Abstract:
Describes is a memory system that utilizes motherboard traces in a way that permits maximum utilization of system data lines while accommodating varying numbers of memory modules. It is possible in a system such as this to utilize all individual sets of point-to-point signaling lines, even when less than all of the available memory sockets are occupied. Memory modules with configurable data widths support a relatively wide mode in which one module utilizes all available system data lines, or a relatively narrow mode in which multiple, narrower modules split the available system data lines between them.

Description:
BACKGROUND 
       [0001]      FIG. 1  (prior art) shows an example of a conventional memory system  100 . In this example, memory system  100  resides on a computer motherboard  105  and is actually a subsystem of the motherboard. System  100  includes a plurality of female electrical connectors  110 , each of which accepts a memory module  115  (only one of which is shown here). Each memory module  115  contains a plurality of memory devices  120 , typically packaged as discrete integrated circuits (ICs). Memory devices  120  are usually some type of read/write memory, such as Dynamic Random Access Memories (DRAMS), Static Random Access Memories (SDRAMs), Flash RAM, or other types. Read-Only Memories (ROM) devices might also be used. 
         [0002]    Motherboard  105  includes a memory controller  125  connected via conductive traces  130  to connectors  110 . Memory controller  125  communicates with memory modules  115  through conductive traces  130 . Memory controller  125  also has an interface (not shown) that communicates with other components on the motherboard, allowing those components to read from and write to memory. 
         [0003]    Each memory module  115  typically contains a fixed-width data path interface. The fixed-width nature of the interface is generally a result of a desire to create an industry standard interface that can accommodate interoperable modules from a large number of suppliers. 
         [0004]    System  100  works with different numbers of memory modules  115  installed, and with modules having different memory capacities and/or organizations. However, a system such as this is normally designed for a specific system data path width, i.e., for a specified number of data bit lines from controller  125  to memory modules  115 . 
         [0005]    Memory devices can be targeted to a wide variety of markets with very different sets of cost and performance constraints; consequently, the optimal device width can vary significantly from one application to the next. Unfortunately, these variations make it difficult for memory suppliers and distributors to accurately predict the customer demand mix for memory devices of various widths. Inaccuracies in demand-mix prediction can cause supply/demand imbalances and inventory management difficulties, which in turn can lead to pricing instability and highly variable profit margins. Furthermore, a memory device manufacturer may find that optimizing the cost for each target device width means a different design at the die level and potentially at the package level. This can increase the time-to-market and level of financial and engineering resources required to deliver each of these products to market. 
         [0006]    Fixed-width devices have other drawbacks related to inflexible data path configuration. Because the system memory interface width and memory device interface widths are fixed, the addition of more memory devices or modules to the system typically requires multiple ranks, which generally necessitates the use of a multi-drop datapath topology. Adding more drops to the system tends to degrade signaling performance. 
         [0007]    One way to reduce time-to-market and resource requirements is to create a common die design and package pinout that can support a variety of device data path widths. Some memory manufacturers support this capability through memory designs that allow configurations to be postponed until relatively late in the manufacturing process. A configuration is typically selected through one of several possible schemes, such as fuse or anti-fuse programmability, wire-bonding options, or upper level metal mask changes. This flexibility allows the device to be tested at the target width and sold as a fixed-width device. 
         [0008]    Another way to reduce time-to-market and resource requirements associated with fixed-width memories is to use a memory design in which the width (e.g., the number of data pins) can be dynamically changed to suit the needs of a particular system. One such memory design is depicted in U.S. Patent Application No. 5,893,927 to William P. Hovis, which is incorporated herein by reference.  FIG. 2 , taken from the Hovis patent, illustrates a conventional synchronous dynamic random access memory (SDRAM)  200  having a programmable device width. SDRAM  200  includes a clock generator  205  that provides clock signals to various components of SDRAM  200 . A command decoder  210  receives chip select /CS, row enable /RAS, column enable /CAS and write command /W inputs. (The “/” preceding the signal names identifies the signals as active low. Overbars are used in the figures for the same purpose.) Command decoder  210  recognizes, for example, a write command when /CS, /CAS, and /W are simultaneously asserted (i.e., logic low). Command decoder  210  then outputs the command to some control logic  215 , which controls the operation of the other components of SDRAM  200  based on the received command. 
         [0009]    Besides the commands of /CS, /RAS, /CAS and /W, command decoder  210  also recognizes commands based on a combination of /CS, /RAS, /CAS, and /W. For instance, command decoder  210  decodes the simultaneous receipt of /CS, /RAS, /CAS, and /W as a mode register set command. When a mode register set command is received, control logic  215  causes a mode register  220  to latch the address data on address inputs A0-A10 and BA0-BA1. 
