Patent Publication Number: US-7899983-B2

Title: Buffered memory module supporting double the memory device data width in the same physical space as a conventional memory module

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under DARPA, HR0011-07-9-0002. THE GOVERNMENT HAS CERTAIN RIGHTS IN THIS INVENTION. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present application relates generally to an improved data processing system. More specifically, the present application is directed to a buffered memory module supporting double the memory device data width in the same physical space as a conventional memory module. 
     2. Description of Related Art 
     Contemporary high performance computing main memory systems are generally composed of one or more dynamic random access memory (DRAM) devices, which are connected to one or more processors via one or more memory control elements. 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 main memory device(s), 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-before-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. 
     Furthermore, with the movement to multi-core and multi-threaded processor designs, new requirements are being made for the memory subsystem to supply very large data bandwidths and memory capacity into a single processor socket. At a system level, the bandwidth and memory capacity available from the memory subsystem is directly proportional to the number of dual in-line memory modules (DIMMs) that are installed in the system and the number of independent memory channels connected to the DIMMs. Due to the large increases in the number of cores and threads in a processor socket, a system that at one time only required four or eight DIMMs on each processor socket now may require two to four times the number of independent DIMMs. This in turn would drive system packaging to larger and larger packages. In a dense computing environment where there may be hundreds of processor racks, increasing the package size for a system may not be a viable option. 
     A conventional fully buffered DIMM includes a memory hub device that interfaces between a memory controller of a processor and dynamic random access memory (DRAM) on the DIMM. This memory hub device includes a high-frequency, high-bandwidth bus structure or memory channel between the memory hub device and the processor. The memory hub device also includes a second high-frequency, high-bandwidth point-to-point interface to the next DIMM in a daisy-chain configuration and a lower-bandwidth multi-drop eight-byte interface to the DRAMs on the DIMM. The bandwidth capability of the memory channel that is feeding the DIMM is significantly larger than the bandwidth capability of the interface to the DRAMs on the DIMM creating a mismatch of bandwidths. 
     A mismatch of bandwidths normally results in loss of performance in the system. That is, even though the processor is able to send access requests to the memory hub device using the high-bandwidth memory channel, the memory hub device is limited in its access to the DRAMS by lower-bandwidth memory interface. The industry standard solution to this is to install another DIMM on the daisy-chain interface. With this configuration the bandwidth from two memory hub devices may be combined to more efficiently use the bandwidth of the channel to the memory controller. However, the link between the memory hub devices results in added latency on read operations, which results in lower system performance. Additionally, there are many system configurations that do not have the physical space for a second DIMM socket. Without the space for the second socket there is no solution to efficiently use the bandwidth on the memory channel. In addition, for systems that target very dense computing environments, there may not be enough DIMM connectors for all the memory channels on the processor interface, let alone providing multiple DIMMs per memory channel. 
     SUMMARY 
     In order to increase the memory bandwidth through a memory module, the illustrative embodiments implement multiple memory device data interfaces in a memory hub device of a memory module that interfaces between a memory controller of a processor and memory devices on the memory module. Providing multiple memory device data interfaces on the memory hub device results in a more even match between the bandwidth on a memory channel coupled to the memory module and the bandwidth of the memory device data interface of a single memory module. Additionally, the multiple memory device data interfaces on the memory hub device also double the storage capacity of the memory module. 
     The illustrative embodiments provide mechanisms for enhancing the memory bandwidth available through a buffered memory module. One illustrative embodiment provides multiple memory device data interfaces in a memory hub device of a memory module that interfaces between a memory controller of a processor and memory devices on the memory module. Another illustrative embodiment provides for using a high-frequency, high-bandwidth point-to-point interface or memory channel that generally connects a memory hub device to another memory module in a daisy-chain configuration as an independent memory channel onto the memory module. A further illustrative embodiment provides a memory module stacking implementation that pins out data buses of two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access. 
     The illustrative embodiments provide a memory system that comprises a memory hub device integrated into a memory module. The illustrative embodiments provide a first memory device data interface integrated in the memory hub device that communicates with a first set of memory devices integrated in the memory module. The illustrative embodiments provide a second memory device data interface integrated in the memory hub device that communicates with a second set of memory devices integrated in the memory module. In the illustrative embodiments, the first set of memory devices are spaced in a first plane and coupled to a substrate of the memory module. In the illustrative embodiments, the second set of memory devices are spaced in a second plane above the first plane and coupled to the substrate. In the illustrative embodiments, data buses of the first set of memory devices are coupled to the substrate separately from data buses of the second set of memory devices. 
     In the illustrative embodiments, at least one of chip select signals, clock enable signals, calibration signals, or on-die termination signals of the first set of memory devices and the second set of memory devices are coupled together to the substrate. In the illustrative embodiments, the second set of memory devices may be stacked over the first set of memory devices. 
     In the illustrative embodiments, a first memory access request may be processed via the first memory device data interface and a second memory access request may be processed via the second memory device data interface at substantially a same time in a parallel manner using a chip select signal. 
     In the illustrative embodiments, the memory system may further comprise a memory channel for communicating with the first set of memory devices and second set of memory devices via the memory hub device. In the illustrative embodiments, the memory system may further comprise a memory controller in communication with the memory channel. In the illustrative embodiments, the memory controller may generate memory access requests, receives memory access requests, and responds to memory access requests. 
     In the illustrative embodiments, the memory hub device may further comprise a memory hub controller coupled to the first memory device data interface and the second memory device data interface. In the illustrative embodiments, the memory hub controller may respond to access request packets from one of a memory controller external to the memory module or a downstream memory hub device of another memory module by responsively driving one or more of the first set of memory devices or the second set of memory devices using a memory device address and control bus and directing one of a read data flow selector or a write data flow selector. 
     In the illustrative embodiments, the memory hub controller may drive the read data flow selector to a multiplexer of the memory hub device to select outputting read data directly, from one of the first memory device data interface or the second memory device data interface, or output read data from a read data queue. In the illustrative embodiments, the memory hub controller may drive the write data flow selector to a first multiplexer and a second multiplexer for selecting either a direct input from a link interface of the memory hub device or an input from a write data queue of the memory hub device. In the illustrative embodiments, the first multiplexer may provide an output to the first memory device data interface and the second multiplexer may provide an output to the second memory device data interface. 
     In the illustrative embodiments, the memory hub controller, in response to a write access request packet, may issue a first memory write access request to the first set of memory devices via the first memory device data interface and substantially immediately issue a second memory write access request to the second set of memory devices via the second memory device data interface. In the illustrative embodiments, the memory hub controller, in response to a read access request packet, may issue a first memory read access request to the first set of memory devices via the first memory device data interface and substantially immediately issue a second memory read access request to the second set of memory devices via the second memory device data interface. 
