Multi-rank partial width memory modules

[0001] This application is a continuation of U.S. application Ser. No. 13/411,489, filed Mar. 2, 2012, now U.S. Pat. No. 8,446,781, which is a continuation of U.S. patent application Ser. No. 11/939,432, filed Nov. 13, 2007, now U.S. Pat. No. 8,130,560, and claims the benefit of U.S. Provisional Application No. 60/865,623, filed Nov. 13, 2006, the entirety of which are incorporated by reference.

[0002] The present invention relates to memory modules and more particularly to multi-rank memory modules.

[0003] The memory controller of typical computer systems communicates with memory [e.g. dynamic random access memory (DRAM)] circuits that are positioned on dual inline memory modules (DIMMs). A set of signal lines, collectively referred to as a memory bus or memory channel, is used to connect the memory controller to one or more DIMMs. The memory bus typically includes a data bus, an address bus, a control bus, and clock signals. In a parallel memory bus topology, the memory bus connects to a plurality of DIMMs in parallel.

[0004] Each DIMM in a parallel memory bus topology connects to all the data signals, all the address signals, and some or all of the control and clock signals of the memory bus. Thus, the data bus width of a DIMM matches that of the memory bus. However, the number of data I/O signals of a DRAM circuit is usually smaller than the number of data I/O signals of a DIMM. In other words, the data bus width of a DRAM circuit is smaller than the data bus width of a DIMM. Therefore, a plurality of DRAM circuits are operated in parallel to match the data widths of the individual DRAM circuits to that of the DIMM. The plurality of DRAM circuits that are controlled by a common control signal (e.g. a chip select signal) and respond in parallel to an access from the memory controller are referred to as a rank of DRAM circuits. DIMMs with one and two ranks are commonly available.

[0005] In a parallel memory bus topology, the impedance discontinuity and the bus loading caused by a DIMM affects the signal integrity of the memory bus. Consequently, as the frequency of operation of the memory bus increases, the maximum number of DIMMs that can be supported by a memory bus decreases. For example, at a 533 MHz data rate, four DIMMs per channel are feasible. However, it is expected that only two DIMMs per channel may be feasible at a 1333 MHz data rate. Thus, computer system designers are increasingly being forced to choose between memory bandwidth and memory capacity.

[0006] There is thus a need for addressing these and/or other issues associated with the prior art.

[0007] A system is provided for multi-rank, partial width memory modules. A memory controller is provided. Additionally, a memory bus is provided. Further, a memory module with a plurality of ranks of memory circuits is provided, the memory module including a first number of data pins that is less than a second number of data pins of the memory bus.

[0019] FIG. 1A shows a system 170 for multi-rank, partial width memory modules, in accordance with one embodiment. As shown, a memory controller 172 is provided. Additionally, a memory bus 174 is provided. Further, a memory module 176 with a plurality of ranks of memory circuits 178 is provided, the memory module 176 including a first number of data pins that is less than a second number of data pins of the memory bus.

[0020] In the context of the present description, a rank refers to at least one circuit that is controlled by a common control signal. The number of ranks of memory circuits 178 may vary. For example, in one embodiment, the memory module 176 may include at least four ranks of memory circuits 178. In another embodiment, the memory module 176 may include six ranks of memory circuits 178.

[0021] Furthermore, the first number and the second number of data pins may vary. For example, in one embodiment, the first number of data pins may be half of the second number of data pins. In another embodiment, the first number of data pins may be a third of the second number of data pins. Of course, in various embodiments the first number and the second number may be any number of data pins such that the first number of data pins is less than the second number of data pins.

[0022] In the context of the present description, a memory controller refers to any device capable of sending instructions or commands, or otherwise controlling the memory circuits 178. Additionally, in the context of the present description, a memory bus refers to any component, connection, or group of components and/or connections, used to provide electrical communication between a memory module and a memory controller. For example, in various embodiments, the memory bus 174 may include printed circuit board (PCB) transmission lines, module connectors, component packages, sockets, and/or any other components or connections that fit the above definition.

[0023] Furthermore, the memory circuits 178 may include any type of memory device. For example, in one embodiment, the memory circuits 178 may include dynamic random access memory (DRAM). Additionally, in one embodiment, the memory module 176 may include a dual in-line memory module (DIMM).

[0024] Strictly as an option, the system 170 may include at least one buffer chip (not shown) that is in communication with the memory circuits 178 and the memory bus 174.