         [0010]    The data on address inputs A0-A10, generally, represent either a row or column address, whereas the data on address inputs BA0-BA1, generally, represent a bank address. The bank address inputs BA0-BA1 specify one of the memory banks A-D discussed in detail below. During the mode register set operation, however, the data on address inputs A0-A10 and BA0-BA1 represent commands. Hereinafter, the address inputs and the data thereon will generically be referred to as address inputs. 
         [0011]    SDRAM  200  includes a row address buffer and refresh counter  225  and a column address buffer and burst counter  230 , both of which connect to address inputs A0-A10 and BA0-BA1. The row address buffer portion latches the address inputs at row-access-strobe (RAS) time and provides the row address to the appropriate row decoder  235 . The refresh counter portion refreshes the memory. The column address buffer portion latches the address inputs at column-access-strobe (CAS) time and provides the column address to the appropriate column decoder  240 . The burst counter portion controls the reading/writing of more than one column based on a pre-set burst length. 
         [0012]    The memory of SDRAM  200  is divided into four memory banks A-D that can be independently and simultaneously selected. Each memory bank A-D has associated therewith a row decoder  235 , a sense amplifier  255 , and a column decoder  240 . Based on the address latched by the row address buffer and refresh counter  225 , one of row decoders  235  enables a row of bits in the corresponding bank. An associated sense amplifier  255  latches the columns of this row via sense amplification, and the associated column decoder  240  outputs one or more bits depending on the device width and burst length. Sense amplifier  255  typically represents a combination of column I/O amplifiers arranged along an edge of the array of banks and lower-level sense amplifiers interleaved between memory cells. 
         [0013]    SDRAM  200  includes configuration logic  260  for setting the device width. Configuration logic  260  connects to mode register  220 , and from there receives a memory-width configuration value stored in register  220  during device configuration. Based on this information, configuration logic  260  configures a data control circuit  265 , a latch circuit  270 , and an input/output (I/O) buffer  275  to obtain the device width associated with the memory-width configuration value. Specifically, configuration logic  260  controls switches and multiplexers in data control circuit  265  such that the number of active I/O drivers corresponds to the programmed device width. 
         [0014]    Data control circuit  265  is connected to each column decoder  240 , and to data I/O pin(s) DQ(s) via latch circuit  270  and input/output buffer  275 . During a read operation, sense amplifiers  255  and column decoders  240  output data to data control circuit  265  based on the row enabled by decoder  235 , the column enabled by decoder  240 , and the burst length. Data control circuit  265  then routes the data to the number of I/O drivers set based on the device width. The data from the I/O drivers is then latched by the latch circuit  270 , buffered by I/O buffer  275 , and output on the data I/O pin(s) DQ(s). The number of I/O pin(s) DQ(s) corresponds to the device width. 
         [0015]    During a write operation, SDRAM  200  receives data over the I/O pin(s) DQ(s). This data is buffered by I/O buffer  275 , latched by latch circuit  270 , and received by data control circuit  265 . Data control circuit  265  sends the data to the appropriate column decoders  240  for storage in the memory banks A-D according to the enabled row and column. 
         [0016]    SDRAM  200  also includes an input DQM to latch circuit  270  for every 8 bits of input/output. For instance a x16 SDRAM will have two inputs DQM0 and DQM1. When enabled, the input DQM prevents reading or writing the remainder of a burst. In this manner, the burst length can be controlled. 
         [0017]    Each read operation presents an entire row of data to sense amps  255 . Each write operation similarly involves an entire row. In SDRAM  200 , changing the memory width merely changes the number of bits selected from the accessed row: the narrower the memory configuration, the fewer bits are selected from the accessed row. Since the power required to perform a row access does not change with changes in device width, the relative power efficiency of row accesses reduces with memory width. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  (prior art) shows an example of a conventional memory system  100 . 
           [0019]      FIG. 2  (prior art) illustrates a conventional synchronous dynamic random access memory (SDRAM)  200  having a programmable device width. 
           [0020]      FIG. 3  depicts a variable-width memory  300  in accordance with an embodiment of the invention. 
           [0021]      FIG. 4A  details a portion of an embodiment of memory  300  of  FIG. 3 . 
           [0022]      FIG. 4B  details a portion of another embodiment memory  300  of  FIG. 3 . 
           [0023]      FIGS. 5A-5C  depict various width configurations of a memory module  500  that includes four variable-width memories  300  of the type described above in connection with  FIGS. 3 and 4 . 
           [0024]      FIGS. 6A and 6B  depict a computer motherboard  600  adapted to use a variable-width memory in accordance with an embodiment of the invention. 
           [0025]      FIG. 7  depicts a portion  700  of motherboard  600  detailing the signal-line configuration. 
           [0026]      FIG. 8  depicts portion  700  of  FIG. 7  with a memory module  800  and shorting module  810  installed. 
           [0027]      FIGS. 9A-9D  depict memory configurations that respectively accommodate one, two, three, or four memory modules. 
           [0028]      FIG. 10  (prior art) shows a floor plan of a conventional 1 Gb DRAM  1000  having a 16-bit wide data path D0-D15. 
           [0029]      FIGS. 11A and 11B  depict a high-level floor plan of a DRAM  1100  featuring a configurable core. 
           [0030]      FIG. 12  depicts a specific implementation of a configurable core  1200  and associated circuitry. 