     In the illustrative embodiments, read data from the first memory read access request and the second memory read access request may be directly transferred to the memory controller. In the illustrative embodiments, read data from the first memory read access request and the second memory read access request may be queued prior to be transferred to the memory controller. 
     In the illustrative embodiments, the memory system may be part of a data processing device. In the illustrative embodiments, the memory system may be a main memory of a data processing device. In the illustrative embodiments, the memory module may be a dual in-line memory module (DIMM) or a single in-line memory module (SIMM). 
     These and other features and advantages of the present invention will be described in, or will become apparent to those of ordinary skill in the art in view of, the following detailed description of the exemplary embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, as well as a preferred mode of use and further objectives and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of an exemplary data processing system in which aspects of the illustrative embodiments may be implemented; 
         FIG. 2  depicts an exemplary synchronous memory module, such as a dual in-line memory module (DIMM); 
         FIG. 3  illustrates an exemplary data processing system coupled to a subsystem of memory modules; 
         FIG. 4  depicts an exemplary block diagram of a memory hub device of a memory module; 
         FIG. 5  depicts a buffered memory module within a memory system that comprises multiple memory device data interfaces in accordance with one illustrative embodiment; 
         FIGS. 6A and 6B  depict the use of a high-frequency, high-bandwidth point-to-point interface or memory channel within a memory system that generally connects to another memory module as an independent memory channel onto the memory module in accordance with an illustrative embodiment; 
         FIG. 7  depicts an exemplary memory device stack configuration in accordance with an illustrative embodiment; and 
         FIGS. 8 and 9  depict memory module stacking implementations within a memory system in accordance with the illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     The illustrative embodiments provide mechanisms for enhancing the memory bandwidth available through a buffered memory module. As such, the mechanisms of the illustrative embodiments may be used with any of a number of different types of data processing devices and environments. For example, the memory system of the illustrative embodiments may be utilized with data processing devices such as servers, client data processing systems, stand-alone data processing systems, or any other type of data processing device. Moreover, the memory systems of the illustrative embodiments may be used in other electronic devices in which memories are utilized including printers, facsimile machines, storage devices, flashdrives, or any other electronic device in which a memory is utilized. In order to provide a context for the description of the mechanisms of the illustrative embodiments, and one example of a device in which the illustrative embodiments may be implemented,  FIG. 1  is provided hereafter as an exemplary diagram of data processing environment in which embodiments of the present invention may be implemented. It should be appreciated that  FIG. 1  is only exemplary and is not intended to assert or imply any limitation with regard to the environments in which aspects or embodiments of the present invention may be implemented. Many modifications to the depicted environments may be made without departing from the spirit and scope of the present invention. 
     With reference now to  FIG. 1 , a block diagram of an exemplary data processing system is shown in which aspects of the illustrative embodiments may be implemented. Data processing system  100  is an example of a computer in which computer usable code or instructions implementing the processes for illustrative embodiments of the present invention may be located. 
     In the depicted example, data processing system  100  employs a hub architecture including north bridge and memory controller hub (NB/MCH)  102  and south bridge and input/output (I/O) controller hub (SB/ICH)  104 . Processing unit  106 , main memory  108 , and graphics processor  110  are connected to NB/MCH  102 . Graphics processor  110  may be connected to NB/MCH  102  through an accelerated graphics port (AGP). 
     In the depicted example, local area network (LAN) adapter  112  connects to SB/ICH  104 . Audio adapter  116 , keyboard and mouse adapter  120 , modem  122 , read only memory (ROM)  124 , hard disk drive (HDD)  126 , CD-ROM drive  130 , universal serial bus (USB) ports and other communication ports  132 , and PCI/PCIe devices  134  connect to SB/ICH  104  through bus  138  and bus  140 . PCI/PCIe devices may include, for example, Ethernet adapters, add-in cards, and PC cards for notebook computers. PCI uses a card bus controller, while PCIe does not. ROM  124  may be, for example, a flash binary input/output system (BIOS). 
     HDD  126  and CD-ROM drive  130  connect to SB/ICH  104  through bus  140 . HDD  126  and CD-ROM drive  130  may use, for example, an integrated drive electronics (IDE) or serial advanced technology attachment (SATA) interface. Super I/O (SIO) device  136  may be connected to SB/ICH  104 . 
     An operating system runs on processing unit  106 . The operating system coordinates and provides control of various components within the data processing system  100  in  FIG. 1 . As a client, the operating system may be a commercially available operating system such as Microsoft® Windows® XP (Microsoft and Windows are trademarks of Microsoft Corporation in the United States, other countries, or both). An object-oriented programming system, such as the Java™ programming system, may run in conjunction with the operating system and provides calls to the operating system from Java™ programs or applications executing on data processing system  100  (Java is a trademark of Sun Microsystems, Inc. in the United States, other countries, or both). 
     As a server, data processing system  100  may be, for example, an IBM® eServer™ System p™ computer system, running the Advanced Interactive Executive (AIX™) operating system or the LINUX® operating system (eServer, System p, and AIX are trademarks of International Business Machines Corporation in the United States, other countries, or both while LINUX is a trademark of Linus Torvalds in the United States, other countries, or both). Data processing system  100  may be a symmetric multiprocessor (SMP) system including a plurality of processors in processing unit  106 . Alternatively, a single processor system may be employed. 
     Instructions for the operating system, the object-oriented programming system, and applications or programs are located on storage devices, such as HDD  126 , and may be loaded into main memory  108  for execution by processing unit  106 . The processes for illustrative embodiments of the present invention may be performed by processing unit  106  using computer usable program code, which may be located in a memory such as, for example, main memory  108 , ROM  124 , or in one or more peripheral devices  126  and  130 , for example. 
     A bus system, such as bus  138  or bus  140  as shown in  FIG. 1 , may be comprised of one or more buses. Of course, the bus system may be implemented using any type of communication fabric or architecture that provides for a transfer of data between different components or devices attached to the fabric or architecture. A communication unit, such as modem  122  or network adapter  112  of  FIG. 1 , may include one or more devices used to transmit and receive data. A memory may be, for example, main memory  108 , ROM  124 , or a cache such as found in NB/MCH  102  in  FIG. 1 . 
     Those of ordinary skill in the art will appreciate that the hardware in  FIG. 1  may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash memory, equivalent non-volatile memory, or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in  FIG. 1 . Also, the processes of the illustrative embodiments may be applied to a multiprocessor data processing system, other than the SMP system mentioned previously, without departing from the spirit and scope of the present invention. 