[0025] In one embodiment, the buffer chip may be utilized to transform data signals associated with the memory bus 174. For example, the data signals may be transformed from a first data rate to a second data rate which is two times the first data rate.

[0026] Additionally, data in the data signals may be transformed from a first data width to a second data width which is half of the first data width. In one embodiment, the data signals may be associated with data transmission lines included in the memory bus 174. In this case, the memory module 176 may be connected only some of a plurality of the data transmission lines corresponding to the memory bus. In another embodiment, the memory module 176 may be configured to connect to all of the data transmission lines corresponding to the memory bus.

[0027] More illustrative information will now be set forth regarding various optional architectures and features with which the foregoing framework may or may not be implemented, per the desires of the user. It should be strongly noted that the following information is set forth for illustrative purposes and should not be construed as limiting in any manner. Any of the following features may be optionally incorporated with or without the exclusion of other features described.

[0028] FIG. 1B illustrates a two-rank registered DIMM (R-DIMM) 100 built with 8-bit wide (x8) memory (e.g. DRAM, etc.) circuits in accordance with Joint Electron Device Engineering Council (JEDEC) specifications. It should be noted that the aforementioned definitions may apply during the present description.

[0029] As shown, included are a register chip 102, and a plurality of DRAM circuits 104 and 106. The DRAM circuits 104 are positioned on one side of the R-DIMM 100 while the DRAM circuits 106 are positioned on the opposite side of the R-DIMM 100. The R-DIMM 100 may be in communication with a memory controller of an electronic host system as shown. In various embodiments, such system may be in the form of a desktop computer, a lap-top computer, a server, a storage system, a networking system, a workstation, a personal digital assistant (PDA), a mobile phone, a television, a computer peripheral (e.g. printer, etc.), a consumer electronics system, a communication system, and/or any other software and/or hardware, for that matter.

[0030] The DRAM circuits 104 belong to a first rank and are controlled by a common first chip select signal 140. The DRAM circuits 106 belong to a second rank and are controlled by a common second chip select signal 150. The memory controller may access the first rank by placing an address and command on the address and control lines 120 and asserting the first chip select signal 140.

[0031] Optionally, data may then be transferred between the memory controller and the DRAM circuits 104 of the first rank over the data signals 130. The data signals 130 represent all the data signals in the memory bus, and the DRAM circuits 104 connect to all of the data signals 130. In this case, the DRAM circuits 104 may provide all the data signals requested by the memory controller during a read operation to the first rank, and accept all the data signals provided by the memory controller during a write operation to the first rank. For example, the memory bus may have 72 data signals, in which case, each rank on a standard R-DIMM may have nine x8 DRAM circuits.

[0032] The memory controller may also access the second rank by placing an address and command on the address and control lines 120 and asserting the second chip select signal 150. Optionally, data may then be transferred between the memory controller and the DRAM circuits 106 of the second rank over the data signals 130. The data signals 130 represent all the data signals in the memory bus, and the DRAM circuits 106 connect to all of the data signals 130. In this case, the DRAM circuits 106 may provide all the data signals requested by the memory controller during a read operation to the second rank, and accept all the data signals provided by the memory controller during a write operation to the second rank.

[0033] FIG. 2 illustrates a two-rank registered DIMM (R-DIMM) 200 built with 4-bit wide (x4) DRAM circuits in accordance with JEDEC specifications. Again, the aforementioned definitions may apply during the present description.

[0034] As shown, included are a register chip 202, and a plurality of DRAM circuits 204A, 204B, 206A, and 206B. The R-DIMM 200 may be in communication with a memory controller of an electronic host system as shown. The DRAM circuits 204A and 204B belong to a first rank and are controlled by a common first chip select signal 240.

[0035] In some embodiments, the DRAM circuits 204A may be positioned on one side of the R-DIMM 200 while the DRAM circuits 204B are positioned on the opposite side of the R-DIMM 200. The DRAM circuits 206A and 206B belong to a second rank and are controlled by a common second chip select signal 250. In some embodiments, the DRAM circuits 206A may be positioned on one side of the R-DIMM 200 while the DRAM circuits 206B are positioned on the opposite side of the R-DIMM 200.