           [0031]      FIGS. 13A-F  are simplified block diagrams of core  1200  of  FIG. 12  illustrating access timing in a number of memory-access configurations. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 3  depicts a variable-width memory  300  in accordance with an embodiment of the invention. Memory  300  is similar to SDRAM  200  of  FIG. 2 , like-numbered elements being the same. Memory  300  differs from SDRAM  200 , however, in that the memory core organization changes with device width, resulting in reduced power usage for relatively narrow memory configurations. Also advantageous, reorganizing the core for relatively narrow memory widths increases the number of logical memory banks, and consequently reduces the likelihood of bank conflicts. Fewer conflicts means improved speed performance. These and other benefits of the invention are detailed below. 
         [0033]    Much of the operation of memory  300  is similar to SDRAM  200  of  FIG. 2 . A discussion of those portions of memory  300  in common with SDRAM  200  is omitted here for brevity. The elements of  FIG. 3  described above in connection with  FIG. 2  are numbered in the two-hundreds (e.g., 2XX) for convenience. In general, the first digit of numerical designations indicates the figure in which the identified element is introduced. 
         [0034]    Memory  300  includes a configurable memory core  305 . In the example, memory core  305  includes eight physical memory banks PB0-PB7, though the number of physical banks may vary according to need. Physical banks PB0-PB7 are interconnected such that they can be combined to form different numbers of logical banks. In the example, pairs of physical banks (e.g., PB0 and PB1) can be combined to form four logical banks LB0-LB3, collections of four physical banks (e.g., PB0-PB3) can be combined to form two logical banks LB4 and LB5, and all eight physical banks can be combined to form a single logical bank LB0-7. Assuming, for simplicity, that each physical bank PB0-PB7 includes a single data I/O terminal, memory core  305  can be configured as a one-bit-wide memory with eight logical banks, a two-bit-wide memory with four logical banks, a four-bit-wide memory with two logical banks, or an eight-bit-wide memory with one logical bank. 
         [0035]    Some configuration logic  310  controls the configuration of memory core  305  via a data control circuit  315 . Configuration logic  310  also controls the data width through a collection of latches  320  and a collection of I/O buffers  325 . As detailed below, data control circuit  315  includes some data routing logic, such as a crossbar switch, to provide flexible routing between the memory banks and data terminals DQs. The purpose and operation of these blocks is described below in more detail. As noted in  FIG. 3 , the data terminals (DQs) can be configured to have widths of x1, x2, x4, and x8. 
         [0036]      FIG. 4A  shows a specific implementation of a configurable core  400  and associated circuitry. In one embodiment, core  400  is a portion of memory  300  of  FIG. 3 . The number of physical banks is reduced to four physical banks PB0-PB3 in  FIG. 4  for brevity. Memory  300  might include two memory “slices,” each of which comprises a memory core  400 . The manner of extending the memory core of  FIG. 4A  to eight or more banks will be readily apparent to those of skill in the art. 
         [0037]    The components of core  400  are similar to like-numbered elements in  FIG. 3 . For this embodiment, the serialization ratio is 1:1. Serialization ratios greater than 1:1 are possible with the addition of serial-to-parallel (write) and parallel-to-serial (read) conversion circuits. In this example, there are four physical banks PB0-3 supporting four read data bits and four write data bits. Generally, data control circuit  315  contains multiplexing logic for read operations and demultiplexing logic for write operations. The multiplexing logic and demultiplexing logic are designed to allow one, two, or four device data lines DQ0-DQ3 to be routed to the four physical banks PB0-PB3. 
         [0038]    In the one-bit wide configuration, device data line D0 can be routed to/from any of the four physical banks PB0-PB3. In the 2-bit wide configuration (“x2”), device data lines DQ0 and DQ1 can be routed to/from physical banks PB0 and PB1 (collectively, logical bank LB0,1) or physical banks PB2 and PB3 (collectively logical banks LB2,3). Finally, in the 4-bit wide configuration, device data lines DQ0, DQ1, DQ2, and DQ3 can be routed to/from respective physical banks PB0, PB1, PB2, and PB3 (collectively, logical bank LB0-3). Core  400  can thus be configured as a one-, two-, or four-bank memory with respective widths of four (x4), two (x2), and one (x1) data bits. 
         [0039]    Core  400  is a synchronous memory; consequently, each physical bank PB0-PB3 includes an input latch  405  and an output latch  410 . Physical banks PB0-PB3 additionally include respective memory arrays MA0-MA3, sense amplifiers SA0-SA3, and bank-select terminals BS1-BS3. Asserting a bank select signal on one of terminals BS1-BS3 loads the data in the addressed location within the selected memory array into the respective one of sense amplifiers SA1-1A3. 
         [0040]    Latch  320  includes a pair of latches  415  and  420  for each physical bank PB0-PB3. Data control circuit  315  includes five multiplexers  425 ,  430 ,  435 ,  440 , and  445  that communicate data between latch  320  and physical banks PB0-PB3. Multiplexers  425  and  430  are controlled by a write control signal WB; multiplexer  435  is controlled by a read control signal RA; multiplexer  440  is controlled by a write control signal WA; and multiplexer  445  is controlled by two read control signals RA and RB. Write control signals WA and WB and read control signals RA and RB are based on the selected data path width and bits of the requested memory address or transfer phase. Configuration logic  310  ( FIG. 3 ) produces these signals in response to the programmed data width, whether the operation is a read or write operation, and appropriate addressing information. 
         [0041]    Table 1 shows the control values used for data path slice widths of one, two, and four. Table 1 also indicates which of data terminals D0-D3 are used for each data width. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 WRITE 
                 READ 
                   
