Abstract:
A memory system comprises a nonvolatile memory comprising a memory block having multiple pages, and a controller configured to control the nonvolatile memory to store data in the memory block according to a command and logical address received from an external source. The controller is configured to determine whether the logical address is currently mapped to a bad page of the memory block by referring to a bad page map, and as a consequence of determining that the logical address corresponds to the bad page, remaps the logical address to a different page and stores dummy data in the bad page.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2013-0018066 filed on Feb. 20, 2013, the subject matter of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The inventive concept relates generally to electronic memory technologies. More particularly, certain embodiments of the inventive concept relate to memory systems that perform address mapping for a nonvolatile memory device using a bad page map. 
     Memory devices may be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. 
     Examples of volatile memory devices include static random access memory (SRAM) devices, dynamic random access memory (DRAM) devices, and synchronous DRAM (SDRAM) devices. Examples of nonvolatile memory devices include flash memory devices, read only memory (ROM) devices, programmable ROM (PROM) devices, electrically erasable and programmable ROM (EEPROM) devices, and various forms of resistive memory such as phase-change RAM (PRAM), ferroelectric RAM (FRAM), and resistive RAM (RRAM). 
     Most nonvolatile memory devices tend to wear out at a rate determined by usage. For instance, flash memory devices tend to wear out at a rate determined by the number of erase or program operations that have been performed. Where certain memory cells are used more often than others, they may wear out sooner, resulting in localized regions of defective or unreliable cells, such as “bad blocks”, “bad pages”, “bad sectors”, and so on. 
     To preserve reliability in the face of local deterioration, memory systems that incorporate flash memory devices and other types of nonvolatile memory devices typically include mechanisms for managing memory cells that have worn out. One technique is to remap addresses of defective regions to non-defective regions. Such remapping, however, may unduly increase the overhead of memory management, and it may also prevent some pages of memory from being used because they belong to a block that has been deemed worn out. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the inventive concept, a memory system comprises a nonvolatile memory comprising a memory block having multiple pages, and a controller configured to control the nonvolatile memory to store data in the memory block according to a command and logical address received from an external source. The controller is configured to determine whether the logical address is currently mapped to a bad page of the memory block by referring to a bad page map, and as a consequence of determining that the logical address corresponds to the bad page, remaps the logical address to a different page and stores dummy data in the bad page. 
     In another embodiment of the inventive concept, a memory system comprises a nonvolatile memory comprising a memory block having multiple pages, and a controller configured to control the nonvolatile memory to store data in the memory block according to a command and logical address received from an external source. The controller is configured to determine whether the logical address is currently mapped to a bad page of the memory block by referring to a bad page map, and as a consequence of determining that the logical address corresponds to the bad page, remaps the logical address to a different page and stores dummy data in the bad page. The controller is further configured to determine whether a number of times that an erase operation has been performed on the memory block has reached a predetermined reference value, and as a consequence of determining that the number of times that an erase operation has been performed on the memory block has reached the predetermined reference value, update the bad page map based on a bad page list comprising information indicating whether each of multiple different pages is a bad page according to different numbers of erase values. 
     In another embodiment of the inventive concept, a method is provided for operating a memory system comprising a nonvolatile memory. The method comprises controlling the nonvolatile memory to store data in the memory block according to a command and logical address received from an external source, determining whether the logical address is currently mapped to a bad page of the memory block by referring to a bad page map, and, as a consequence of determining that the logical address corresponds to the bad page, remapping the logical address to a different page and stores dummy data in the bad page. 
     These and other embodiments of the inventive concept can potentially increase the lifetime and performance of memory cells by managing remapping operations on a page-by-page basis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate selected embodiments of the inventive concept. In the drawings, like reference numbers indicate like features. 
         FIG. 1  is a block diagram illustrating a memory system in accordance with an embodiment of the inventive concept. 
         FIG. 2  is a diagram illustrating a bad page map in accordance with an embodiment of the inventive concept. 
         FIG. 3  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. 
         FIG. 4  is a table illustrating an example of a bad page list. 
         FIG. 5  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. 
         FIG. 6  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. 
         FIG. 7  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. 
         FIG. 8  is a flowchart illustrating a method of writing data of a nonvolatile memory device in accordance with an embodiment of the inventive concept. 
         FIG. 9  is a block diagram illustrating a solid state drive (SSD) comprising a memory system in accordance with an embodiment of the inventive concept. 
         FIG. 10  is a block diagram illustrating a memory card comprising a nonvolatile memory device in accordance with an embodiment of the inventive concept. 
