#39;s, DLL&#39;s, non-volatile memory, etc) may be comprised of multiple separate chips and/or components, may be combined as multiple separate chips onto one or more substrates, may be combined onto a single package or even integrated onto a single device—based on technology, power, space, cost and other tradeoffs. In addition, one or more of the various passive devices such as resistors, capacitors may be integrated into the support chip packages, or into the substrate, board or raw card itself, based on technology, power, space, cost and other tradeoffs. These packages may include an integrated heat sink or other cooling enhancements, which may be further attached to the immediate carrier or another nearby carrier or heat removal system. 
   Memory devices, hubs, buffers, registers, clock devices, passives and other memory support devices and/or components may be attached to the memory subsystem and/or hub device via various methods including soldered interconnects, conductive adhesives, socket structures, pressure contacts and other methods which enable communication between the two or more devices via electrical, optical or alternate means. 
   The one or more memory modules (or memory subsystems) and/or hub devices may be electrically connected to the memory system, processor complex, computer system or other system environment via one or more methods such as soldered interconnects, connectors, pressure contacts, conductive adhesives, optical interconnects and other communication and power delivery methods. Connector systems may include mating connectors (male/female), conductive contacts and/or pins on one carrier mating with a male or female connector, optical connections, pressure contacts (often in conjunction with a retaining mechanism) and/or one or more of various other communication and power delivery methods. The interconnection(s) may be disposed along one or more edges of the memory assembly and/or placed a distance from an edge of the memory subsystem depending on such application requirements as ease-of-upgrade/repair, available space/volume, heat transfer, component size and shape and other related physical, electrical, optical, visual/physical access, etc. Electrical interconnections on a memory module are often referred to as contacts, or pins, or tabs. Electrical interconnections on a connector are often referred to as contacts or pins. 
   As used herein, the term memory subsystem refers to, but is not limited to: one or more memory devices; one or more memory devices and associated interface and/or timing/control circuitry; and/or one or more memory devices in conjunction with a memory buffer, hub device, and/or switch. The term memory subsystem may also refer to one or more memory devices, in addition to any associated interface and/or timing/control circuitry and/or a memory buffer, hub device or switch, assembled into a substrate, a card, a module or related assembly, which may also include a connector or similar means of electrically attaching the memory subsystem with other circuitry. The memory modules described herein may also be referred to as memory subsystems because they include one or more memory devices and hub devices. 
   Additional functions that may reside local to the memory subsystem and/or hub device include write and/or read buffers, one or more levels of memory cache, local pre-fetch logic, data encryption/decryption, compression/decompression, protocol translation, command prioritization logic, voltage and/or level translation, error detection and/or correction circuitry, data scrubbing, local power management circuitry and/or reporting, operational and/or status registers, initialization circuitry, performance monitoring and/or control, one or more co-processors, search engine(s) and other functions that may have previously resided in other memory subsystems. By placing a function local to the memory subsystem, added performance may be obtained as related to the specific function, often while making use of unused circuits within the subsystem. 
   Memory subsystem support device(s) may be directly attached to the same substrate or assembly onto which the memory device(s) are attached, or may be mounted to a separate interposer or substrate also produced using one or more of various plastic, silicon, ceramic or other materials which include electrical, optical or other communication paths to functionally interconnect the support device(s) to the memory device(s) and/or to other elements of the memory or computer system. 
   Information transfers (e.g. packets) along a bus, channel, link or other naming convention applied to an interconnection method may be completed using one or more of many signaling options. These signaling options may include such methods as single-ended, differential, optical or other approaches, with electrical signaling further including such methods as voltage or current signaling using either single or multi-level approaches. Signals may also be modulated using such methods as time or frequency, non-return to zero, phase shift keying, amplitude modulation and others. Voltage levels are expected to continue to decrease, with 1.5V, 1.2V, 1V and lower signal voltages expected consistent with (but often independent of) the reduced power supply voltages required for the operation of the associated integrated circuits themselves. 
   One or more clocking methods may be utilized within the memory subsystem and the memory system itself, including global clocking, source-synchronous clocking, encoded clocking or combinations of these and other methods. The clock signaling may be identical to that of the signal lines themselves, or may utilize one of the listed or alternate methods that is more conducive to the planned clock frequency(ies), and the number of clocks planned within the various subsystems. A single clock may be associated with all communication to and from the memory, as well as all clocked functions within the memory subsystem, or multiple clocks may be sourced using one or more methods such as those described earlier. When multiple clocks are used, the functions within the memory subsystem may be associated with a clock that is uniquely sourced to the subsystem, or may be based on a clock that is derived from the clock related to the information being transferred to and from the memory subsystem (such as that associated with an encoded clock). Alternately, a unique clock may be used for the information transferred to the memory subsystem, and a separate clock for information sourced from one (or more) of the memory subsystems. The clocks themselves may operate at the same or frequency multiple of the communication or functional frequency, and may be edge-aligned, center-aligned or placed in an alternate timing position relative to the data, command or address information. 
   Information passing to the memory subsystem(s) will generally be composed of address, command and data, as well as other signals generally associated with requesting or reporting status or error conditions, resetting the memory, completing memory or logic initialization and other functional, configuration or related information. Information passing from the memory subsystem(s) may include any or all of the information passing to the memory subsystem(s), however generally will not include address and command information. This information may be communicated using communication methods that may be consistent with normal memory device interface specifications (generally parallel in nature), the information may be encoded into a ‘packet’ structure, which may be consistent with future memory interfaces or simply developed to increase communication bandwidth and/or enable the subsystem to operate independently of the memory technology by converting the received information into the format required by the receiving device(s). 