               
             
          
           
               
                 WIDTH 
                 WA 
                 WB 
                 RA 
                 RB 
                 DATA TERMINALS 
               
               
                   
               
               
                 1 
                 1 
                 1 
                 A0 
                 A1 
                 DQ0 
               
               
                 2 
                 0 
                 1 
                 0 
                 A0 
                 DQ0 &amp; DQ1 
               
               
                 4 
                 0 
                 0 
                 0 
                 0 
                 DQ0, DQ1, DQ2, &amp; DQ3 
               
               
                   
               
             
          
         
       
     
         [0042]    When a width of one is selected during a read operation, the configuration logic  310  allows data from any one of the four physical banks PB0-PB3 to be presented at data terminal DQ0. Control signals RA and RB determine which data-bit signals will be presented at any given time. Control signals RA and RB are set (at this data width) to equal the two least-significant bits (A1, A0) of the memory address corresponding to the current read operation. 
         [0043]    When a width of one is selected during a write operation, the circuit accepts the data bit signal from data terminal DQ0 and routes it to all four physical banks PB0-PB3 simultaneously. Control signals WA and WB are both set to a logical value of one to produce this routing. Other logic circuits (not shown) within configuration logic  310  control which of input latches  405  and  410  are active during any single write operation, so that each data bit signal is latched into the appropriate physical bank. For a given physical bank, only one of latches  405  and  410  is active during any given memory cycle. 
         [0044]    When a width of two is selected during a read operation, configuration logic  310  allows two of the four data bit signals associated with physical banks PB0-PB3 to be present at data terminals DQ0 and DQ1. To obtain this result, control signal RA is set to 0, and control signal RB is equal to the lower bit (A0) of the memory address corresponding to the current read operation. Control signal RB determines which of two pairs of data bit signals ( 0  and  1  or  2  and  3 ) are presented at data terminals DQ0 and DQ1 during a given read operation. 
         [0045]    When a width of two is selected during a write operation, configuration logic  310  accepts the data bit signals from physical banks PB0 and PB 1 and routes them either to data terminals DQ0 and DQ1 or DQ2 and DQ3. In this configuration, physical banks PB0 and PB1 collectively form one logical bank LB0,1 and physical banks PB2 and PB3 collectively form a second logical bank LB2,3. Control signals WA and WB are set to 0 and 1, respectively, to obtain this result. 
         [0046]    A width of four is selected by setting all of the control signals (RA, RB, WA, and W,) to 0. Read and write data signals are then passed directly between physical banks PB0-PB3 and corresponding data terminals DQ0-DQ3. 
         [0047]    For each row access, data moves from memory arrays MA0-MA3 to their respective sense amplifiers SA0-SA3. Core  400  minimizes the power required to perform a row access by limiting each row access to the selected physical bank(s). To this end, bank-select signals on lines BS0-BS3 are only asserted to selected banks. 
         [0048]    Configuration logic  310  determines which of physical banks PB0-PB3 are selected, and consequently which bank-select signals are asserted, based upon the selected device width and memory address. The following Table 2 summarizes the logic within configuration logic  310  that generates the appropriate bank-select signals. 
         [0000]                                                  TABLE 2                           ADDRESS LINES A1:A0            WIDTH   00   01   10   11               1   BS0   BS1   BS2   BS3       2   BS0 &amp; BS1   BS2 &amp; BS3   BS0 &amp; BS1   BS2 &amp; BS3       4   BS0-BS3   BS0-BS3   BS0-BS3   BS0-BS3                    
When core  400  is configured to have a width of one, the two least-significant address bits A0 and A1 are decoded to select one of physical banks PB0-PB3; when core  400  is configured to have a width of two, address bit A0 enables the physical banks within either of logical banks LB0,1 or LB2,3; and when core  400  is configured to have a width of four, address bits A0 and A1 are ignored and all physical banks PB0-PB3 are selected (i.e., enabled).
 