[0036] In various embodiments, the DRAM circuits 204A and 206A may be stacked on top of each other, or placed next to each other on the same side of a DIMM PCB, or placed on opposite sides of the DIMM PCB in a clamshell-type arrangement. Similarly, the DRAM circuits 204B and 206B may be stacked on top of each other, or placed next to each other on the same side of the DIMM PCB, or placed on opposite sides of the board in a clamshell-type arrangement.

[0037] The memory controller may access the first rank by placing an address and command on address and control lines 220 and asserting a first chip select signal 240. Optionally, data may then be transferred between the memory controller and the DRAM circuits 204A and 204B of the first rank over the data signals 230. In this case, the data signals 230 represent all the data signals in the memory bus, and the DRAM circuits 204A and 204B connect to all of the data signals 230.

[0038] The memory controller may also access the second rank by placing an address and command on the address and control lines 220 and asserting a second chip select signal 250. Optionally, data may then be transferred between the memory controller and the DRAM circuits 206A and 206B of the second rank over the data signals 230. In this case, the data signals 230 represent all the data signals in the memory bus, and the DRAM circuits 206A and 206B connect to all of the data signals in the memory bus. For example, if the memory bus has 72 data signals, each rank of a standard R-DIMM will have eighteen x4 DRAM circuits.

[0039] FIG. 3 illustrates an electronic host system 300 that includes a memory controller 350, and two standard R-DIMMs 330 and 340. Additionally, the aforementioned definitions may apply during the present description.

[0040] As shown, a parallel memory bus 310 connects the memory controller 350 to the two standard R-DIMMs 330 and 340, each of which is a two-rank DIMM. The memory bus 310 includes an address bus 312, a control bus 314, a data bus 316, and clock signals 318. All the signals in the address bus 312 and the data bus 316 connect to both of the R-DIMMs 330 and 340 while some, but not all, of the signals in the control bus 314 connect to of the R-DIMMs 330 and 340.

[0041] The control bus 314 includes a plurality of chip select signals. The first two of these signals, 320 and 322, connect to the first R-DIMM 330, while the third and fourth chip select signals, 324 and 326, connect to the second R-DIMM 340. Thus, when the memory controller 350 accesses the first rank of DRAM circuits, it asserts chip select signal 320 and the corresponding DRAM circuits on the R-DIMM 330 respond to the access. Similarly, when the memory controller 350 wishes to access the third rank of DRAM circuits, it asserts chip select signal 324 and the corresponding DRAM circuits on the R-DIMM 340 respond to the access. In other words, each memory access involves DRAM circuits on only one R-DIMM.

[0042] However, both of the R-DIMMs 330 and 340 connect to the data bus 316 in parallel. Thus, any given access involves one source and two loads. For example, when the memory controller 350 writes data to a rank of DRAM circuits on the first R-DIMM 330, both of the R-DIMMs 330 and 340 appear as loads to the memory controller 350. Similarly, when a rank of DRAM circuits on the first R-DIMM 330 return data (e.g. in a read access) to the memory controller 350, both the memory controller 350 and the second R-DIMM 340 appear as loads to the DRAM circuits on the first R-DIMM 330 that are driving the data bus 316. Topologies that involve a source and multiple loads are typically capable of operating at lower speeds than point-to-point topologies that have one source and one load.

[0043] FIG. 4 illustrates a four-rank, half-width R-DIMM 400 built using x4 DRAM circuits, in accordance with one embodiment. As an option, the R-DIMM 400 may be implemented in the context of the details of FIGS. 1-3. Of course, however, the R-DIMM 400 may be implemented in any desired environment. Again, the aforementioned definitions may apply during the present description.

[0044] As shown, included are a register chip 402, and a plurality of DRAM circuits 404, 406, 408, and 410. The DRAM circuits 404 belong to the first rank and are controlled by a common chip select signal 420. Similarly, the DRAM circuits 406 belong to the second rank and are controlled by a chip select signal 430. The DRAM circuits 408 belong to the third rank and are controlled by a chip select signal 440, while the DRAM circuits 410 belong to the fourth rank and are controlled by a chip select signal 450.

[0045] In this case, the DRAM circuits 404, 406, 408, and 410 are all x4 DRAM circuits, and are grouped into nine sets of DRAM circuits. Each set contains one DRAM circuit from each of the four ranks The data pins of the DRAM circuits in a set are connected to each other and to four data pins 470 of the R-DIMM 400. Since there are nine such sets, the R-DIMM 400 may connect to 36 data signals of a memory bus. In the case where a typical memory bus has 72 data signals, the R-DIMM 400 is a half-width DIMM with four ranks of DRAM circuits.

