Abstract:
A method and apparatus that coordinates refresh and parity-scanning in DRAM-based devices such that parity-scan operations substitute for refresh operations when both operations are required in the system. The process of parity-scanning automatically refreshes the entries being scanned, subject to refresh and parity-scan interval requirements. As such, refresh and parity-scan operations may be performed in a single operation, which bolsters the scheduling and performance of the two operations.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to dynamic memory, and more particularly to coordinating refresh commands and parity-scan commands in a dynamic random access memory-based device. 
   BACKGROUND OF THE INVENTION 
   An essential semiconductor device is semiconductor memory, such as a random access memory (RAM) device. A RAM allows a memory circuit to execute both read and write operations on its memory cells. Typical examples of RAM devices include dynamic random access memory (DRAM) and static random access memory (SRAM) devices. 
   DRAM is a specific category of RAM containing an array of individual memory cells, where each cell includes a capacitor for holding a charge and a transistor for accessing the charge held in the capacitor. The transistor is often referred to as the access transistor or the transfer device of the DRAM cell. 
     FIG. 1  illustrates a portion of a DRAM memory circuit containing two neighboring DRAM cells  100 . Each cell  100  contains a storage capacitor  102  and an access transistor or transfer device  101 . For each cell  100 , one side of the storage capacitor  102  is connected to a reference voltage (illustrated as a ground potential for convenience purposes). The other side of the storage capacitor  102  is connected to the drain of the transfer device  101 . The gate of the transfer device  101  is connected to a word line  104 . The source terminal of the transfer device  101  is connected to a bit line  103 . With the memory cell  100  components connected in this manner, it is apparent that the word line  104  controls access to the storage capacitor  102  by allowing or preventing the signal (representing a logic “0” or a logic “1”) carried on the bit line  103  to be written to or read from the storage capacitor  102 . Thus, each cell  100  contains one bit of data (i.e., a logic “0” or logic “1”). 
   Another form of memory is the content addressable memory (CAM) device. A CAM is a memory device that accelerates any application requiring fast searches of a database, list, or pattern, such as in database machines, image or voice recognition, or computer and communication networks. CAMs provide benefits over other memory search algorithms by simultaneously comparing the desired information (i.e., data in a comparand register) against an entire list of pre-stored entries. As a result of their unique searching algorithm, CAM devices are frequently employed in network equipment, particularly router and switches, computer systems and other devices that require rapid content searching. 
   In order to perform a memory search in the above-identified manner, CAMs are organized differently than other memory devices (e.g., DRAM). For example, data is stored in a RAM in a particular location, called an address. During a memory access, the user supplies an address and writes into or reads the data at the specified address. 
   In a CAM, however, data is stored in locations in a somewhat random fashion. The locations can be selected by an address bus, or the data can be written into the first empty memory location. Every memory location includes one or more status bits that maintain state information regarding the memory location. For example, each memory location may include a valid bit whose state indicates whether the memory location stores valid information, or whether the memory location does not contain valid information (and is therefore available for writing). 
   Once information is stored in a CAM memory location, the information may be found by comparing every bit of memory with data in the comparand register. When the content stored in the CAM memory location does not match the data in the comparand register, a local match detection circuit returns a no match indication. When the content stored in the CAM memory location matches the data in the comparand register, the local match detection circuit returns a match indication. If one or more local match detect circuits return a match indication, the CAM device returns a “match” indication. Otherwise, the CAM device returns a “no-match” indication. In addition, the CAM may return the identification of the address location in which the desired data is stored (or one of such addresses if more than one address contained matching data). Thus, with a CAM, the user supplies the data and gets back the address if there is a match found. 
     FIG. 2  is a circuit diagram showing a conventional DRAM-based CAM cell  200  that includes two one-transistor DRAM cells  210   a ,  210   b  and a four-transistor comparator circuit  220  made up of transistors Q 2 , Q 3 , Q 5  and Q 6 . The DRAM cells  210   a ,  210   b  are used to store values in the CAM cell  200 . Generally, the content of the first DRAM cell  210   a  is the logical complement of the content of the second DRAM cell  210   b . However, the cells  210   a ,  210   b  may also store the same values, i.e., “1”, “1”, or “0”, “0”, when so desired. 