     Moreover, the data processing system  100  may take the form of any of a number of different data processing systems including client computing devices, server computing devices, a tablet computer, laptop computer, telephone or other communication device, a personal digital assistant (PDA), or the like. In some illustrative examples, data processing system  100  may be a portable computing device which is configured with flash memory to provide non-volatile memory for storing operating system files and/or user-generated data, for example. In other illustrative embodiments, data processing device  100  may be any type of digital commercial product that utilizes a memory system in accordance with the illustrative embodiments, as discussed hereafter. For example, data processing device  100  may be a printer, facsimile machine, flash memory device, wireless communication device, game system, portable video/music player, or any other type of consumer electronic device. Essentially, data processing system  100  may be any known or later developed data processing system without architectural limitation. 
     Furthermore, data processing device  100  may employ many different types of memory for main memory  108 . In some illustrative embodiments, main memory  108  may be a memory module, such as a dual in-line memory module (DIMM), single in-line memory module (SIMM), or other memory module or card structure. In general, a DIMM refers to a small circuit board or substrate that is comprised primarily of random access memory (RAM) integrated circuits, or dies, on one or both sides, i.e. planar surfaces, of the circuit board/substrate with signal and/or power pins along both sides of a common edge of the circuit board/substrate. A SIMM refers to a small circuit board or substrate composed primarily of RAM integrated circuits, or dies, on one or both sides, i.e. planar surfaces, of the circuit board/substrate and pins generally along both long edges, with each pin connected to the pin directly (or slightly offset from the pin) on the adjacent side. 
     As mentioned above, main memory  108  may be accessed by NB/MCH  102  using a high-frequency, high-bandwidth point-to-point interface or other known interfaces such as multi-drop. The interface on the memory module however is limited to the lower-bandwidth multi-drop eight-byte interface to the memory devices of the contemporary memory module. Thus, the illustrative embodiments provide mechanisms for enhancing the memory bandwidth available through a memory module. While the preferred embodiment is directed to a DIMM, the mechanisms described in the illustrative embodiment may be used with other memories, such as a SIMM, a memory card, a QUIMM (Quad inline memory module), or other carrier or assembly having electrical and dimensional attributes optimally suited for a given system environment. 
     In order to increase the memory bandwidth through a memory module, the illustrative embodiments implement multiple memory device data interfaces in a memory hub device of a memory module that interfaces between a memory controller of a processor and memory devices on the memory module. Providing multiple memory device data interfaces on the memory hub device results in a more even match between the bandwidth on a memory channel coupled to the memory module and the bandwidth of the memory device data interface of a single memory module. Additionally, the multiple memory device data interfaces on the memory hub device also double the storage capacity of the memory module. 
     The illustrative embodiment also provide for using a high-frequency, high-bandwidth point-to-point interface or memory channel that generally connects a memory hub device to another memory module in a daisy-chain configuration as an independent memory channel onto the memory module. Another illustrative embodiment provides a memory module stacking implementation that pins out data buses of two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access. A pin out is a term used in electronics to describe how an electrical connector, such as a memory module, is wired. An electrical connector typically consists of several electrical contacts or pins that can be used to carry electrical power or signals. Due to the wide variety of applications and manufacturers, a wide selection of electrical connectors exists with different types and numbers of contacts or pins. The pin out of an electrical connector identifies each individual pin. Proper identification of pins ensures that the signals and power are transmitted across the electrical connectors. 
       FIG. 2  depicts an exemplary memory module, such as a dual in-line memory module (DIMM). Memory module  200  depicted in  FIG. 2  may be part of main memory in a data processing device or system, such as main memory  108  in data processing system  100  of  FIG. 1 . Memory module  200  depicts a front planar side and a back planar side of a DIMM design for nine synchronous dynamic random access memory (SDRAM) chips  202 , which may also be referred to as memory devices. In the depiction of  FIG. 2 , the backside view of the DIMM (top of drawing) may be rotated down such that the notches, or keys, on the edges are aligned with the notches, or keys, on the edges of the front side view of the DIMM (bottom of drawing). 
     In the depicted example, SDRAM chips  202  are arranged on the front and back sides of printed circuit board  204  with corresponding buffer  206  centrally disposed on each side. Thus, SDRAM chips  202  may be referred to as being disposed on a right side and a left side, relative to buffer  206 , of the front side and on a right side and a left side, relative to buffer  206 , of the back side. When viewed as an assembled memory module, connector pins  208  on the front side of printed circuit board  204  are disposed along a common edge with connector pins  210  on the back side of printed circuit board  204 . 
     Keys  212  provide a positive mechanical interlock for systems solely supporting DRAM or SDRAM. In the exemplary embodiment, systems supporting both DRAM and SDRAM would have no connector key in this position. A side edge key may be used to inform the controller of the type of memory technology employed, e.g., flash write, EPROM, etc. or in other embodiments, may be used to identify operating voltage or other operational features for which a mechanical means is optimal to prevent system or module damage. Memory module  200  may be coupled to a memory controller of a data processing system, which controls the reading and writing of data from and to memory module  200 . The DIMM depicted in  FIG. 2  includes 168 pins in the exemplary illustration, whereas subsequent DIMMs may be constructed with pincounts ranging from 100 pins to over 300 pins, and in alternate exemplary embodiments, pins may be placed on more than one edge to permit interconnection to alternate interfaces (e.g. test, diagnostic, characterization, add-on memory/extended memory, etc). 
       FIG. 3  illustrates an exemplary data processing system coupled to a subsystem of memory modules. Data processing system  300  includes processor  302 , with memory controller  304  and cache  306  integrated thereon, and one or more memory modules  308 , such as memory module  200  of  FIG. 2 . Each of the memory modules  308  may include a memory hub device  310  connected to one or more memory devices  312 . Each of memory modules  308  connects via bus structures  314  or memory channels that are connected to processor  302  through a cascade interconnect bus structure, which may also be referred to as a hub-and-spoke topology. Memory controller  304  is interconnected to memory hub devices  310  of the memory modules  308  via one or more memory channels  314 . Memory hub devices  310  may also be interconnected to other memory hub devices  330  of other memory modules  340  in an nth group of DIMMs  320  or to a standalone repeater hub device using memory channel  315 . 
     Each memory hub device  310  and  330  provides one or more low speed connection(s) to groups of memory devices  312  following, for example, the fully buffered DIMM standard. The connections to the memory devices may include both common and independent signals to the one or more memory devices, with the signals comprising one or more of data, address, command, control, status, reset, and other signals present in contemporary or future memory devices. Multiple identically configured memory modules  308  are logically grouped together into module groups  318  and  320 , and may be operated on in unison or with a subset of the modules selected based on the commands issued by memory controller  304  to provide for optimal latency, bandwidth, and error correction effectiveness for system memory cache line transfer, diagnostics, and other communication modes to the memory storage. 