   Initialization of the memory subsystem may be completed via one or more methods, based on the available interface buses, the desired initialization speed, available space, cost/complexity objectives, subsystem interconnect structures, the use of alternate processors (such as a service processor) which may be used for this and other purposes, etc. In one embodiment, the high speed bus may be used to complete the initialization of the memory subsystem(s), generally by first completing a training process to establish reliable communication, then by interrogation of the attribute or ‘presence detect’ data associated with the various components and/or characteristics associated with that subsystem, and ultimately by programming the appropriate devices with information associated with the intended operation within that system. In a cascaded system, communication with the first memory subsystem would generally be established, followed by subsequent (downstream) subsystems in the sequence consistent with their position along the cascade interconnect bus. 
   A second initialization method would include one in which the high speed bus is operated at one frequency during the initialization process, then at a second (and generally higher) frequency during the normal operation. In this embodiment, it may be possible to initiate communication with all of the memory subsystems on the cascade interconnect bus prior to completing the interrogation and/or programming of each subsystem, due to the increased timing margins associated with the lower frequency operation. 
   A third initialization method might include operation of the cascade interconnect bus at the normal operational frequency(ies), while increasing the number of cycles associated with each address, command and/or data transfer. In one embodiment, a packet containing all or a portion of the address, command and/or data information might be transferred in one clock cycle during normal operation, but the same amount and/or type of information might be transferred over two, three or more cycles during initialization. This initialization process would therefore be using a form of ‘slow’ commands, rather than ‘normal’ commands, and this mode might be automatically entered at some point after power-up and/or re-start by each of the subsystems and the memory controller by way of POR (power-on-reset) logic included in each of these subsystems. 
   A fourth initialization method might utilize a distinct bus, such as a presence detect bus (such as the one defined in U.S. Pat. No. 5,513,135 to Dell et al., of common assignment herewith), an 12C bus (such as defined in published JEDEC standards such as the 168 Pin DIMM family in publication 21-C revision 7R8) and/or the SMBUS, which has been widely utilized and documented in computer systems using such memory modules. This bus might be connected to one or more modules within a memory system in a daisy chain/cascade interconnect, multi-drop or alternate structure, providing an independent means of interrogating memory subsystems, programming each of the one or more memory subsystems to operate within the overall system environment, and adjusting the operational characteristics at other times during the normal system operation based on performance, thermal, configuration or other changes desired or detected in the system environment. 
   Other methods for initialization can also be used, in conjunction with or independent of those listed. The use of a separate bus, such as described in the fourth embodiment above, also offers the advantage of providing an independent means for both initialization and uses other than initialization, such as described in U.S. Pat. No. 6,381,685 to Dell et al., of common assignment herewith, including changes to the subsystem operational characteristics on-the-fly and for the reporting of and response to operational subsystem information such as utilization, temperature data, failure information or other purposes. 
   With improvements in lithography, better process controls, the use of materials with lower resistance, increased field sizes and other semiconductor processing improvements, increased device circuit density (often in conjunction with increased die sizes) will help facilitate increased function on integrated devices as well as the integration of functions previously implemented on separate devices. This integration will serve to improve overall performance of the intended function, as well as promote increased storage density, reduced power, reduced space requirements, lower cost and other manufacturer and customer benefits. This integration is a natural evolutionary process, and may result in the need for structural changes to the fundamental building blocks associated with systems. 
   The integrity of the communication path, the data storage contents and all functional operations associated with each element of a memory system or subsystem can be assured, to a high degree, with the use of one or more fault detection and/or correction methods. Any or all of the various elements may include error detection and/or correction methods such as CRC (Cyclic Redundancy Code), EDC (Error Detection and Correction), parity or other encoding/decoding methods suited for this purpose. Further reliability enhancements may include operation re-try (to overcome intermittent faults such as those associated with the transfer of information), the use of one or more alternate or replacement communication paths to replace failing paths and/or lines, complement-re-complement techniques or alternate methods used in computer, communication and related systems. 
   The use of bus termination, on buses as simple as point-to-point links or as complex as multi-drop structures, is becoming more common consistent with increased performance demands. A wide variety of termination methods can be identified and/or considered, and include the use of such devices as resistors, capacitors, inductors or any combination thereof, with these devices connected between the signal line and a power supply voltage or ground, a termination voltage or another signal. The termination device(s) may be part of a passive or active termination structure, and may reside in one or more positions along one or more of the signal lines, and/or as part of the transmitter and/or receiving device(s). The terminator may be selected to match the impedance of the transmission line, or selected via an alternate approach to maximize the useable frequency, operating margins and related attributes within the cost, space, power and other constraints. 
   Technical effects and benefits include an improvement in memory subsystem reliability and fault tolerance while providing a flexible set of memory configurations. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof In addition, it will be understood that the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 
   The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
   As will be appreciated by one skilled in the art, the present invention may be embodied as a system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer-usable program code embodied in the medium. 
   Any combination of one or more computer-usable or computer-readable medium(s) may be utilized. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CDROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-usable medium may include a propagated data signal with the computer-usable program code embodied therewith, either in baseband or as part of a carrier wave. The computer usable program code may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc. 
   Computer program code for carrying out operations of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
   The present invention is described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
   These computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
   The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
   The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.