   The first DRAM cell  210   a  includes transistor Q 1  and capacitor CA, which combine to form storage node A that receives a data value from bit line BL 1  at node U during write operations, and applies the stored data value to the gate terminal of transistor Q 2  of the comparator circuit  220 . Transistor Q 2  is connected in series with transistor Q 3 , which is controlled by a data signal transmitted on data line D 1 , between a match line M and a discharge line D. The second DRAM cell  210   b  includes transistor Q 3  and capacitor CB, which combine to form storage node B that receives a data value from bit line BL 2  at node V, and applies the stored data value to the gate terminal of transistor Q 5  of the comparator circuit  220 . Transistor Q 5  is connected in series with transistor Q 6 , which is controlled by a data signal transmitted on inverted data line D 1 #, between the match line M and the discharge line D. 
     FIG. 3  is a block diagram of a portion of a CAM device  300  that includes a plurality of CAM cells such as the CAM cell  200  illustrated in  FIG. 2 . For purposes of simplicity, only a portion of the CAM device  300  is illustrated in  FIG. 3 . In particular, some well known components such as e.g., the previously discussed comparand register, control logic, and input/output logic are not illustrated merely to simplify  FIG. 3 . The CAM device  300  includes two arrays  310   a ,  310   b  of CAM cells  200 . Each array  310   a ,  310   b  includes its own bit lines (i.e., BL 11 –BL 16  for the first array  310   a , BL 21 –BL 26  for the second array  310   b ) and word lines (i.e., WL 11 –WL 13  for the first array  310   a  and WL 21 –W 23  for the second array  310   b ). Each word line WL 11 –WL 13 , WL 21 –WL 23  is also coupled to a respective word line driver  320   a ,  320   b . Similarly, each bit line BL 11 –BL 16 , BL 21 –BL 26  is also coupled to respective bit line drivers (not illustrated). The CAM device  300  also includes a plurality of sense amplifiers  330 . Each sense amplifier  330  is coupled to the CAM cells  200  connected to two different bit lines (e.g., bit lines BL 11 , BL 21 ) from two different arrays  310   a ,  310   b . This type of architecture, where a sense amplifier is coupled to bit lines from different arrays, is generally known as an open bit line architecture. 
   One of the drawbacks associated with DRAM cells is that the charge on the storage capacitors may naturally decay over time, even if the capacitors remain electrically isolated. Thus, DRAM cells require periodic refreshing. Additionally, refreshing is also required after a memory cell has been accessed, for example, as part of a read operation. 
   In DRAM-based devices, refresh commands are issued periodically to keep the contents of the DRAM memory array maintained at their previously stored values. The refresh operations have the effect of restoring charges lost from DRAM cells due to leakage currents. Refresh operations are also essential for ensuring that data in the DRAM memory is not corrupted over time. For proper DRAM operation, the device adheres to minimum operating specifications and maintains a periodic interval for issuing refresh commands. 
   Refresh commands may be issued explicitly to the DRAM-based device from another device such as e.g., a memory controller. Refresh operations may also be internally generated during idle cycles, with the controller concurrently ensuring that a sufficient number of idle cycles are interspersed in the command stream to meet the device&#39;s refresh requirements. This is known as a “self-refresh.” 
   Although refreshes allow DRAM memory to retain previously stored values, soft-errors and coupling defects can still occur that would falsely toggle a stored bit. If this erroneous toggling has occurred, future refresh operations would refresh the false value. One method of mitigating the problem of false toggling of stored data bits includes storing parity bits with the data. The memory array is periodically parity-scanned to determine if any errors have occurred. Parity-scanning may be performed internally whenever there are idle cycles. System hardware running in the background monitors the command stream to see when it can insert a parity-scan command. However, unlike refresh operations, parity-scanning is typically performed on a best-effort policy (as opposed to all inclusive policy). This means that all rows may not have been scanned by the end of the parity-scan time interval. 
   Accordingly, there is a desire and need to coordinate refresh and parity-scan operations in a DRAM-based device. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a method and apparatus for coordinating refresh and parity-scan operations in a DRAM-based device. 