[0046] FIG. 5 illustrates a six-rank, one-third width R-DIMM 500 built using x8 DRAM circuits, in accordance with another embodiment. As an option, the R-DIMM 500 may be implemented in the context of the details of FIGS. 1-4. Of course, however, the R-DIMM 500 may be implemented in any desired environment. Additionally, the aforementioned definitions may apply during the present description.

[0047] As shown, included are a register chip 502, and a plurality of DRAM circuits 504, 506, 508, 510, 512, and 514. The DRAM circuits 504 belong to the first rank and are controlled by a common chip select signal 520. Similarly, the DRAM circuits 506 belong to the second rank and are controlled by a chip select signal 530. The DRAM circuits 508 belong to the third rank and are controlled by a chip select signal 540, while the DRAM circuits 510 belong to the fourth rank and are controlled by a chip select signal 550. The DRAM circuits 512 belong to the fifth rank and are controlled by a chip select signal 560. The DRAM circuits 514 belong to the sixth rank and are controlled by a chip select signal 570.

[0048] In this case, the DRAM circuits 504, 506, 508, 510, 512, and 514 are all x8 DRAM circuits, and are grouped into three sets of DRAM circuits. Each set contains one DRAM circuit from each of the six ranks The data pins of the DRAM circuits in a set are connected to each other and to eight data pins 590 of the R-DIMM 500. Since there are three such sets, the R-DIMM 500 may connect to 24 data signals of a memory bus. In the case where a typical memory bus has 72 data signals, the R-DIMM 500 is a one-third width DIMM with six ranks of DRAM circuits.

[0049] FIG. 6 illustrates a four-rank, half-width R-DIMM 600 built using x4 DRAM circuits and buffer circuits, in accordance with yet another embodiment. As an option, the R-DIMM 600 may be implemented in the context of the details of FIGS. 1-5. Of course, however, the R-DIMM 600 may be implemented in any desired environment. Again, the aforementioned definitions may apply during the present description.

[0050] As shown, included are a register chip 602, a plurality of DRAM circuits 604, 606, 608, and 610, and buffer circuits 612. The DRAM circuits 604 belong to the first rank and are controlled by a common chip select signal 620. Similarly, the DRAM circuits 606 belong to the second rank and are controlled by a chip select signal 630. The DRAM circuits 608 belong to the third rank and are controlled by a chip select signal 640, while the DRAM circuits 610 belong to the fourth rank and are controlled by a chip select signal 650.

[0051] In this case, the DRAM circuits 604, 606, 608, and 610 are all x4 DRAM circuits, and are grouped into nine sets of DRAM circuits. Each set contains one DRAM circuit from each of the four ranks, and in one embodiment, the buffer chip 612. The data pins of the DRAM circuits 604, 606, 608, and 610 in a set are connected to a first set of pins of the buffer chip 612, while a second set of pins of the buffer chip 612 are connected to four data pins 670 of the R-DIMM 600. The buffer chip 612 reduces the loading of the multiple ranks of DRAM circuits on the data bus since each data pin of the R-DIMM 600 connects to only one pin of a buffer chip instead of the corresponding data pin of four DRAM circuits.

[0052] Since there are nine such sets, the R-DIMM 600 may connect to 36 data signals of a memory bus. Since a typical memory bus has 72 data signals, the R-DIMM 600 is thus a half-width DIMM with four ranks of DRAM circuits. In some embodiments, each of the DRAM circuit 604, 606, 608, and 610 may be a plurality of DRAM circuits that are emulated by the buffer chip to appear as a higher capacity virtual DRAM circuit to the memory controller with at least one aspect that is different from that of the plurality of DRAM circuits.

[0053] In different embodiments, such aspect may include, for example, a number, a signal, a memory capacity, a timing, a latency, a design parameter, a logical interface, a control system, a property, a behavior (e.g. power behavior), and/or any other aspect, for that matter. Such embodiments may, for example, enable higher capacity, multi-rank, partial width DIMMs. For the sake of simplicity, the address and control signals on the R-DIMM 600 are not shown in FIG. 6.

[0054] FIG. 7 illustrates an electronic host system 700 that includes a memory controller 750, and two half width R-DIMMs 730 and 740, in accordance with another embodiment. As an option, the electronic host system 700 may be implemented in the context of the details of FIGS. 1-6. Of course, however, the electronic host system 700 may be implemented in any desired environment. Additionally, the aforementioned definitions may apply during the present description.