     In the exemplary embodiment, memory controller  304  translates system requests for memory access into packets according to a memory hub device communication protocol. Typically, memory write packets contain at least a command, address, and associated data. Memory read packets typically contain at least a command and address, and imply that an expected packet will be returned which contains the requested data and/or information related to the read request. Memory controller  304  sends the memory write packets and memory read packets to memory hub device  310  of a memory module  308 . Memory hub device  310  routes the packets to a corresponding memory device  312  associated with memory hub device  310  or another memory hub device  330  of another memory module  340 , or a standalone repeater hub device. The details of how memory hub device  310  may route the packets in this manner will be provided with reference to  FIG. 4  hereafter. 
       FIG. 4  depicts an exemplary block diagram of a memory hub device of a memory module. Memory hub device  402 , such as memory hub device  310  of  FIG. 3 , may be connected to a memory controller (not shown), such as memory controller  304  of  FIG. 3 , through memory channel  408 , which may be a multi-drop bus structure, point-to-point bus structure, or the like, that may further include a cascade connection to one or more additional memory hub devices or standalone repeater hub device. In the exemplary embodiment, memory channel  408  is a high bandwidth bus structure on which memory access requests are transmitted and received by the memory controller through the memory channel to and from memory hub device  402 . 
     Exemplary memory hub device  402  comprises link interface  404  that receives high-speed memory access requests from an upstream or downstream memory hub device (not shown) or from a memory controller (not shown) via memory channel  408  or  409 . Link interface  404  also provides the means to re-synchronize, translate, and re-drive high-speed memory access requests to memory devices  406  and/or to re-drive the high-speed memory access requests downstream or upstream on memory channel  409  as applicable using known memory system communication protocols. Link interface  404  may also receive read data packets from a downstream or upstream memory hub device (not shown) on memory channel  409 . Link interface  404  may select between the read data packets from the downstream or upstream memory hub device and the data from memory devices  406  internal to memory hub device  402  using known memory system communication protocols, and then send the data upstream or downstream on memory channel  408 . 
     Memory hub controller  414  responds to access request packets, i.e. write packets and read packets, by responsively driving memory devices  406  using memory device address and control bus  416 . Memory hub controller  414  also controls data flow by directing read data flow selector  418  and write data flow selector  420 . Link interface  404  decodes the data packets received from the memory controller and directs the address and command information to memory hub controller  414 . Memory write data from link interface  404  may be temporarily stored in write data queue  422  before being provided to multiplexer  440 . Alternatively, the memory write data may be directly driven to multiplexer  440  via internal bus  424 . Memory hub controller  414  uses the address of the write data and control information from the write packet to control write data flow selector  420  and, thus, multiplexer  440  such that multiplexer  440  sends the memory write data from write data queue  422 , where the address specific write data may be stored, or internal bus  424  if the address specific write data is sent directly from link interface  404 . The memory write data may then be sent via internal bus  426  to memory device data interface  410 . Memory device data interface  410  then sends the memory write data to memory devices  406  via memory device data bus  412 . While all of memory devices  406  receive the write data, only the memory device having the address of the write data actually stores the write data. In the exemplary embodiments, memory device data interface  410  is an eight-byte data interface that manages the technology-specific data interface with memory devices  406 , and further controls the bi-directional memory device data bus  412 . However, memory device data interface  410  may be comprised of more or less bytes based on the application requirements, alternate reliability structures (requiring more or less data bits), mechanical (and other) limitations or the like. 
     As an example of the command flow for a write command, when the memory controller, such as memory controller  304  of  FIG. 3 , issues a write command to memory devices  406  on memory hub device  402 , the memory controller will transmit both a write command and write data to memory hub device  402  via memory channel  408 . Link interface  404  decodes the address information associated with the write data and, if the write data is targeted to memory devices  406 , link interface  404  moves the write data to a buffer in write data queue  422 . The selection of a buffer may be determined in many ways, such as a first in first out queuing method, a buffer implicitly defined in the write command, or other buffer management implementation. Memory hub device  402  generally stores the write data in write data queue  422  prior to the write command being issued, but, depending on the protocol of memory devices  406  and memory channel  408 , some or all of the write data may be transferred directly from link interface  404  to memory device data interface  410  via multiplexer  440  under control of memory hub controller  414  and write data flow selector  420 . Memory hub controller  414  uses the address of the write data and write command to control write data flow selector  420  and, thus, multiplexer  440  so that multiplexer  440  sends the memory write data from write data queue  422 , where the address specific write data may be stored, or internal bus  424  if the address specific write data is sent directly from link interface  404 . 
     After the write data has been transferred, the memory controller will issue a write command to link interface  404  on memory channel  408 . Control logic in link interface  404  will in parallel forward the write command to downstream memory hub devices on memory channel  409  and further decode the write command to determine if the write command is targeted at memory devices  406  attached to memory hub device  402 . If the write command is targeted for memory devices  406 , link interface  404  forwards the write command to memory hub controller  414  to be executed via internal bus  435 . Memory hub controller  414  converts the write command into the correct protocols for memory devices  406  installed on memory module. Memory hub controller  414  sends the write command to memory devices  406  over memory device address and control bus  416 . While all of memory devices  406  receive the write data command, only the memory device with the address of the write data actually executes the write command. If the write data is stored in write data queue  422 , memory hub controller  414  transfers, at an appropriate time, the write data from write data queue  422  to memory device data interface  410  using write data flow selector  420 . Memory device data interface  410  forwards the write data to memory devices  406  on memory device data bus  412 . 
     Memory read data may also be provided from memory devices  406  to memory device data interface  410  via memory device data bus  412 . Memory device data interface  410  may provide the memory read data to multiplexer  450  directly via internal bus  430  or indirectly via read data queue  428  and internal bus  430 . Multiplexer  450  outputs data to link interface  404  using read data flow selector  418  under control of memory hub controller  414 . Memory hub controller  414  uses the address of the read data to control read data flow selector  418  and, thus, multiplexer  450  so that multiplexer  450  sends memory read data from read data queue  428 , where the address specific read data may be stored, or internal bus  430  if the address specific read data is to be sent directly to link interface  404 . Link interface  404  may then transmit the memory read data upstream on memory channel  408  to a memory controller in a processor as one or more read reply packet(s). 