   The process of parity-scanning involves reading entries from the memory array, and checking that the parity bits that were stored with the entries are still consistent with current parity calculations. For most DRAM devices, read operations automatically refresh the entry being read, restoring charges that may have leaked away from the DRAM cells over time. Therefore, as parity-scan involves reading the array as part of its process, it may be considered as a viable substitute for refresh operations whose sole purpose is to renew rather than check the content of stored entries, but not vice versa. 
   The above and other features and advantages are achieved in various embodiments of the invention by providing a method and apparatus that coordinates refresh and parity-scanning commands in DRAM-based devices such that parity-scan operations substitute for refresh operations when both operations are required in the system. The process of parity-scanning automatically refreshes the entries being scanned, subject to refresh and parity-scan interval requirements. As such, refresh and parity-scan operations may be performed in a single operation, which bolsters the scheduling and performance of the two operations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The forgoing and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings, in which: 
       FIG. 1  is a circuit diagram depicting conventional dynamic random access memory (DRAM) cells; 
       FIG. 2  is a circuit diagram of a conventional six-transistor DRAM based CAM cell; 
       FIG. 3  is a block diagram depicting a conventional CAM device; 
       FIG. 4  is a diagram illustrating an exemplary circuit for coordinating refresh and parity-scanning operations constructed in accordance with an embodiment of the invention; 
       FIG. 5  is a block diagram depicting an exemplary semiconductor chip implementing the refresh and parity scan logic illustrated in  FIG. 4 ; 
       FIG. 6  is a simplified block diagram depicting a packet router employing the  FIG. 5  semiconductor chip in accordance with another exemplary embodiment of the invention; and 
       FIG. 7  is a block diagram depicting a processor system in accordance with another exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention overlaps and coordinates the refresh and parity-scan operations in DRAM devices and DRAM-based devices (such as e.g., CAM devices). The two operations are not treated as separate processes. Both operations are periodic and involve reading the contents of the DRAM memory. As used herein, “parity refresh” or “refresh scan” refers to any operation that performs both refresh and parity-scan functions on the DRAM memory cells. 
   The following definitions are used herein: numRows=number of rows per bank; numBanks=number of banks; numRowsTot=numRows*numBanks=number of total rows; tRef=maximum refresh interval for a row; tScanTot=maximum time interval by the end of which it is desirable to have all rows parity-scanned; and tScan=tScanTot/numRows=maximum parity-scan interval for a row. 
   If tRef&lt;tScan, refresh operations will occur more frequently than parity-scan operations. Each refresh command, whether generated internally as a self-refresh operation or issued explicitly from another device, may cause rows at the same row address (but residing in different banks) to be refreshed simultaneously. Alternatively, each refresh command may cause only one row in one bank to be refreshed at a time, proceeding from one bank to the next only when all rows in the former bank have been refreshed. The first approach of refreshing rows at the same row address simultaneously requires fewer refresh commands to refresh the entire memory array, but will consume more power per refresh than the second approach. 
   Likewise, each parity-scan command, whether generated internally or issued from another device, may cause rows at the same row address (but residing in different banks) to be parity-scanned simultaneously, or may cause only one row in one bank to be parity-scanned at a time, proceeding from one bank to the next only when all rows in the former have been scanned. The first approach of scanning rows at the same row address simultaneously requires fewer parity-scan commands to scan the entire memory array, but will consume more power per parity-scan and may require more circuitry to implement than the second approach. 
   The scenario where only one row in one bank is refreshed or parity-scanned at a time may be logically treated in the following discussions as numBanks=1 (although physically the device may have multiple banks; numBanks is used in this situation to denote the number of banks that are being refreshed or parity scanned simultaneously), and numRows=numRowsTot, since it would require numRowsTot refreshes or parity-scanning operations to go through the entire array once. 
   Referring to the drawings, an exemplary embodiment of a circuit  400  for coordinating refresh and parity-scanning is shown in  FIG. 4 . The circuit  400  may be considered as having four portions: configuration register fields  400   a , command register fields  400   b , status register fields  400   c  and control logic  400   d.    