         [0049]    The circuit of  FIG. 4A  is just one example of many possible designs. Other embodiments will benefit from other configurations. For example, it is possible to use more or less elaborate data routing schemes to account for the different connection needs for memory systems with more or fewer modules. Moreover, multiple memory cores  400  may be used to construct devices with greater than four device data connections. For example, a device having sixteen device data connections could use four memory cores while supporting three programmable widths; namely, 16, 8, or 4-bits widths. There are many possible alternatives for the number and width of physical and logical banks, the number of device data connections per device, serialization ratios, and data-path widths. 
         [0050]    All data to and from memory core  400  passes through data terminal DQ0 in the x1 mode, terminals DQ0 and DQ1 in the x2 mode, and terminals DQ0-DQ3 in the x4 mode.  FIG. 4B  depicts an embodiment  450  that benefits from a more flexible routing scheme in which the data terminals DQ0-DQ3 can be routed to different input/output pins of the memory module upon which core  305  is mounted. Embodiment  450  substitutes data control circuit  315  of  FIG. 4A  with a more flexible crossbar switch  460 . In the depicted embodiment, the data terminals to and from physical bank PB0 can be routed to any of data connections DQ0-DQ3 in the x1 mode; the data terminals to and from physical banks PB0 and PB1 can be routed to either data connections DQ0 and DQ1 or data connections DQ2 and DQ3, respectively, in the x2 mode; and the data terminals to and from physical banks PB0-PB3 can be routed to data connections DQ0-DQ3, respectively, in the x4 mode. U.S. Pat. Nos. 5,530,814 and 5,717,871 describe various types of crossbar switches, and are incorporated herein by reference. 
         [0051]      FIG. 5A  depicts a memory module  500  that includes four variable-width memories  502  of the type described above in connection with  FIGS. 3 ,  4 A, and  4 B. Module  500 , typically a printed circuit board, also includes a number of conductive traces  505  that convey data between the data pins (3, 2, 1, 0) of memories  502  and corresponding module pins  510 . In  FIG. 5A , each memory  502  is configured to be one-bit wide, and the resulting four data bits are connected to four consecutive ones of pins  510 . The selected traces are identified as bold lines; the selected module pins are crosshatched. 
         [0052]      FIG. 5B  depicts the same memory module  500  of  FIG. 5A ; unlike in  FIG. 5A , however, each memory  502  is configured to be two-bits wide, and the resulting eight data bits are connected to eight consecutive ones of pins  510 . The memory module  500  of  FIG. 5B  is thus configured to be twice as wide (and half as deep) as the same module  500  of  FIG. 5A . As in  FIG. 5A , the selected traces are identified as bold lines; the selected pins are crosshatched. 
         [0053]      FIG. 5C  depicts the same memory module  500  of  FIGS. 5A and 5B ; unlike in  FIGS. 5A and 5B , however, each memory  502  is configured to be four-bits wide, and the resulting sixteen data bits are connected to sixteen consecutive ones of pins  510 . The memory module  500  of  FIG. 5C  is thus configured to be twice as wide (and half as deep) as the same module  500  of  FIG. 5B  and four times as wide (and one forth as deep) as the same memory module  500  of  FIG. 5A . Once again, the selected traces are identified as bold lines; the selected pins are crosshatched. 
         [0054]      FIGS. 6A and 6B  depict a computer motherboard (or system backplane)  600  adapted to use a variable-width memory in accordance with an embodiment of the invention. Motherboard  600  includes a memory controller  605  and a plurality of electrical receptacles or connectors  610  and  615 . The connectors are memory module sockets, and are configured to receive installable/removable memory modules  620  and  625 . 
         [0055]    Each of memory modules  620  and  625  comprises a module backplane  630  and a plurality of integrated memory circuits  635 . Each memory module also includes first and second opposed rows of electrical contacts (module pins)  640  along opposite surfaces of its backplane. Only one row of contacts  640  is visible in  FIG. 6A . There are corresponding rows of connector contacts (not visible in  FIG. 6A ) in each of connectors  610  and  615 . 
         [0056]    A plurality of signal lines, or “traces,” extends between memory controller  605  and electrical connectors  610  and  615  for electrical communication with memory modules  620  and  625 . More specifically, there are a plurality of sets of signal lines, each set extending to a corresponding, different one of connectors  610  and  615 . A first set of signal lines  645  extends to first electrical connector  610 , and a second set of signal lines  650  extends to second electrical connector  615 . Motherboard  600  also has a third set of signal lines  655  that extends between the two connectors. 
         [0057]    In the embodiment shown, the signal lines comprise system data lines—they carry data that has been read from or that is to be written to memory modules  620  and  625 . It is also possible that other signal lines, such as address and control lines, would couple to the memory modules through the connectors. These additional signal lines could have a different interconnection topology than what is shown for signal lines  645 ,  650 , and  655 . 
         [0058]    The routing of the signal lines is more clearly visible in  FIG. 6B , in which memory modules  620  and  625  have been omitted for clarity. The illustrated physical routing is shown only as a conceptual aid—actual routing is likely to be more direct, through multiple layers of a printed-circuit board. 
         [0059]      FIG. 7A  depicts a portion  700  of motherboard  600  detailing the signal-line configuration. This view shows cross-sections of connectors  610  and  615 . Electrical conductors, traces, and/or contacts are indicated symbolically in  FIG. 7A  by relatively thick solid or dashed lines. Each of the three previously described sets of signal lines is represented by a single one of its conductors, which has been labeled with the reference numeral of the signal line set to which it belongs. The respective lines of a particular set of signal lines are routed individually in the manner shown. 
         [0060]    As discussed above, each connector  610  and  615  has first and second opposed rows of contacts.  FIG. 7A  shows individual contacts  705  and  710  corresponding respectively to the two contact rows of each connector. It is to be understood that these, again, are representative of the remaining contacts of the respective contact rows. 
         [0061]    As is apparent in  FIG. 7A , the first set of signal lines  645  extends to first contact row  705  of first connector  610 . The second set of signal lines  650  extends to the first contact row  705  of second connector  615 . In addition, a third set of signal lines  655  extends between the second contact row  710  of first connector  610  and second contact row  710  of second connector  615 . The third set of signal lines  655  is represented by a dashed line, indicating that these lines are used only in certain configurations; specifically, signal lines  655  are used only when a shorting module is inserted into connector  610  or  615 . Such a shorting module, the use of which will be explained in more detail below, results in both sets of signal lines  645  and  650  being configured for communications with a single memory module. 
         [0062]    The system of  FIG. 7A  can be configured to include either one or two memory modules.  FIG. 8  illustrates the first configuration, which includes a memory module  800  in the first connector  610  and a shorting module  810  in the second connector  615 . The shorting module has shorting conductors  815 , corresponding to opposing pairs of connector contacts, between the first and second rows of the second connector. Inserting shorting module  810  into connector  615  connects or couples the second set  650  of signal lines to the second contact row  705  of first connector  610  through the third set of signal lines  655 . In this configuration, the two sets of signal lines  645  and  650  are used collectively to communicate between memory controller  605  and memory module  800 . 