     An example of the command flow for a read command, when memory hub device  402  receives a read command on memory channel  408 , control logic in link interface  404  will in parallel forward this read command to any downstream memory hub device on memory channel  409 , and further decode the read command to determine if the read command is targeted at memory device  406  attached to memory hub device  402 . If link interface  404  determines that the read command is targeted for memory hub device  402 , link interface  404  forwards the read command using internal bus  435  to memory hub controller  414  to be executed. Memory hub controller  414  converts the read command into the correct protocols for memory devices  406  installed on the memory module. Memory hub controller  414  then sends the read command to memory devices  406  over memory device address and control bus  416 . While all of memory devices  406  receive the read data command, only the memory device with the address of the read data actually executes the read command and sends the read data to memory device data interface  410 . Memory devices  406  execute the read command and transfer a read data packet to memory device data interface  410  over memory device data bus  412 . 
     Under control of memory hub controller  414 , memory device data interface  410  transfers the read data packet to either read data queue  428  or directly to link interface  404  to be transferred back to the memory controller using memory channel  408 . Memory hub controller  414  uses the address of the read data to control read data flow selector  418  and, thus, multiplexer  450  so that multiplexer  450  sends the memory read data from read data queue  428 , where the address specific read data may be stored, or internal bus  430  if the address specific read data is to be sent directly to link interface  404 . If the read data is stored in read data queue  428 , memory hub controller  414  will decide when to move the stored data to link interface  404  depending on the state of read data queue  428  and the state of link interface  404 . If there is already data in read data queue  428  pending transfer to link interface  404 , then memory hub controller  414  directs the new read data to read data queue  428 . Memory hub controller  414  directs data out of read data queue  428  in a first in, first out manner. Additionally, if link interface  404  is busy moving data from memory channel  409 , then memory hub controller  414  delays the transfer of read data until there is an opening on memory channel  408 . Any known method may be used to manage read data queue  428 . 
     The illustrative embodiments provide mechanisms for enhancing the memory bandwidth available through a buffered memory module. One illustrative embodiment provides multiple memory device data interfaces in a memory hub device of a memory module that interfaces between a memory controller of a processor and memory devices on the memory module. Another illustrative embodiment provides for using a high-frequency, high-bandwidth point-to-point interface or memory channel that generally connects a memory hub device to another memory module in a daisy-chain configuration as an independent memory channel onto the memory module. A further illustrative embodiment provides a memory module stacking implementation that pins out data buses of two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access. 
       FIG. 5  depicts a buffered memory module within a memory system that comprises multiple memory device data interfaces in accordance with one illustrative embodiment. In order to increase the bandwidth from a memory hub device to the memory devices that are coupled to the memory hub, the illustrative embodiments implement a second eight-byte data interface in the memory hub device of the memory module. 
     With reference to  FIG. 5 , exemplary memory hub device  502  includes, in addition to the elements particular to the illustrative embodiments, elements that are similar to elements depicted in memory hub device  402  of  FIG. 4 . Thus, elements in  FIG. 5  that are not specifically described as operating differently from elements in  FIG. 4  are intended to operate in a similar manner as their corresponding elements in  FIG. 4 . For example, memory hub device  502  includes link interface  504 , memory devices  506 , and memory channels  508  and  509 , each of which operate in a similar manner to that described with the corresponding elements in  FIG. 4 . However, in this implementation, the single memory device data interface  410  of  FIG. 4  is replaced with memory device data interface  510  and memory device data interface  511 . This is a significant and innovative addition that enables dramatic performance benefits on a single module, by better matching the slower memory device data buses  512  and  513  with the high speed communications capability of memory channel  508 . In addition, multiple operations can be initiated nearly in parallel (or in parallel, given a modified command structure), thereby dramatically increasing system performance, with minimal memory subsystem power increases (as compared to the use of multiple memory modules/channels, etc). 
     Also in  FIG. 5 , memory devices  506  are either divided in half, with one half of memory devices  506  coupled to memory device data interface  510  using bi-directional memory device data bus  512  and the other half of memory devices  506  are coupled to memory device data interface  511  using bi-directional memory device data bus  513 , or a second set of memory devices  506  is added to the memory module and connected to the new memory device data interface  511 . Memory device data interface  510  and memory device data interface  511  each manage the technology-specific data interface with their portion of memory devices  506  and control their respective one of bi-directional memory device data bus  512  or bi-directional memory device data bus  513 . Memory hub controller  514  responds to access request packets by responsively driving memory devices  506  using memory device address and control buses  516  or  517 . Memory hub controller  514  also controls data flow by directing read data flow selector  518  and write data flow selectors  520  and  521 . Memory hub controller  514  uses the address and control information of the read or write data to control read data flow selector  518  and write data flow selectors  520  and  521  and, thus, multiplexers  550 ,  540  and  541 , respectively. 
     Further, using write data flow selector  520 , memory hub controller  514  may send write data via internal bus  526  to memory device data interface  510  and onto memory devices  506  using bi-directional memory device data bus  512 , or using write data flow selector  521 , memory hub controller  514  may send write data via internal bus  527  to memory device data interface  511  and onto memory devices  506  using bi-directional memory device data bus  513 . By providing memory device data interfaces  510  and  511 , memory hub controller  514  may send write data requests at a faster rate, as opposed to known systems where one write access request had to finish prior to a second write access request being issued. That is, while memory device data interface  510  handles one write data request, memory device data interface  511  may simultaneously handle another write data request. 
     For example, memory controller  532  may send two write requests on memory channel  508  to link interface  504  using a single command transfer (generally over multiple clock cycles) comprising multiple commands/data or multiple transfers (e.g. back-to-back commands and data depending on the command structure of a given structure/application). Link interface  504  decodes the write requests and directs the address and command information to memory hub controller  514  via internal bus  535 . Memory hub controller  514  uses the address of the write data to send control signals to multiplexer  540  or  541  using write data flow selectors  520  or  521  to select the correct data for the write command. Memory device data interfaces  510  and  511  receive the write data from multiplexers  540  and  541  via internal buses  526  and  527  and forward the write data to memory devices  506  across memory device data buses  512  and  513 . Memory hub controller  514  also sends control signals to memory devices  506  across memory device address and control bus  516  or  517 , depending on the address of the write data, to execute the write commands. While all of memory devices  506 , which are coupled to the specific memory device address and control bus  516  or  517  on which memory hub controller  514  sent the write command, receive the write data command, only the memory device with the address of the write data actually executes the write command. Thus, while known systems would execute two write commands in series using only memory device data bus  512 , the illustrative embodiments execute in parallel using memory device data buses  512  and  513 . 