   The configuration register fields  400   a  include tRef and tScan (defined above) configuration register fields  401 ,  403 , which may be either hardwired or initialized to certain values upon reset. A continue_scan register field  402  (described below) is also included. The command register fields  400   b  include parity_scan and self_refresh enable register fields  407 ,  408 . The status register fields  400   c  include starting_scan_address and scan_completed register fields  409 ,  410 . The use of the configuration register fields  400   a , command register fields  400   b  and status register fields  400   c  is discussed below in more detail. 
   The control logic  400   d  includes minimum logic  404 , an interval counter  405 , a row/page counter  406 , coordinating logic  411 , self-refresh logic  412 , parity-scan logic  413 , and four multiplexers  414 ,  415 ,  416 ,  417 . The components within the control logic  400   d  are described below in more detail. 
   Minimum logic  404  is a comparator that selects the smaller of the tRef and tScan configuration register fields  401 ,  403  and sends that value to the interval counter  405 . The number of rows per bank (numRows) is hardwired or stored in the interval counter  405 . The interval counter  405  takes the value selected by the minimum logic  404  and divides that value by numRows to generate a wrap-around limit, such that whenever the interval counter  405  reaches this limit and returns to zero, an increment signal is sent to the row/page counter  406 . The row/page counter  406  tracks the next row to be refreshed or parity-scanned based on the increment signal. 
   The self-refresh logic  412  and parity-scan logic  413  use the row address tracked by row/page counter  406  to generate sequences of command, bank enable, and/or row/page addresses, which are appropriate for their respective operations. The first three multiplexers  414 ,  415 ,  416 , as controlled by the coordinating logic  411 , select the appropriate command (i.e., refresh or read), bank enable, or row/page address and output the selected information to the fourth multiplexer  417 . The fourth multiplexer  417 , as controlled by the coordinating logic  411 , selects and outputs one of the outputs from the first three multiplexers  414 ,  415 ,  416 , or an output from other command/address sources or decoders. 
   In addition, the parity-scan logic  413  contains hardware to check the parities returned from memory to ensure data integrity. The parity-scan enable and self-refresh enable register fields  407 ,  408  may be provided to turn off either the parity-scan or self-refresh logic  413 ,  412 . The coordinating logic  411  checks the command stream for idle cycles to insert parity-scan or refresh commands, upon selecting between the two operations. 
   The row after the last row that was refreshed or scanned (before the current tScanTot period expires) can be saved into the starting_scan_address status register field  409 . This value then serves as the starting address for the next scan iteration. The scan_completed status register field  410  may be provided to indicate that all rows have been scanned before the end of a scan period, if set. 
   There are four scenarios where refresh operations may be implemented. Two of the scenarios include internally generated refresh and parity-scan operations within the device for the cases where tRef&lt;tScan and tRef&gt;tScan. The other two scenarios use externally generated refreshes, while parity-scanning is internally generated, for the cases where tRef&lt;tScan and tRef&gt;tScan. 
   For the first scenario (i.e., internally generated refresh and parity-scan for tRef&lt;tScan), the rate at which refresh operations occur is high enough (usually guaranteed by the presence of adequate idle cycles from external logic) that parity-scanning would most likely be completed through all of the rows within a tScanTot period. At least initially, parity-scan commands can be issued for the dual-purpose of performing scan and refresh operations. Upon completion of scanning through all rows and before tScanTot has expired, system requirements may specify:
     (1) A wrap-around to the starting row, and continue with parity-scanning. This would effectively replace refresh operations with parity-scan operations, until the parity-scan logic  413  is disabled (by clearing the parity_scan enable command register field  407 ); or   (2) The stopping of parity-scanning operations and continuing with refresh operations until the start of the next tScanTot period. This may result from the parity-scans consuming more power than desired. In this case, the coordinating logic  411  would switch to the output of the self-refresh logic  412 , whenever there are idle cycles, for the remainder of the current tScanTot period. The preference for which approach to follow may be indicated in the continue_scan configuration register field  402 .   

   For the second scenario (i.e., internally generated refresh for tRef&gt;tScan), the external logic usually guarantees enough idle cycles to meet refresh requirements, but may or may not guarantee enough idle cycles to satisfy the higher parity-scan frequency. As previously described, the policy for parity-scan may be best-effort only, and it is possible that not all of the rows will be scanned within a tScanTot period. However, since at least the required refresh frequency is guaranteed to be met, parity-scanning can be used for the dual-purpose of scan and refresh throughout the device&#39;s operation, until the parity-scan logic  413  is disabled, at which point the coordinating logic  411  switches the outputs of the first three multiplexers  414 ,  415 ,  416  to the output of the self-refresh logic  412  exclusively. 