         FIG. 11  is a drawing illustrating various systems comprising a memory card in accordance with embodiments of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the inventive concept are described below with reference to the accompanying drawings. These embodiments are presented as teaching examples and should not be construed to limit the scope of the inventive concept. 
       FIG. 1  is a block diagram illustrating a memory system  10  in accordance with an embodiment of the inventive concept. 
     Referring to  FIG. 1 , memory system  10  comprises a nonvolatile memory device  100  and a host  101 . Nonvolatile memory device  100  comprises a controller  110  and a nonvolatile memory  120 . Controller  110  comprises a controller processor  111  and a controller memory. 
     During typical operation, nonvolatile memory device  100  classifies a page in which data is damaged or at risk or damage as a bad page. Then, in an address mapping operation, nonvolatile memory device  100  does not map a logical address to a physical address of the page classified as a bad page. 
     Because nonvolatile memory device  100  prevents data from being stored in bad pages, it can have improved lifetime and improved accuracy. In a write operation, nonvolatile memory device  100  programs dummy data in a page classified as a bad page to improve a write speed. 
     Host  101  is configured to access nonvolatile memory device  100 . Host  101  relies on nonvolatile memory device  100  to store data that is generated or used by various functions it performs. In other words, nonvolatile memory device  100  stores data processed by host  101 . 
     Controller  110  provides an interface between nonvolatile memory  120  and host  101 . Controller  110  drives firmware to control nonvolatile memory  120 . Controller  110  controls read, write, and erase operations of nonvolatile memory  120  using the firmware in response to a request of host  101 . 
     Controller processor  111  controls operations of controller  110 . In certain implementations, controller processor  111  drives firmware for controlling nonvolatile memory  120 . 
     Controller memory  112  can operate as a working memory of controller  110 , a buffer memory between host  101  and nonvolatile memory  120  and a cache memory of nonvolatile memory  120 . 
     Nonvolatile memory  120  stores data under control of controller  110 . The type of nonvolatile memory  120  may be, for instance, ROM, programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), flash memory, PRAM, MRAM, RRAM, or FRAM, for example, although it is not limited to these types of memory. 
     In a write operation, host  101  provides write-requested data and a logical address of the data to nonvolatile memory device  100 . Nonvolatile memory device  100  stores the data in nonvolatile memory  120  in response to a request of host  101 . 
     A flash translation layer (FTL) is stored in controller memory  112  of controller  110 . A bad page map representing a bad page is also stored in controller memory  112 . The FTL and the bad page map may be loaded from a nonvolatile memory into controller  110  into controller memory  112 , or from nonvolatile memory  120  into controller memory  112 . 
     Where controller  110  receives a write request, controller  110  maps a logical address LA provided from host  101  to a physical address PA of nonvolatile memory  120  using the FTL. In a mapping operation, controller  110  precludes a logical address from being mapped to a physical address of a bad page by referring to the bad page map. 
     In a write operation, controller  110  controls nonvolatile memory  120  so that dummy data is written in a bad page. Where multiple pages are programmed at a time, controller  110  controls nonvolatile memory  120  so that a bad page and a normal page are programmed together with each other by programming dummy data in a bad page. 
     Controller  110  updates a bad page map in response to a program/erase cycle or elapsed time. Controller  110  updates a bad page map to continuously classify as bad pages those where data is damaged or at risk of damage. 
     Nonvolatile memory device  100  can classify a page in which data is damaged or at risk of damage as a bad page to prevent a logical address from being mapped to the classified bad page. The management of damaged memory cells on a page-by-page basis can potentially improve the lifetime of nonvolatile memory device  100  compared with management on a block-by-block basis. 
     Although nonvolatile memory device  100  is described as selecting and managing a bad page, the relevant unit could alternatively be defined by a word line. For example, nonvolatile memory device  100  may classify a word line connected to a page in which data is damaged or at risk of damage as a bad word line and can prevent a logical address from being mapped to pages connected to the classified bad word line. In this case, nonvolatile memory device  100  can classify a bad page as a page sharing a word line with a bad page to exclude that a logical address is mapped. 
     Nonvolatile memory device  100  can use various algorithms to classify a page in which data is damaged or at risk of damage as a bad page. Nonvolatile memory device  100  can continuously update a bad page map. 
       FIG. 2  is a diagram illustrating a bad page map in accordance with an embodiment of the inventive concept. In the embodiment of  FIG. 2 , the bad page map has a form of bitmap. However, this is as an illustration and the inventive concept is not limited to the bitmap. Alternatively, for instance, the bad page map may have a form of a list or tree structure. 