         [0063]    In a two-module configuration, shorting bar  810  is replaced with a second memory module  800 . If modules  800  are adapted in accordance with the invention to support two width configurations and to include one half of the module pins  640  on either side, then there is no need for a switch matrix like data control circuit  315  of  FIG. 4A  or crossbar switch  460  of  FIG. 4B . Instead, merely including shorting module  810  provides the memory controller access to the module pins  640  on both sides of the one module  800 . Alternatively, including two memory modules  800  will provide the memory controller access to the same half of the module pins  640  (those on the left-hand side of connector  610 ) on both memory modules; the other half of the module pins  640  are not used. More complex routing schemes can likewise be employed to support additional modules and width configurations. The two-module configuration thus provides the same data width as the single-module configuration, with each module providing half the width. 
         [0064]    For a more detailed discussion of motherboard  600 , see U.S. patent application Ser. No. 09/797,099 filed Feb. 28, 2001, entitled “Upgradeable Memory System with Reconfigurable Interconnect,” by Richard E. Perego et al., which issued Oct. 27, 2009, as U.S. Pat. No. 7,610,447 and is incorporated herein by reference. 
         [0065]    In some embodiments, the access configurations of the memory modules are controllable and programmable by memory controller  605  in the manner described above in connection with  FIGS. 3 ,  4 A,  4 B,  5 A, and  5 B. In such embodiments, the memory controller may be adapted to detect which connectors have installed memory modules, and to set the configuration of each module accordingly. This allows either one or two memory module to be used in a system without requiring manual configuration steps. If one module is used, it may be configured to use two signal-line sets for the best possible performance. If two memory modules are present, they may each be configured to use one signal-line set. This idea can be extended to support memory systems that can accommodate more than two memory modules, though the routing scheme becomes more complex with support for additional modules. 
         [0066]    The integrated memory circuit can be configured for the appropriate access mode using control pins. These control pins might be part of the signal line sets  645 ,  650 , and  655 , or they might be part of a different set of signal lines. These control pins might be dedicated to this configuration function, or they might be shared with other functions. Also, the integrated memory circuit might utilize programmable fuses to specify the configuration mode. Integrated memory circuit configurability might also be implemented, for example, by the use of jumpers on the memory modules. Note that the memory capacity of a module remains the same regardless of how it is configured. However, when it is accessed through one signal line set it requires a greater memory addressing range than when it is accessed through two signal line sets. Also note that the two configurations shown in  FIGS. 6-8  could also be implemented with a shorting connector instead of a shorting module. A shorting connector shorts its opposing contacts when no module is inserted (the same result as when the connector  615  of  FIG. 7B  has a shorting module inserted). A shorting connector with a memory module inserted is functionally identical to the connector  610  in  FIG. 7 . 
         [0067]    As noted above, the general signal line scheme can be generalized for use with n connectors and memory modules. Generally stated, a system such as this uses a plurality of signal-line sets, each extending to a respective module connector. At least one of these sets is configurable or bypassable to extend to a connector other than its own respective connector. Stated alternatively, there are 1 through n sets of signal lines that extend respectively to corresponding connectors 1 through n. Sets 1 through n−1 of the signal lines are configurable to extend respectively to additional ones of the connectors other than their corresponding connectors.  FIGS. 9A-9D  illustrate this generalization, in a memory system  900  in which n=4. 
         [0068]    Referring first to  FIG. 9A , this configuration includes a memory controller  905 ; four memory slots or connectors  910 ,  915 ,  920 , and  925 ; and four signal line sets  930 ,  935 ,  940 , and  945 . Each signal line set is shown as a single line, and is shown as a dashed line when it extends beneath one of the connectors without connection. Physical connections of the signal line sets to the connectors are shown as solid dots. Inserted memory modules are shown as diagonally hatched rectangles, with solid dots indicating signal connections. Note that each inserted memory module can connect to up to four signal line sets. The number of signal line sets to which it actually connects depends upon the connector into which it is inserted. The connectors are identical components, but appear different to the memory modules because of the routing pattern of the four signal line sets on the motherboard. 
         [0069]    Each signal line set extends to a corresponding connector. Furthermore, signal lines sets  935 ,  940 , and  945  are extendable to connectors other than their corresponding connectors: signal line set  935  is extendable to connector  925 ; signal line set  940  is extendable to both connectors  920  and  925 ; signal line set  945  is extendable to connector  925 . More specifically, a first signal line set  930  extends directly to a first memory connector  925  without connection to any of the other connectors. It connects to corresponding contacts of the first contact row of connector  925 . A second signal line set  935  extends directly to a second memory connector  920 , where it connects to corresponding contacts of the first contact row. The corresponding contacts of the second contact row are connected to corresponding contacts of the first contact row of first connector  925 , allowing the second signal line set to bypass second connector  920  when a shorting module is placed in connector  920 . 
         [0070]    A third signal line set  940  extends directly to a third memory connector  915 , where it connects to corresponding contacts of the first contact row. The corresponding contacts of the second contact row are connected to corresponding contacts of the first contact row of connector  920 . The corresponding second contact row contacts of connector  920  are connected to the corresponding contacts of the first contact row of connector  925 . 
         [0071]    A fourth signal line set  945  extends directly to a fourth memory connector  910 , where it connects to corresponding contacts of the first contact row of connector  910 . The corresponding contacts of the second contact row are connected to corresponding contacts of the first contact row of first connector  925 . 
         [0072]    This configuration, with appropriate use of shorting or bypass modules, accommodates one, two, three, or four physically identical memory modules. Each memory module permits simultaneous access through one, two, or four of its four available signal line sets. In the configuration of  FIG. 9A , a single memory module is inserted in first connector  925 . This memory module is configured to permit simultaneous accesses on all of its four signal line sets, which correspond to all four signal line sets. Connectors  910 ,  915 , and  920  are shorted by inserted shorting modules as shown so that signal line sets  935 ,  940 , and  945  extend to connector  925 . 