[0055] As shown, a parallel memory bus 710 connects the memory controller 750 to the two half width R-DIMMs 730 and 740, each of which is a four-rank DIMM. The memory bus includes an address bus 712, a control bus 714, a data bus 716, and clock signals 718. All the signals in the address bus 712 connect to both of the R-DIMMs 730 and 740 while only half the signals in the data bus 716 connect to each R-DIMM 730 and 740. The control bus 714 includes a plurality of chip select signals.

[0056] The chip select signals corresponding to the four ranks in the system, 720, 722, 724, and 726, connect to the R-DIMM 730 and to the R-DIMM 740. Thus, when the memory controller 750 accesses the first rank of DRAM circuits, it asserts the chip select signal 720 and the corresponding DRAM circuits on the R-DIMM 730 and on the R-DIMM 740 respond to the access. For example, when the memory controller 750 performs a read access to the first rank of DRAM circuits, half the data signals are driven by DRAM circuits on the R-DIMM 730 while the other half of the data signals are driven by DRAM circuits on the R-DIMM 740.

[0057] Similarly, when the memory controller 750 wishes to access the third rank of DRAM circuits, it asserts the chip select signal 724 and the corresponding DRAM circuits on the R-DIMM 730 and the R-DIMM 740 respond to the access. In other words, each memory access involves DRAM circuits on both the R-DIMM 730 and the R-DIMM 740. Such an arrangement transforms each of the data signals in the data bus 716 into a point-to-point signal between the memory controller 750 and one R-DIMM.

[0058] It should be noted that partial width DIMMs may be compatible with systems that are configured with traditional parallel memory bus topologies. In other words, all the data signals in the data bus 716 may be connected to the connectors of both DIMMs. However, when partial width DIMMs are used, the memory circuits on each DIMM connect to only half the data signals in the data bus.

[0059] In such systems, some of the data signals in the data bus 716 may be point-to-point nets (i.e. without stubs) while other signals in the data bus 716 may have stubs. To illustrate, assume that all the signals in data bus 716 connect to the connectors of R-DIMM 730 and R-DIMM 740. When two half-width R-DIMMs are inserted into these connectors, the data signals in the data bus 716 that are driven by the DRAM circuits on the R-DIMM 740 are point-to-point nets since the memory controller 750 and the DRAM circuits on the R-DIMM 740 are located at either ends of the nets.

[0060] However, the data signals that are driven by the DRAM circuits on the R-DIMM 730 may have stubs since the DRAM circuits on the R-DIMM 730 are not located at one end of the nets. The stubs correspond to the segments of the nets between the two connectors. In some embodiments, the data signals in the data bus 716 that are driven by the DRAM circuits on the R-DIMM 730 may be terminated at the far end of the bus away from the memory controller 750. These termination resistors may be located on the motherboard, or on the R-DIMM 740, or in another suitable place.

[0061] Moreover, the data signals that are driven by the DRAM circuits on the R-DIMM 740 may also be similarly terminated in other embodiments. Of course, it is also possible to design a system that works exclusively with partial width DIMMs, in which case, each data signal in the data bus 716 connects to only one DIMM connector on the memory bus 710.

[0062] FIG. 8 illustrates an electronic host system 800 that includes a memory controller 840, and three one-third width R-DIMMs 850, 860, and 870, in accordance with another embodiment. As an option, the electronic host system 800 may be implemented in the context of the details of FIGS. 1-7. Of course, however, the electronic host system 800 may be implemented in any desired environment. Still yet, the aforementioned definitions may apply during the present description.

[0063] As shown, a parallel memory bus 880 connects the memory controller 840 to the three one-third width R-DIMMs 850, 860, and 870, each of which is a six-rank DIMM. The memory bus 880 includes an address bus (not shown), a control bus 814, a data bus 812, and clock signals (not shown). All the signals in the address bus connect to all three R-DIMMs while only one-third of the signals in the data bus 812 connect to each of the R-DIMMs 850, 860, and 870.

[0064] The control bus 814 includes a plurality of chip select signals. The chip select signals corresponding to the six ranks in the system, 820, 822, 824, 826, 828, and 830, connect to all three of the R-DIMMs 850, 860, and 870. Thus, when the memory controller 840 accesses the first rank of DRAM circuits, it asserts the chip select signal 820 and the corresponding DRAM circuits on the R-DIMM 850, on the R-DIMM 860, and on the R-DIMM 870 respond to the access.