     Referring to  FIG. 2 , the bad page map comprises bits corresponding to each physical page of nonvolatile memory  120 . In the bad page map, a bit corresponding to a page classified as a bad page may be set to be 1. A bit corresponding to a page classified as a normal page may be set to be 0. With reference to the bad page map, in the case that a page is represented by a bad page, controller  110  may not map a logical address to a physical address of the corresponding page. 
       FIG. 3  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. In the method of  FIG. 3 , a bad page map is updated according to the number of program/erase cycles performed on a selected block as well as a bad page list. The bad page list indicates pages determined to be unreliable (i.e., damaged or at risk of being damaged) when reaching a certain number program/erase cycles. 
     As an example,  FIG. 4  is a table illustrating a bad page list. In this example, the bad page list comprises bits representing whether each page is determined to be a bad page or not with respect to different numbers program/erase cycles. For example, a “1” under column labeled “10” indicates that a corresponding page is damaged or at risk of being damaged after 10 program/erase cycles. 
     In general, pages of nonvolatile memory  120  have different physical characteristics from each other. A physical characteristic of each of the pages of nonvolatile memory  120  can be evaluated in advance using experimental data with respect to a predetermined sample. Accordingly, the bad page list can be generated with reference to an evaluated physical characteristic of each page. 
     In a data processing operation, control processor  111  loads a bad page list in controller memory  112 . Controller processor  111  performs a mapping operation with reference to the bad page list. The bad page list is loaded from nonvolatile memory  120  into controller memory  112 . The bad page list may be loaded from a nonvolatile memory in controller  110  into controller memory  112 . 
     Referring to  FIG. 3 , the method performs a program or erase operation on a selected page or memory block of nonvolatile memory  120  (S 110 ). Then, the method determines, for the selected page or pages among the selected memory block, whether a corresponding program/erase count is greater than or equal to a corresponding reference value (S 110 ). The corresponding reference value may be determined by consulting a bad page list. Typically, the corresponding reference value, for the selected page or a page among the selected block, is a lowest number of program/erase cycles in the bad page list for which that page is marked as being unreliable. For instance, using the example bad page list of  FIG. 4 , the reference value for a fifth page in the list is “10” because the page has a “1” under the column marked “10”. 
     If the count is not greater than or equal to the reference value (S 120 =No), the count is increased (S 125 ). After the count increases, the method is completed. Otherwise, if the count is greater than or equal to the reference value, the bad page map is updated with reference to the bad page list (S 130 ). The bad page map classifies pages determined to be bad pages in the bad page list with respect to the current count with reference to the bad page list. Finally, the count is increased (S 140 ), and method is completed. 
     As indicated by the above description, the method of  FIG. 3  updates of the bad page map based on information stored in the bad page list, in combination with the number of program/erase cycles of each page. 
       FIG. 5  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. In the method of  FIG. 5 , the bad page map is updated in response to a bit error rate (BER). 
     Referring to  FIG. 5 , a write operation is performed on nonvolatile memory  120  (S 210 ). Thereafter, data stored in pages on which the write operation of step S 210  is performed is read out (S 220 ). A BER is calculated for the data read from each page. 
     Next, pages representing a bit error rate higher than a predetermined threshold value are classified as bad pages (S 230 ). The bad page map is updated based on results of the classification. The predetermined threshold value is stored in nonvolatile memory device  100 . The predetermined threshold value may be set in response to an external signal provided from host  101 . 
     A threshold voltage of a memory cell in each page of nonvolatile memory  120  is changed with the lapse of time. A threshold voltage of a memory cell is changed by an interference phenomenon by adjacent memory cells. Examples of the interference phenomenon are a F-poly coupling and lateral charge spreading. 
     Besides interference by adjacent memory cells, a threshold voltage of a memory cell may be changed by read disturbance. A threshold voltage of a memory cell may be changed by a reduction of the quantity of charges of a memory cell over time. 
     A threshold voltage of a memory cell may be changed by various factors besides the factors described above. A threshold voltage may be changed by factors such as a process fail, distortion due to channel instability and program disturbance. 
     Because a threshold voltage of a memory cell is changed as time passes, data stored in each page of nonvolatile memory  120  may become unstable as time goes by. Reliability of data stored in each page of nonvolatile memory  120  may become highest immediately after a write operation is performed. 
     The method of  FIG. 5  determines whether a page is a bad page or not on the basis of a bit error rate of data read from the page right after a write operation is performed. Because the method reads out data right after a write operation is performed, it can reduce an effect by read disturbance or factors changing a threshold voltage in the process of judging whether a page is a bad page. 