   If all of the rows were not scanned by the end of a scan period (as indicated by comparing the starting_scan_address  409  and the output of row/page counter  406 ), then the scan_completed status register field  410  can be cleared so that a host ASIC or external controller can poll it and become notified. The next scan period would begin with the next row to be scanned, with its address stored in the starting_scan_address register field  409 , so that all rows can be refreshed equally during the process. 
   The third scenario (i.e., externally generated refresh for tref&lt;tScan) is similar to the first scenario (i.e., internally generated refresh for tRef&lt;tScan). A difference arises, however, from having the refreshes explicitly issued to the device from an external mechanism. As explained above, parity-scan operations may initially be used to substitute for the refresh operations until all rows have been scanned (prior to the end of tScanTot period). Thereafter, either refresh or parity-scan commands can be used to satisfy refresh requirements, as indicated by the continue_scan configuration field  402 . If the continue-scan field is set, indicating to continue using parity-scan to substitute for refresh in spite of the fact that all rows have already been scanned within the current tScanTot interval, then the refresh logic may be disabled. Upon receiving an external refresh command, the coordinating logic may direct the parity-scan logic to send a parity-scan command in its place. 
   The fourth scenario (i.e., externally generated refresh for tRef&gt;tScan) is similar to the second scenario (i.e., internally generated refresh for tRef&gt;tScan). A difference arises, however, from having the refreshes explicitly issued to the device from an external mechanism. As explained above, parity-scan operations may be used to substitute for the refresh operations throughout the DRAM-based device&#39;s operation (at least when parity-scan is enabled). If all of the rows are not scanned by the end of tScanTot period, it is preferable that scanning in the next scan period be resumed from the current row location, rather than starting from the first row; this way, all rows can be refreshed equally. The self-refresh logic  412  may be disabled, and the command stream is fed to both the coordinating logic  411  and the first three multiplexers  414 ,  415 ,  416 . 
     FIG. 5  is a semiconductor chip  500  embodying the refresh and parity-scanning logic of the invention. The semiconductor chip  500  incorporates refresh/parity scan logic  400  constructed in accordance with the invention as shown in  FIG. 4 . The semiconductor chip  500  also comprises control logic  502 , an input/output port  503 , addressing logic  504 , read/write buffers  505  and DRAM-based memory cells  506 . 
     FIG. 6  is a simplified block diagram of an embodiment of a packet router  600  as may be used in a communications network, such as, e.g., part of the Internet backbone. The router  600  contains a plurality of input lines and a plurality of output lines. When data is transmitted from one location to another, it is sent in a form known as a packet. Oftentimes, prior to the packet reaching its final destination, that packet is first received by a router, or some other device. The router  600  then decodes that part of the data identifying the ultimate destination and decides which output line and what forwarding instructions are required for the packet. 
   Generally, devices such as CAMs utilizing DRAM based memory devices are very useful in router applications because historical routing information for packets received from a particular source and going to a particular destination is stored in the DRAM of the CAM device in the router. As a result, when a packet is received by the router  600 , the router already has the forwarding information stored within its CAM. Therefore, only that portion of the packet that identifies the sender and recipient need be decoded in order to perform a search of the CAM to identify which output line and instructions are required to pass the packet onto a next node of its journey. 
   Still referring to  FIG. 6 , router  600  contains the added benefit of employing a semiconductor memory chip containing an array of cascaded CAM devices with DRAM memory, and also employing hardware for overlapping and coordinating refresh and parity-scanning in the DRAM memory, such as semiconductor chip  500  depicted in  FIG. 5 . 
     FIG. 7  illustrates an exemplary processing system  700  that utilizes a hardware device including for example, the device on semiconductor chip  500  of  FIG. 5 . The processing system  700  includes one or more processors  701  coupled to a local bus  704 . A memory controller  702  and a primary bus bridge  703  are also coupled the local bus  704 . The processing system  700  may include multiple memory controllers  702  and/or multiple primary bus bridges  703 . The memory controller  702  and the primary bus bridge  703  may be integrated as a single device  706 , which may include the hardware on semiconductor chip  500 . 