     Similarly, by providing exemplary memory device data interfaces  510  and  511 , read data requests may be sent at a faster rate as well, as opposed to previous implementations where only one read request could be issued at a time. Memory read data from memory devices  506  through memory device data interface  510  or memory device data interface  511  may be queued in the read data queue  528  or directly transferred to link interface  504  via internal bus  530  using multiplexer  550  and read data flow selector  518  under the controller of memory hub controller  514  based on the address of the read data, to be transmitted upstream on memory channel  508  to memory controller  532  in a processor as a read reply packet. Logic  534  in memory controller  532  will schedule the number of read operations that are issued to ensure that read data queue  528  in memory hub device  502  is not overrun with data. In the exemplary embodiment, memory controller  532  manages the number of read operations by tracking the number of read operations sent to memory hub device  502  and the number of read data packets that it has received from memory hub device  502 . Given a finite number of read data queue locations, memory controller  532  limits the number of outstanding read operations, which are read operations that memory controller  532  has not received read data for, so that there is always space in read data queue  528  for the read operations in flight. 
     For example, memory controller  532  may send two read access requests on the memory channel  508  to link interface  504  using a single command transfer (generally over multiple clock cycles) comprising multiple commands/data or multiple transfers (e.g. back-to-back commands and data depending on the command structure of a given structure/application). Link interface  504  decodes and the read access requests and sends the address and command information to memory hub controller  514 . Memory hub controller  514  generates control signals for memory devices  506  and sends the control signals to memory devices  506  across memory device address and control buses  516  and  517 , depending on the address of the read data. While all memory devices  506 , which are coupled to the specific memory device address and control bus  516  or  517  on which memory hub controller  514  sends the read command, receive the read data command, only the memory device with the address of the read data actually executes the read command and sends the read data to memory device data interface  510 . To complete the read access request, memory devices  506  send the requested read data to memory device data interfaces  510  and  511  at either a predictable or unpredictable time, based on the memory hub device and system structure, which may include a tagging of data or some other method to permit operability with unpredictable access times. Memory hub controller  514  generates control signals using read data flow selector  518  based on the address of the read data to control multiplexer  550  thereby selecting read data to be read from read data queue  528  or directly from one of memory device data interfaces  510  or  511 . Link interface  504  receives the read data from multiplexer  550  via internal bus  530  and sends the read data to memory controller  532  over memory channel  508 . Since, in the exemplary embodiment, memory channel  508  may only receive read data from either memory device data interface  510  or  511  at one time, memory hub controller  514  uses read data flow selector  518  to control multiplexer  550  thereby selecting read data from one of the memory device data interfaces to send directly to memory channel  508  and the read data from the other memory device data interface may be temporarily stored in read data queue  528 . Once the read data from the first memory device data interfaces has been completely transmitted across memory channel  508 , memory hub controller  514  uses read data flow selector  518  to control multiplexer  550  thereby selecting the read data from read data queue  528  to be transmitted across memory channel  508  to memory controller  532 . 
     By providing memory device data interfaces  510  and  511 , read data requests and write data requests may be sent to memory hub device  502  at a faster rate, as opposed to previous implementations where only one data request, read or write, could be issued at a time with delay between subsequent accesses in response to the limited memory bandwidth due to the single memory device data interface  510 . In addition to the operations listed above, other cases exist such as a memory read request may be issued to memory hub device  502  that targets one of memory device data interfaces  510  or  511  and at the same time a memory write request is issued to memory hub device  502  that targets the other one of memory device data interfaces  510  or  511 . The read and write commands will execute as described above with memory hub controller  514  issuing commands to one memory device data interface to execute a read operation and commands to the second memory device data interface to execute a write operation. The dual memory device data interfaces provide for any combination of two operations to be executed in parallel verses in series in known memory hub devices. 
     The wider interface provided by bi-directional memory device data buses  512  and  513  results in a more even match between the bandwidth on memory channel  508  and the bandwidth on memory device data interfaces  510  and  511  of a single memory module. The memory module fits in the same socket as a conventional buffered memory module and provides up to two times the available bandwidth in the same physical space in the system than that of a conventional buffered memory module due to the addition of one or more of memory device data interfaces  511  and bi-directional memory device data buses  513 , memory device address and control buses  517 , etc. Additionally, because memory device data interface  511  and bi-directional memory device data bus  513  are in the same memory hub device with memory device data interface  510  and bi-directional memory device data bus  512 , additional latency impacts that are experienced in a memory hub device with a single memory device data interface may be reduced when accessing the memory module. That is, since the bandwidth of memory hub device  502  has been increased by adding memory device data interface  511  and bi-directional memory device data bus  513 , the mismatch in bandwidth of memory hub device  502  to the bandwidth of the memory channel has decreased, thereby increasing the rate at which memory hub device  502  may process memory access requests. 
     Furthermore, with the addition of memory device data interface  511  on memory hub device  502 , the storage capacity of the memory module within a memory system may be doubled while maintaining the increased bandwidth described earlier. That is, with a single memory device data interface as described in  FIG. 4 , the total memory module capacity is determined by the technology and number of memory devices  506  and the desired frequency of operation. Given these parameters only a set number of memory devices  506  may be installed on a given memory module, thus, setting the maximum capacity of that memory module. With the addition of memory device data interface  511  and bi-directional memory device data bus  513 , the memory module may support two times the number of memory devices  506  given the same guidelines used in the industry standard memory module. For example, a conventional memory module that includes a single memory device data interface may support 9, 18, 36, 72, or some other quantity of memory devices, as applicable to a system environment. By providing a second memory device data interface, the illustrative embodiments provide capacity for doubling the conventional 9 memory devices to 18 memory devices, the conventional 18 memory devices to 36 memory devices, the conventional 36 memory devices to 72 memory devices, the conventional 72 memory devices to 144 memory devices, and so on. 
     Additionally, memory hub device  502  may support both a compatibility mode and an enhanced bandwidth mode. In the compatibility mode, memory device data interface  511  and the coupled ones of memory devices  506  appear to memory controller  532  as a second memory module coupled to memory hub device  502 . In the compatibility mode, bi-directional memory device data buses  512  and  513  run as independent data buses as they would if they were on two independent memory modules. For example, in the compatibility mode, link interface  504  decodes read commands for memory hub device  502  and read commands for a second memory hub device that would have been attached on memory channel  509 . Commands that would have been targeted for memory hub device  502  are directed to memory device data interface  510  and commands targeted for the second memory hub device in the daisy-chain are directed to memory device data interface  511 . Memory hub controller  514  manages the data flow on the read commands through read data queue  528  to schedule the returning read data to memory controller  532  so that it would appear like there are two memory hub devices in the system instead of a single memory hub device. This allows current memory controller designs to use this memory module and get the advantages of the added bandwidth per memory module socket, the lower latency of a single memory hub device, and the added capacity in a conventional memory module slot. The memory controller would be informed, generally at power-up, of the structure defined above, and as such, would be aware of the reduced latency when accessing the “second” memory hub (which is not integrated on the first memory hub. 