       FIG. 6  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. In the method of  FIG. 6 , the bad page map is updated in response to a program/erase cycle and a BER. 
     Referring to  FIG. 6 , a program or erase operation is performed on nonvolatile memory  120  (S 310 ). Thereafter, a count and a reference value are compared with each other (S 320 ). The count represents the number of program/erase cycles that have been performed on a selected block. If the count does not reach the reference value (S 320 =No), the count increases and the method ends (S 325 ). The reference value may be stored in nonvolatile memory device  100 . The reference value may be set in response to an external signal provided from host  101 . If the count is greater than the reference value (S 320 =Yes), data stored in pages of the selected block is read out (S 330 ). A bit error rate is calculated for the read data. 
     Next, pages having a bit error rate higher than a predetermined threshold value are classified as bad pages (S 340 ). On the basis of a classification result, a bad page map is updated. Thereafter, a value of the count is reset and the update operation is over (S 350 ). 
     As indicated by the above description, the method of  FIG. 6  determines whether a page is a bad page or not on the basis of a BER when the number of program/erase cycles reaches the predetermined reference value. In the method, a BER measured in response to a program/erase cycle with respect to the selected block may be considered. 
       FIG. 7  is a flowchart illustrating a method of updating a bad page map in accordance with an embodiment of the inventive concept. In the method of  FIG. 7 , the bad page map is updated in response to time that elapsed after a write operation is performed and a BER. 
     Referring to  FIG. 7 , a program or erase operation is performed on nonvolatile memory  120  (S 410 ). Next, an elapsed time and a reference value are compared with each other in a selected page (S 420 ). The elapsed time is time that elapsed after a write operation is performed in the selected page or after the elapsed time is reset. If the elapsed time has not reached the reference value (S 420 =No), the method ends. The reference value is stored in nonvolatile memory device  100 . The reference value may be set in response to an external signal being provided from host  101 . 
     If the elapsed time is greater than the reference value (S 420 =Yes), data stored in pages of the selected block is read out (S 430 ). A bit error rate is calculated for the read data. 
     Pages having a bit error rate higher than a predetermined threshold value are classified as bad pages (S 440 ). On the basis of a classification result, a bad page map is updated. Thereafter, the elapsed time is reset and the method ends (S 450 ). 
     As indicated by the above description, the method of  FIG. 7  periodically determines whether a page is a bad page or not at every predetermined time interval. For example, the method can classify pages having a bit error rate higher than a predetermined threshold value when time of a certain percentage, for example, 80%, of data retention time elapsed as a bad page. According to the above method, a bit error rate measured in response to time that elapsed after a write operation is performed may be considered. 
       FIG. 8  is a flowchart illustrating a method of writing data of a nonvolatile memory device in accordance with an embodiment of the inventive concept. In the method of  FIG. 8 , nonvolatile memory device  100  prevents a logical address from being mapped to a physical address of a page classified as a bad page with reference to a bad page map. Because nonvolatile memory device  100  does not use a page in which data is at risk of damage as a storage space, it may have improved lifetime and improved accuracy. 
     Referring to  FIG. 8 , a write request is provided from host  101  (S 510 ). Host  101  may provide a write command, file data of write-requested data and a logical address to nonvolatile memory device  100 . 
     Next, a logical address provided from host  101  is mapped to a physical address of nonvolatile memory  120  (S 520 ). Nonvolatile memory device  100  maps a logical address to a physical address with reference to the bad page map. Nonvolatile memory device  100  prevents a logical address from being mapped to a physical address classified as a bad page with reference to the bad page map. Subsequently, the write-requested data is written in an area of nonvolatile memory  120  corresponding to the physical address to which the logical address is mapped (S 530 ). 
     As indicated by the above description, in the method of  FIG. 8 , nonvolatile memory device  100  classifies a page in which data is damaged or at risk of damage as a bad page and stores a classification result in a bad page map. Nonvolatile memory device  100  prevents a page classified as a bad page from being used as a data storage space with reference to the bad page map in the mapping process. Because nonvolatile memory device  100  does not store data in the bad page, it may have improved lifetime and improved accuracy. 
       FIG. 9  is a block diagram illustrating an SSD  1000  comprising a memory system in accordance with an embodiment of the inventive concept. 
     Referring to  FIG. 9 , SSD  1000  comprises a host  1100  and a SSD  1200 . Host  1100  comprises a host interface  1121 , a host controller  1120  and a DRAM  1130 . 