   The memory controller  702  is also coupled to one or more memory buses  707 . Each memory bus accepts memory components  708 . Any one of memory components  708  may alternatively contain a hardware device such as the device described in connection with  FIG. 4 . 
   The memory components  708  may be a memory card or a memory module. The memory components  708  may include one or more additional devices  709 . For example, in a SIMM or DIMM, the additional device  709  might be a configuration memory, such as a serial presence detect (SPD) memory and may additionally or alternatively contain a hardware device such as the device described in connection with  FIG. 4 . The memory controller  702  may also be coupled to a cache memory  705 . The cache memory  705  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  701  may also include cache memories, which may form a cache hierarchy with cache memory  705 . If the processing system  700  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  702  may implement a cache coherency protocol. If the memory controller  702  is coupled to a plurality of memory buses  707 , each memory bus  707  may be operated in parallel, or different address ranges may be mapped to different memory buses  707 . 
   The primary bus bridge  703  is coupled to at least one peripheral bus  710 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  710 . These devices may include a storage controller  711 , a miscellaneous I/O device  714 , a secondary bus bridge  715 , a multimedia processor  718 , and a legacy device interface  720 . The primary bus bridge  703  may also be coupled to one or more special purpose high speed ports  722 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system  700 . 
   The storage controller  711  couples one or more storage devices  713 , via a storage bus  712 , to the peripheral bus  710 . For example, the storage controller  711  may be a SCSI controller and storage devices  713  may be SCSI discs. The I/O device  714  may be any sort of peripheral. For example, the I/O device  714  may be a local area network interface, such as an Ethernet card. The secondary bus bridge  715  may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge  715  may be a universal serial port (USB) controller used to couple USB devices  717  to the processing system  700 . The multimedia processor  718  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to additional devices such as speakers  719 . The legacy device interface  720  is used to couple legacy devices  721 , for example, older styled keyboards and mice, to the processing system  700 . 
   The processing system  700  illustrated in  FIG. 7  is only an exemplary processing system with which the invention may be used. While  FIG. 7  illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system  700  to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU  701  coupled to memory components  708  and/or memory devices  709 . The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices. 
   The parity-scan/refresh logic of the invention is preferably intended to be used as part of a DRAM-based memory, which in turn may be a component within a computer or other hardware system using DRAM to store and retrieve information, such as e.g., routers. If the DRAM-based memory is used within a computer system, the memory may be integrated into the microprocessor chip, i.e., as an on-chip cache; may operate on a separate chip that interfaces with the microprocessor via a high-speed processor bus; or may operate as an I/O device communicating with the microprocessor via an I/O bus such as the PCI bus. If the DRAM-based memory is used within an application-specific system such as routers, then it may be integrated as a subcomponent within a microprocessor or a host ASIC controlling its operation, or may interface to them via a system bus. 
   If the DRAM-based memory (and therefore the parity-scan/refresh logic of the invention) is used within a computer system, then the command and configuration registers may be written by a microprocessor. For example, if the memory design is a device residing on a PCI bus, the microprocessor can write to these registers by issuing a PCI write command. Likewise, the status registers may be read by a microprocessor in this case with a PCI read command. If the DRAM-based memory resides on the same chip as the microprocessor, then logic internal to the chip can set the command and configuration registers appropriately as well as read the status registers directly. 
   If the DRAM-based memory (and therefore the parity-scan/refresh logic of the invention) is used within an application-specific system such as a router, then the command and configuration registers may be written by a microprocessor within the router, or a host ASIC which controls the operation of the memory device. 
   While certain embodiments of the invention have been described and illustrated above, the invention is not limited to these specific embodiments as numerous modifications, changes and substitutions of equivalent elements can be made without departing form the spirit and scope of the invention. For example, although the invention has been described in connection with a specific circuit employing a configuration of register fields and logic, the invention may be practiced with many other configurations without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not to be considered as limited by the specifics of the particular structures and processes which have been described and illustrated herein, but is only limited by the scope of the appended claims.