     In the enhanced mode, memory hub device  502  may run memory device data interface  510  and memory device data interface  511  as two independent eight-byte memory ports, as described above, or a single sixteen-byte memory port. When running as a single sixteen-byte memory port, data read from memory devices  506  through memory device data interface  510  and memory device data interface  511  is buffered using read data queue  528 , which collects the data and feeds it to memory channel  508  through link interface  504 . Read data queue  528  is necessary since the read bandwidth from the sixteen-byte memory port interface may be higher than the read bandwidth on memory channel  508 . For example, in the enhanced mode, memory hub controller  514  stores read data from both memory device data interfaces  510  and  511  into read data queue  528 . Memory hub controller  514  transmits the stored read data from read data queue  528  to link interface  504  at a rate that link interface  504  may accept. The read data stored in read data queue  528  will be sent to link interface  504  in a first in, first out (FIFO) manner. Thus, memory hub controller  514  sends 16 bytes of read data on a first beat of the read access request to memory controller  532  across memory channel  508 , followed by a second 16 bytes of read data on a second beat, continuing in this manner until all the read data is transferred. The enhanced mode allows memory controller  532  to better manage the resource on the memory module for higher overall data bandwidth actively using both memory device data interfaces on every command. 
     Thus, these illustrative embodiments provide a better match between the bandwidth of memory channel  508  and the bandwidth provided by memory device data interface  510  and memory device data interface  511 . Additionally, by providing memory device data interface  510  and memory device data interface  511  on a single memory module, the illustrative embodiments provide for doubling the storage capacity of the memory module by allowing the number of memory devices on the memory module to double. 
     A memory module employing memory hub device  502  of  FIG. 5  provides for an interface to a memory controller and an interface to another memory hub device while interfacing to double the number of memory devices. However, it is possible to provide one or more memory hub devices that quadruple the number of memory devices of a single memory module by using the interface normally connected to another memory hub device in another manner.  FIGS. 6A and 6B  depict the use of exemplary high-frequency, high-bandwidth point-to-point interface or memory channel within a memory system that generally connects to another memory module as an independent memory channel onto the memory module in accordance with an illustrative embodiment. 
     Turning now to  FIG. 6A , memory interface  600  comprises memory module  602  that includes memory hub devices  604  and  606 , such as memory hub device  502  of  FIG. 5 . In memory module  602 , each of memory hub devices  604  and  606  are coupled to memory devices  608  using memory device data interfaces  609  in a similar manner to that described in  FIG. 5 . Memory hub devices  604  and  606  operate in a similar manner to that described in  FIG. 5 . That is, memory hub devices  604  and  606  respond to access requests from memory controller  610  by writing data to and reading data from memory devices  608 . 
     However, in this illustrative embodiment, instead of link interface  605  of memory hub device  604  being interconnected to memory controller  610  via upstream independent memory channel  612  as well as interconnected to another memory hub device via downstream independent memory channel  614 , link interface  605  of memory hub device  604  interconnects only to memory controller  610  using only independent memory channel  612 . The illustrative embodiments reassign independent memory channel  614 , such that downstream memory channel  509  of  FIG. 5 , does not connect to memory hub device  606 . Rather, memory hub device  606  connects to independent memory channel  614 . Thus, memory hub devices  604  and  606  are each independently coupled to memory controller  610  via independent memory channels  612  and  614 . By interconnecting memory module  602  with independent memory channels  612  and  614 , a conventional memory module socket may provide up to four times the bandwidth and up to four times the memory capacity with a fully equipped memory module, which allows for extremely dense and high performance computer systems. 
     While exemplary memory interface  600  depicts interconnecting memory hub devices  604  and  606 , such as memory hub device  502  described in  FIG. 5 , the illustrative embodiments also anticipate other designs of a memory hub device employing two independent memory channels and four memory device data interfaces such as that shown in  FIG. 6B . In  FIG. 6B , exemplary memory module  620  is depicted using memory hub device  622 , which include two independent memory channels  624  and  626  coupled to memory controller  628  via link interfaces  629  and four memory device data interfaces  630  each independently coupled to a set of memory devices  632 . A first memory channel  624  may be coupled to a first set of two of the four memory device data interfaces  630  while a second memory channel  626  may be coupled to a second set of two of the four memory device data interfaces  630 . In this way, the bandwidth to memory devices  632  may be multiplied by up to four relative to a memory module having a memory hub device with only one memory device data interface, such as shown in  FIG. 4 . Moreover up to four times the number of memory devices may be included in the memory module when compared to the memory module in  FIG. 4 . 
     Thus, by using the high-frequency, high-bandwidth point-to-point interface or memory channel that generally connects to another memory module as an independent memory channel onto the memory module, the illustrative embodiments provide the capability of a single memory module having up to four times the bandwidth and up to four times the memory capacity if fully equipped. 
     Using either the enhanced memory module of  FIG. 5 ,  6 A, or  6 B may require a larger number of independent memory device data sites on the memory module to support the larger number of memory hub device to memory device data buses that are being driven by the memory hub device(s). A data site is the physical space on a memory module where the memory devices are located. For example, with four eight-byte data interfaces, such as memory device data interfaces  609  of  FIG. 6A , a memory module would require physical space or data sites for 72 memory devices. Known memory modules only have data sites for 36 memory devices on a memory module. Thus, for each eight-byte data bus on the memory module, eighteen memory device data sites are required assuming a four-bit wide configuration of the memory device chips. This number of memory device data sites may be reduced in half to nine memory device data sites if an eight-bit wide memory device configuration is used. However, providing an eight-bit wide memory device data site would reduce the reliability of the memory module for some subset of memory device failures. 
     This reduction in reliability is due to failing mechanisms in a memory device chip where the full memory device chip fails. For a four-bit wide memory device, this results in a four-bit data error to the error correction code in the memory controller. In known systems a four-bit error is generally correctable by the error correction logic so this device failure will not cause a system failure. If the memory device is an eight-bit wide device, the failure will result in an eight-bit data error to the error correction code in the memory controller. In general an eight-bit error is not correctable by standard error correction codes. Although more advanced codes could be designed to correct an eight-bit error they will either require larger data words into the correction logic or more memory devices off the memory module and either of these two options result in additional system costs and complexities. 
     To avoid a reduction of reliability, high-end systems generally use only four-bit wide memory device chips. For the very dense memory module implementations, a total of 72 memory device data sites are required to populate all the memory device data buses. The physical implementation of such a dense memory module implementation may not be possible due to the available height for the memory module. One solution is to configure the memory devices in a stacked configuration. 