[0065] For example, when the memory controller 840 performs a read access to the first rank of DRAM circuits, one-third of the data signals are driven by DRAM circuits on the R-DIMM 850, another one-third of the data signals are driven by DRAM circuits on the R-DIMM 860, and the remaining one-third of the data signals are driven by DRAM circuits on the R-DIMM 870. In other words, each memory access involves DRAM circuits on all three of the R-DIMMs 850, 860, and 870. Such an arrangement transforms each of the data signals in the data bus 812 into a point-to-point signal between the memory controller 840 and one R-DIMM.

[0066] In various embodiments, partial-rank, partial width, memory modules may be provided, wherein each DIMM corresponds to a part of all of the ranks in the memory bus. In other words, each DIMM connects to some but not all of the data signals in a memory bus for all of the ranks in the channel. For example, in a DDR2 memory bus with two R-DIMM slots, each R-DIMM may have two ranks and connect to all 72 data signals in the channel. Therefore, each data signal in the memory bus is connected to the memory controller and the two R-DIMMs.

[0067] For the case of the same memory bus with two multi-rank, partial width R-DIMMs, each R-DIMM may have four ranks but the first R-DIMM may connect to 36 data signals in the channel while the second R-DIMM may connect to the other 36 data signals in the channel. Thus, each of the data signal in the memory bus becomes a point-to-point connection between the memory controller and one R-DIMM, which reduces signal integrity issues and increases the maximum frequency of operation of the channel. In other embodiments, full-rank, partial width, memory modules may be built that correspond to one or more complete ranks but connect to some but not all of the data signals in the memory bus.

[0068] FIG. 9 illustrates a two-full-rank, half-width R-DIMM 900 built using x8 DRAM circuits and buffer circuits, in accordance with one embodiment. As an option, the R-DIMM 900 may be implemented in the context of the details of FIGS. 1-8. Of course, however, the R-DIMM 900 may be implemented in any desired environment. Again, the aforementioned definitions may apply during the present description.

[0069] As shown, included are a register chip 902, a plurality of DRAM circuits 904 and 906, and buffer circuits 912. The DRAM circuits 904 belong to the first rank and are controlled by a common chip select signal 920. Similarly, the DRAM circuits 906 belong to the second rank and are controlled by chip select signal 930.

[0070] The DRAM circuits 904 and 906 are all illustrated as x8 DRAM circuits, and are grouped into nine sets of DRAM circuits. Each set contains one DRAM circuit from each of the two ranks, and in one embodiment, the buffer chip 912. The eight data pins of each of the DRAM circuits in a set are connected to a first set of pins of the buffer chip 912, while a second set of pins of the buffer chip 912 are connected to four data pins 970 of the R-DIMM 900. The buffer chip 912 acts to transform the eight data signals from each DRAM circuit operating at a specific data rate to four data signals that operate at twice the data rate and connect to the data pins of the R-DIMM, and vice versa. Since there are nine such sets, the R-DIMM 900 may connect to 36 data signals of a memory bus.

[0071] In the case that a typical memory bus has 72 data signals, the R-DIMM 900 is a half-width DIMM with two full ranks of DRAM circuits. In some embodiments, each DRAM circuit 904 and 906 may be a plurality of DRAM circuits that are emulated by the buffer chip to appear as a higher capacity virtual DRAM circuit to the memory controller with at least one aspect that is different from that of the plurality of DRAM circuits. In different embodiments, such aspect may include, for example, a number, a signal, a memory capacity, a timing, a latency, a design parameter, a logical interface, a control system, a property, a behavior (e.g. power behavior), and/or any other aspect, for that matter. Such embodiments may, for example, enable higher capacity, full-rank, partial width DIMMs. For the sake of simplicity, the address and control signals on the R-DIMM 900 are not shown in FIG. 9.

[0072] FIG. 10 illustrates an electronic host system 1000 that includes a memory controller 1050, and two half width R-DIMMs 1030 and 1040, in accordance with one embodiment. As an option, the electronic host system 1000 may be implemented in the context of the details of FIGS. 1-9. Of course, however, the electronic host system 1000 may be implemented in any desired environment. Additionally, the aforementioned definitions may apply during the present description.