     Host  1100  stores data in SSD  1200  or reads data stored in SSD  1200 . Host controller  1120  transmits a signal SGL such as a command, an address, a control signal and an ID representing category of file to SSD  1200  through SSD  1200 . DRAM  1130  is a main memory of host  1100 . 
     SSD  1200  exchanges signal SGL with host  1100  through host interface  1211  and receives power from a power supply through a power connector  1221 . SSD  1200  comprises multiple nonvolatile memories  1201 ˜ 120   n , a SSD controller  1210  and an auxiliary power supply  1220 . Nonvolatile memories  1201 ˜ 120   n  may be embodied by a PRAM, a MRAM, an ReRAM, a FRAM, etc. besides a NAND type flash memory. 
     Nonvolatile memories  1201 ˜ 120   n  are used as a storage medium. Nonvolatile memories  1201 ˜ 120   n  may be connected to SSD controller  1210  through multiple channels CH 1 ˜CHn, with one or more nonvolatile memories connected to one channel. Nonvolatile memories  1201 ˜ 120   n  may also be connected to the same data bus. 
     SSD controller  1210  exchanges signal SGL with host  1100  through host interface  1211 . Signal SGL may comprise a command, an address, data, etc. SSD controller  1210  writes data in a corresponding nonvolatile memory or reads data from a corresponding nonvolatile memory according to a command of host  1100 . 
     Auxiliary power supply  1220  is connected to host  1100  through power connector  1221 . Auxiliary power supply  1220  can be provided with power from host  1100  to be charged. auxiliary power supply  1220  may be located inside SSD  1200  or outside SSD  1200 . For example, auxiliary power supply  1220  may be located in a main board and may provide an auxiliary power to SSD  1200 . 
     SSD  1200  classifies a page in which data is at risk of damage as a bad page and stores a classification result in a bad page map. SSD  1200  prevents a page classified as a bad page from being used as a data storage space with reference to the bad page map in the mapping process. Because SSD  1200  does not store data in the bad page, it may have improved lifetime and improved accuracy. 
       FIG. 10  is a block diagram illustrating a memory card  2000  comprising a nonvolatile memory device in accordance with an embodiment of the inventive concept. Memory card  2000  may be, for example, a MMC card, a SD card, a multiuse card, a micro SD card, a memory stick, a compact SD card, an ID card, a PCMCIA card, a SSD card, a chip card, a smart card, a USB card, etc. 
     Referring to  FIG. 10 , memory card  2000  comprises an interface part  2100  performing an interface with the outside, a controller  2200  having a buffer memory and controlling an operation of memory card  2000  and at least one of nonvolatile memory devices  2300 . Controller  2200  is a processor and can control write and read operations of nonvolatile memory device  2300 . Controller  2200  couples to nonvolatile memory device  2300  and interface part  2100  through a data bus DATA and an address bus ADDRESS. 
     Memory card  2000  classifies a page in which data is at risk of damage as a bad page and stores a classification result in a bad page map. Memory card  2000  prevents the page classified as a bad page from being used as a data storage space with reference to the bad page map in a mapping process. Because memory card  2000  does not store data in the bad page, it may have improved lifetime and improved accuracy. 
       FIG. 11  is a drawing illustrating various systems using a memory card in accordance with embodiments of the inventive concept. 
     Referring to  FIG. 11 , memory card  2000  may be used in a video camera, a television, an audio device, a game device, an electronic music device, a cellular phone, a computer, a personal digital assistant (PDA), a voice recorder and a PC card. 
     A nonvolatile memory device in accordance with an embodiment of the inventive concept can be mounted using various types of packages such as package on package (PoP), ball grid array (BGA), chip scale package (CSP), plastic leaded chip carrier (PLCC), plastic dual in-line package (PDIP), die in waffle pack, die in wafer form, chip on board (COB), ceramic dual in-line package (CERDIP), plastic metric quad flat pack (MQFP), thin quad flat pack (TQFP), small outline (SOIC), shrink small outline package (SSOP), thin small outline (TSOP), thin quad flatpack (TQFP), system in package (SIP), multi chip package (MCP), wafer-level fabricated package (WFP) and wafer-level processed stack package (WSP). 
     Because the above described nonvolatile memory devices are managed by page unit, their lifetime and accuracy can be improved. 
     The foregoing is illustrative of embodiments and is not to be construed as limiting thereof. Although a few embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the scope of the inventive concept. Accordingly, all such modifications are intended to be included within the scope of the inventive concept as defined in the claims.