       FIG. 7  depicts an exemplary memory device stack configuration in accordance with an illustrative embodiment. In stack configuration  700 , a first layer of memory devices  702  are secured to printed circuit board  704 , then a second layer of memory devices  706  are secured on top of memory devices  702  and secured to printed circuit board  704  forming chip stack  708 . Printed circuit board  704  may be any type of mounting surface or substrate. While there are many memory device stack configurations, such as the exemplary stacking configuration shown in  FIG. 7 , those configurations electrically connect the data buses from the memory devices in the stack together and pin out separate chip select signals for each memory device. The chip select signal is a signal that selects which memory device to access, for example, selecting a chip closest to the substrate (which may be referred to as the bottom chip) or selecting a memory device mounted on top of the bottom chip (which may be referred to as the top chip). Thus, a memory device stack using a four-bit wide memory device will fit in the same footprint as an unstacked four-bit wide memory device and be recognized by a memory controller as two independent memory devices coupled electrically to the same data bus. However, since the data buses from the stacked memory devices are electrically connected together, the stacked memory devices have to be accessed independently using a chip select signal instead of the memory devices having the capability of being accessed in parallel. 
       FIGS. 8 and 9  depict memory module stacking implementations within a memory system in accordance with an illustrative embodiment.  FIG. 8  depicts an exemplary ball-out for two memory devices in a memory device stack that electrically connects the data buses from the two memory devices in the stack together and pins out separate chip selects for each memory device. A ball-out describes all of the electrical connections for coupling a memory device to the substrate of the memory module.  FIG. 9  depicts a memory module stacking implementation that pins out the data buses of two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access. In these illustrative embodiments,  FIGS. 8 and 9  depict an exemplary ball-out for a double-data-rate three (DDR3) four-bit two-high memory device stack. Although, there are numerous pins for the depicted ball-outs, for ease of explanation the illustrative embodiments discuss only the pins that will pin out the data buses of two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access. 
     With regard to  FIG. 8 , for a standard DDR3 four-bit two-high memory device stack, ball-out  800  includes two clock enable signals  802  (labeled CKE 0  and CKE 1 ) to turn the clock to the memory devices on and off, two calibration signals  804  (labeled ZQ 0  and ZQ 1 ) to calibrate the memory devices, two on-die termination signals  806  (labeled ODT 0  and ODT 1 ) to turn the electrical termination to the memory devices on and off, and two chip select signals  808  (labeled CS 0  bar and CS 1  bar) that provide reading from either the top or the bottom memory device of the stacked memory devices. Ball-out  800  also includes data bus lines  810  (labeled DQ 0 , DQ 1 , DQ 2 , and DQ 3 ), collectively referred to as the data bus, to write and read data from the respective one of the stacked memory devices based on the enabled chip select signal  808 , and strobe signals  812  (labeled DQS and DQS bar) which work as a pair to identify when data on data bus lines  810  is valid. Thus, with ball-out  800  the memory devices are required to be accessed independently since the data buses from the stacked memory devices are electrically connected together through data bus lines  810 . 
     With regard to  FIG. 9 , for an improved DDR3 four-bit two-high memory device stack, ball-out  900  includes clock enable signal  902  (labeled CKE) to turn the clock to the memory devices on and off, calibration signal  904  (labeled ZQ) to calibrate the memory devices, on-die termination signal  906  (labeled ODT) to turn the electrical termination to the memory devices on and off, and chip select signal  908  (labeled CS bar) to provide reading the stacked memory devices. Since ball-out  900  electrically connects the chip select signals, as well as the clock enable signals, calibrations signals, and on-die terminations signal, of the two four-bit wide memory devices together for parallel access, the secondary clock enable signal, calibration signal, on-die termination signal, and chip select signal shown in  FIG. 8  are changed to no connects (NC)  909 . Ball-out  900  also includes data bus lines  910  (labeled DQ 0 , DQ 1 , DQ 2 , and DQ 3 ), collectively referred to as data bus  0 , to write and read data from a first memory device of the stacked memory devices. In this implementation, previous no connect (NC) pins are reassigned as data bus lines  911  (labeled DQ 4 , DQ 5 , DQ 6 , and DQ 7 ), collectively referred to as data bus  1 , to write and read data from the second memory device of the stacked memory devices. 
     Ball-out  900  also reassigns strobe signals DQS and DQS bar as strobe signals  912  (labeled DQS 0  and DQS 0  bar) for the first memory device of the stacked memory devices, which work as a pair to identify when data on the data bus  0  is valid. Additionally, ball-out  900  also reassigns previous NC pins as strobe signals  913  (labeled DQS 1  and DQS 1  bar) for the second memory device of the stacked memory devices, which work as a pair to identify when data on the data bus  1  is valid. Thus, with ball-out  900  the memory devices are accessed in parallel using data bus lines  910  and data bus lines  911 , also referred to as data bus  0  and data bus  1  respectively, since the data buses of the stacked memory devices are separately pinned out. 
     Thus, pinning out the data buses of the two stacked four-bit wide memory devices separately while electrically connecting the chip selects signals together results in the two four-bit wide memory devices appearing to a memory controller as a single eight-bit wide memory device that provides two-times the density. Using this configuration provides four eight-byte data buses on a memory module in only 36 memory device data sites and looks like a memory module using eight-bit wide memory device chips but have the reliability of a memory module with 72 memory device data sites using four-bit wide data chips. 
     It should be appreciated that  FIGS. 8-9  are only exemplary and are not intended to assert or imply any limitation with regard to the ways that data buses may be separately connected while access in parallel using commonly connected chip select signals. Many modifications to the depicted connections may be made without departing from the spirit and scope of the present invention. 
     Thus, the illustrative embodiments provide mechanisms for enhancing the memory bandwidth available through a buffered memory module. In one illustrative embodiment, a memory hub device of a memory module provides multiple memory device data interfaces that interface between a memory controller of a processor and memory devices on the memory module. A memory hub device with multiple memory device data interfaces provides a better match between the bandwidth on a memory channel coupled to the memory module and the bandwidth of the memory device data interface of a single memory module. In a second illustrative embodiment, a memory hub device with multiple memory device data interfaces provides for doubling the storage capacity of the memory module. In a third illustrative embodiment, a memory module is provided that provides for two high-frequency, high-bandwidth point-to-point interfaces to a memory controller. In a fourth illustrative embodiment, a memory stacking implementation is provided that pins out the data buses of the two stacked four-bit wide memory devices separately but electrically connects the chip select signals of the two four-bit wide memory devices together for parallel access of the stacked memory devices. 
     The description of the present invention has been presented for purposes of illustration and description, and 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. The embodiment was chosen and described in order to best explain the principles of the invention, 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.