         [0073]      FIG. 9B  illustrates a second configuration in which connectors  910  and  915  are shorted by inserting shorting modules. Thus, signal line sets  930  and  945  extend to connector  925  and the inserted memory module is configured to permit simultaneous accesses on these two signal line sets. Signal line sets  935  and  940  extend to connector  920  and the inserted memory module is configured to permit simultaneous accesses on these two signal line sets. 
         [0074]      FIG. 9C  illustrates a third configuration in which connector  910  is shorted by inserting a shorting module, and memory modules are positioned in connectors  915 ,  920 , and  925 . Signal line sets  930  and  945  extend to connector  925  and the inserted memory module is configured to permit simultaneous accesses on these two signal line sets. Signal line set  935  extends to connector  920  and the inserted memory module is configured to permit accesses on this signal line set. Signal line set  940  extends to connector  915  and the inserted memory module is configured to permit accesses on this signal line set. 
         [0075]      FIG. 9D  illustrates a fourth configuration, with a memory module in each of the four available memory connectors. Each module is connected to use a respective one of the four signal line sets, with no shorting modules in use. 
         [0076]    An interesting aspect of a memory device with programmable data access width relates to the characteristic of the device that its bandwidth may generally be reduced as its data width is narrowed. As device bandwidth is reduced, opportunities increase for altering the device&#39;s memory array configuration to provide greater independence between array partitions. 
         [0077]      FIG. 10  shows an example of a conventional 1 Gb density DRAM  1000  with a 16-bit wide data path D0-D15.  FIG. 10  shows a high-level floor plan of the DRAM die, including left (“L”) and right (“R”) bank subdivisions, row decoders, column decoders, I/O sense amps (I/O), and data pin locations D0-D7 and D8-D15. A pair of regions  1005  and  1012  within memory banks B0-L and B0-R (i.e., the left and right halves of bank 0) indicates a sample page location for an 8 KB page within bank zero. 4 KB worth of sense amp circuitry for the left and right halves of DRAM  1000  are accessed in parallel via a pair of multiplexers  1010  and  1015  to form an 8 KB page. In this design, data from left and right halves of the die are accessed in parallel to meet the device peak bandwidth requirement. This also allows the data paths for the left and right halves of the die to be largely independent. (This aspect of some embodiments is discussed in more detail below in connection with  FIG. 12 .) 
         [0078]      FIGS. 11A and 11B  depict a high-level floor plan of a DRAM  1100  featuring a configurable core in accordance with one embodiment. DRAM  1100  can operate as DRAM  1000  of  FIG. 10 , but can also be configured to reduce peak device bandwidth by a factor of two. Such a bandwidth reduction allows the full amount of device bandwidth to be serviced by either the left half ( FIG. 11A ) or right half ( FIG. 11B ) of the device. In this embodiment, the eight active device data connections D0-D7—shown in bold—are located on the left side of the die, requiring that a data path  1105  be provided from the right side memory array to the left side data connections D0-D7. With the memory array divided into left and right halves, it becomes feasible to manage banks on each side independently. In this case, the 16-bit wide device that supported eight independent banks accessed via data terminals D0-D15 (like DRAM  1000  of  FIG. 10 ) can be reconfigured as an 8-bit wide device supporting 16 independent banks, with data access provided via either data terminals D0-D7 or D8-D15. 
         [0079]    There is typically some incremental circuit overhead associated with increasing the bank count of the device, setting a practical limit to the number of independent banks that could potentially be supported. However, a performance improvement related to the increased number of banks may justify some increase in device cost. 
         [0080]    In the embodiment of  FIGS. 11A and 11B , device page size is reduced for the 8-bit wide configuration (4 KB) relative to the 16-bit wide configuration (8 KB). Reducing the page size is attractive from a power consumption perspective because fewer sense amps are activated during a RAS operation. In addition to activating fewer sense amps, it is also possible to subdivide word lines using a technique known as “sub-page activation.” In this scheme, word lines are divided into multiple sections, one or more of which are activated for a particular RAS operation. This technique typically adds some incremental die area overhead in exchange for reduced power consumption and potentially improved array access or cycle times. 
         [0081]    The examples highlighted in  FIGS. 11A and 11B  are intended to illustrate the concept of how a configurable array organization can be used to reduce power consumption and increase the number of logical memory banks Write transactions are not described for this embodiment, although the same principles of power reduction and memory bank count apply to writes as well. The basic principles of configurable array organization can be exploited regardless of the type or capacity of memory device. 
         [0082]      FIG. 12  depicts a specific implementation of a configurable core  1200  and associated circuitry, the combination of which may be integrated to form a memory component. Core  1200  is similar to core  450  of  FIG. 4B , like-named elements being the same. Core  1200  provides the same functionality as core  450 , but the configuration and switching logic is modified to afford users the ability to partition the four physical banks PB0-PB3 into two separately addressable memories, each of which can be either one or two bits wide. Some elements are omitted from the depiction of  FIG. 12  for brevity. For example, core  1200  may also include registers  405  and  410 . 
         [0083]    Physical bank PB0 includes a row decoder RD0, a memory array MA0, a sense amp SA0 (actually a collection of sense amplifiers), and a column decoder CD0. Each of the remaining physical banks PB1-PB3 includes identical structures. The row decoders, memory banks, sense amps, and column decoders are omitted from  FIG. 4B  for brevity, but are included in  FIG. 12  to illustrate an addressing scheme that enables core  1200  to independently access logical blocks LB0,1 and LB2,3. 
         [0084]    Address buffers  225  and  230 , introduced in  FIG. 3 , connect directly to the row and column decoders of physical banks PB2 and PB3. Configuration logic  310 , also introduced in  FIG. 3 , connects to the bank-select terminals BS3-0 and to a crossbar switch  1207 . Address buffers  225  and  230  are also selectively connected to the row and column decoders in physical banks PB0 and PB1 via a multiplexer  1205 . 
         [0085]    The configuration and switching logic of core  1200  is extended to include a second set of address buffers (row and column)  1209  and a second set of configuration logic  1210 . Address buffers  1209  connect to the row and column decoders in physical banks PB0 and PB1 via multiplexer  1205 . Configuration logic  1210  connects to crossbar switch  1207 —the data control circuit in this embodiment—and to bank-select terminals BS0 and BS1 via multiplexer  1205 . A configuration-select bus CONF from configuration logic  310  includes three control lines C0-C2 that connect to crossbar switch  1207 . Line C2 additionally connects to the select terminal of multiplexer  1205 . In this embodiment, mode register  220  ( FIG. 3 ) is adapted to store configuration data establishing the levels provided on lines C0-C2. 
         [0086]    Core  1200  supports four operational modes, or “configurations,” in addition to those described above in connection with  FIGS. 3 ,  4 A, and  4 B. These modes are summarized below in Table 3. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 CONF# 
                 CORE CONFIGURATION 
                 C2 
                 C1 
                 C0 
               