[0073] As shown, a parallel memory bus 1010 connects the memory controller 1050 to the two half width R-DIMMs 1030 and 1040, each of which is a two-rank R-DIMM. The memory bus 1010 includes an address bus 1012, a control bus 1014, and a data bus 1016, and clock signals 1018. All the signals in the address bus 1012 connect to both of the R-DIMMs 1030 and 1040 while only half the signals in the data bus 1016 connect to each R-DIMM. The control bus 1014 includes a plurality of chip select signals.

[0074] The chip select signals corresponding to the first two ranks, 1020 and 1022, connect to the R-DIMM 1030 while chip select signals corresponding to the third and fourth ranks, 1024 and 1026, connect to the R-DIMM 1040. Thus, when the memory controller 1050 accesses the first rank of DRAM circuits, it asserts chip select signal 1020 and the corresponding DRAM circuits on the R-DIMM 1030 respond to the access.

[0075] For example, when the memory controller 1050 performs a read access to the first rank of DRAM circuits, the R-DIMM 1030 provides the entire read data on half the data signals in the data bus but at twice the operating speed of the DRAM circuits on the R-DIMM 1030. In other words, the DRAM circuits on the R-DIMM 1030 that are controlled by chip select signal 1020 will return n 72-bit wide data words at a speed off transactions per second.

[0076] The buffer circuits on the R-DIMM 1030 will transform the read data in 2n 36-bit wide data words and drive them to the memory controller 1050 at a speed of 2f transactions per second. The memory controller 1050 will then convert the 2n 36-bit wide data words coming in at 2f transactions per second back to n 72-bit wide data words at f transactions per second. It should be noted that the remaining 36 data signal lines in the data bus 1016 that are connected to the R-DIMM 1040 are not driven during this read operation.

[0077] Similarly, when the memory controller 1050 wishes to access the third rank of DRAM circuits, it asserts chip select signal 1024 and the corresponding DRAM circuits on the R-DIMM 1040 respond to the access such that the R-DIMM 1040 sends back 2n 36-bit wide data words at a speed of 2f transactions per second. In other words, each memory access involves DRAM circuits on only one R-DIMM. Such an arrangement transforms each of the data signals in the data bus 1016 into a point-to-point signal between the memory controller 1050 and one R-DIMM.

[0078] It should be noted that full-rank, partial width DIMMs may be compatible with systems that are configured with traditional parallel memory bus topologies. In other words, all the data signals in the data bus 1016 may be connected to the connectors of both of the R-DIMMs 1030 and 1040. However, when full-rank, partial width DIMMs are used, each DIMM connects to only half the data signals in the data bus 1016. In such systems, some of the data signals in the data bus 1016 may be point-to-point nets (i.e. without stubs) while other signals in the data bus 1016 may have stubs.

[0079] To illustrate, assume that all the signals in data bus 1016 connect to the connectors of the R-DIMM 1030 and the R-DIMM 1040. When two full-rank, half-width R-DIMMs are inserted into these connectors, the data signals in the data bus that are driven by the R-DIMM 1040 are point-to-point nets since the memory controller 1050 and the buffer circuits on the R-DIMM 1040 are located at either ends of the nets. However, the data signals that are driven by the R-DIMM 1030 may have stubs since the buffer circuits on the R-DIMM 1030 are not located at one end of the nets.

[0080] The stubs correspond to the segments of the nets between the two connectors. In some embodiments, the data signals in the data bus that are driven by the R-DIMM 1030 may be terminated at the far end of the bus away from the memory controller 1050. These termination resistors may be located on the motherboard, or on the R-DIMM 1040, or in another suitable place. Moreover, the data signals that are driven by the R-DIMM 1040 may also be similarly terminated in other embodiments. Of course, it is also possible to design a system that works exclusively with full-rank, partial width DIMMs, in which case, each data signal in the data bus connects to only one DIMM connector on the memory bus.

[0081] While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. For example, an un-buffered DIMM (UDIMM), a small outline DIMM (SO-DIMM), a single inline memory module (SIMM), a MiniDIMM, a very low profile (VLP) R-DIMM, etc. may be built to be multi-rank and partial width memory modules. As another example, three-rank one-third width DIMMs may be built. Further, the memory controller and optional buffer functions may be implemented in several ways. As shown here the buffer function is implemented as part of the memory module. The buffer function could also be implemented on the motherboard beside the memory controller, for example. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.