               
                   
               
             
             
               
                 1 
                 Single Address, Variable Width 
                 1 
                 X 
                 X 
               
               
                 2 
                 Separate Addresses, separate 2-bit busses 
                 0 
                 0 
                 0 
               
               
                   
                 (DQ3/DQ2 and DQ1/DQ0) 
               
               
                 3 
                 Separate Addresses, memories share lines 
                 0 
                 0 
                 1 
               
               
                   
                 DQ1 and DQ0 
               
               
                 4 
                 Separate Addresses, memories share lines 
                 0 
                 1 
                 0 
               
               
                   
                 DQ3 and DQ2 
               
               
                 5 
                 Separate Addresses, banks configured to 
                 0 
                 1 
                 1 
               
               
                   
                 be 1-bit wide, data on lines DQ0 and DQ1 
               
               
                   
               
             
          
         
       
     
         [0087]    Core  1200  is operationally identical to core  450  of  FIG. 4B  if each of lines C0-C2 is set to logic one. In that case, the logic one on line C2 causes multiplexer  1205  to pass the address from address buffers  225  and  230  to physical banks PB0 and PB1. The logic levels on lines C0 and C1 are irrelevant in this configuration. 
         [0088]    Driving line C2 to a voltage level representative of a logic zero causes multiplexer  1205  to convey the contents of the second set of address buffers  1209  to physical banks PB0 and PB1, and additionally causes crossbar switch  1207  to respond to the control signals on lines C0 and C1. Logical banks LB0,1 and LB2,3 are thereby separated to provide independent memory access. Logical banks LB0,1 and LB2,3 are separately addressable in each of configurations two through five of Table 3. Though not shown, logical banks LB0,1 and LB2,3 can be adapted to receive either the same clock signal or separate clock signals. 
         [0089]    In configuration number two, crossbar switch  1207  accesses logical bank LB0,1 on lines DQ0 and DQ1 and logical bank LB2,3 on lines DQ2 and DQ3. Core  1200  is therefore divided into a pair of two-bit memories accessed via separate two-bit data busses. 
         [0090]    In configuration number three, crossbar switch  1207  alternatively accesses either logical bank LB0,1 or logical bank LB2,3 via lines DQ0 and DQ1. Core  1200  is therefore divided into two separately addressable two-bit memories that share a two-bit data bus. Configuration number four is similar, but access is provided via lines DQ2 and DQ3. 
         [0091]    Configuration number five divides core  1200  into two separately addressable, one-bit-wide memories. In effect, each pair of physical blocks within logical blocks LB0,1 and LB2,3 is combined to form a single-bit memory with twice the address locations of a parallel configuration. Each of the resulting one-bit-wide memories is then separately accessible via one bus line. 
         [0092]    The modes of Table 3 are not exhaustive. More control signals and/or additional control logic can be included to increase the available memory configurations. For example, configuration number five might be extended to include the ability to select the bus line upon which data is made available, or the two-bit modes could be extended to provide data on additional pairs of bus lines. 
         [0093]    The mode-select aspect allows core  1200  to efficiently support data of different word lengths. Processors, which receive instructions and data from memory like core  1200 , are sometimes asked to alternatively perform complex sets of instructions on relatively small data structures or perform simple instructions on relatively large data structures. In graphics programs, for example, the computationally simple task of refreshing an image employs large data structures, while more complex image processing tasks (e.g., texture mapping and removing hidden features) often employ relatively small data structures. Core  1200  can dynamically switch between configurations to best support the task at hand by altering the contents of mode register  220  ( FIG. 3 ). In the graphics-program example, instructions that contend with relatively large data structures might simultaneously access both logical blocks LB0,1 and LB2,3 in parallel, and instructions that contend with relatively small data structures might access logical blocks LB0,1 and LB2,3 separately using separate addresses. Core  1200  may therefore provide more efficient memory usage. As with cores  400  and  450 , core  1200  minimizes the power required to perform a row access by limiting each row access to the selected physical bank(s). 
         [0094]      FIG. 13A  is a simplified block diagram  1300  of core  1200  of  FIG. 12  illustrating memory access timing in one memory-access mode. In this example, core  1200  is configured to deliver full-width data from combined logical blocks LB2,3 and LB0,1. The pairs of memory blocks within each logical block LB2,3 and LB0,1 are combined for simplicity of illustration. At time T1, the data stored in row address location ADD0 in each of logical blocks LB2,3 and LB0,1 are each loaded simultaneously into respective sense amplifiers SA2/3 and SA0/1. The row address ADD0 used for each logical block is the same. Then, at time T2, the contents at the same column address of the two sense amplifiers are accessed simultaneously with data lines DQ3/2 and DQ1/0 via switch  1207 . Time T1 precedes time T2. 
         [0095]      FIG. 13B  is a block diagram  1310  of core  1200  of  FIG. 12  illustrating access timing in a second memory-access mode. In this example, core  1200  is configured to alternatively deliver half-width data by separately accessing logical blocks LB2,3 and LB0,1. At time T1, the contents of row address ADD0 in logical block LB2,3 loads into sense amplifiers SA2/3. At another time T2 (where T2 may be earlier or later than T1), the contents of row address ADD0 in local block LB0/1 loads into sense amplifiers SA0/1. Of interest, at each of times T1 and T2 only the accessed physical blocks are enabled using the appropriate bank-select signals BS3-0 (see  FIG. 12 ). The content at a column address of sense amplifiers SA2/3 is accessed at time T3 via the data lines DQ0/1. The content at the same column address of sense amplifiers SA0/1 is accessed at another time T4 via the data lines DQ0/1 (where T4 may be earlier or later than T3). Time T1 precedes time T3, and time T2 precedes T4. 
         [0096]      FIG. 13C  is a simplified block diagram  1315  of core  1200  of  FIG. 12  illustrating access timing in a third memory-access configuration. As in the example of  FIG. 13A , core  1200  is configured to deliver full-width data from combined logical blocks LB2,3 and LB0,1; unlike the example of  FIG. 13A , however, diagram  1315  illustrates the case in which logical blocks LB2,3 and LB0,1 are addressed separately. At time T1, the contents of row address ADD0 in logical block LB2,3 and row address ADD1 in logical block LB0,1 are loaded substantially simultaneously into respective sense amplifiers SA2/3 and SA0/1. The term “substantially simultaneous” is used here to indicate the possibility that these two operations are not precisely simultaneous (coincident), but nevertheless overlap. The content at a first column address of sense amplifiers SA2/3 is accessed at time T2 via the data lines DQ0/1. The content at a second column address of sense amplifiers SA0/1 is accessed substantially simultaneously at time T2 via the data lines DQ0/1. Time T1 precedes time T2. 
         [0097]      FIG. 13D  is a block diagram  1320  of core  1200  of  FIG. 12  illustrating access timing in a fourth memory-access mode. With respect to timing, diagram  1320  is similar to diagram  1310  of  FIG. 13B . Diagram  1320  differs from diagram  1310 , however, in that each of logical blocks LB2,3 and LB0,1 is independently addressed. Core  1200  can therefore interleave data from different addresses in logical banks LB2,3 and LB0,1 and provide the resulting data on data lines DQ1 and DQ0. Specifically, at time T1, the contents of row address ADD0 in logical block LB2,3 loads into sense amplifiers SA2/3. At another time T2 (where T2 may be earlier or later or the same as T1), the contents of another row address ADD1 in logical block LB0/1 loads into sense amplifiers SA0/1 (ADD0 and ADD1 may be the same or different). The content at a first column address of sense amplifiers SA2/3 is accessed at time T3 via the data lines DQ0/1. The content at a second column address of sense amplifiers SA0/1 is accessed at another time T4 via the data lines DQ0/1 (where T4 may be earlier or later than T3). Time T1 precedes time T3, and time T2 precedes T4. 
         [0098]      FIG. 13E  is a simplified block diagram  1325  of core  1200  illustrating access timing in a mode that delivers full-width data from combined logical blocks LB2,3 and LB0,1. With respect to timing, diagram  1325  is similar to diagram  1300  of  FIG. 13A . Diagram  1325  differs from diagram  1300 , however, in that each of logical blocks LB2,3 and LB0,1 is independently addressed. 
         [0099]      FIG. 13F  is a simplified block diagram  1330  of core  1200  illustrating access timing in a mode that delivers half-width data from independently addressed logical blocks LB2,3 and LB0,1. The flow of data in diagram  1330  is similar to that of diagram  1320  of  FIG. 13D . However, diagram  1330  differs from diagram  1320  with respect to timing because the contents of address locations ADD0 of logical block LB2,3 and ADD1 of logical block LB0,1 are delivered to respective sense amplifiers SA2/3 and SA0/1 substantially simultaneously. 
         [0100]    Although details of specific implementations and embodiments are described above, such details are intended to satisfy statutory disclosure obligations rather than to limit the scope of the following claims. Thus, the invention as defined by the claims is not limited to the specific features described above. Rather, the invention is claimed in any of its forms or modifications that fall within the proper scope of the appended claims, appropriately interpreted in accordance with the doctrine of equivalents.