Patent Publication Number: US-10783034-B2

Title: Memory system and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-246674, filed on Dec. 22, 2017; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a memory system and a control method. 
     BACKGROUND 
     In a memory system such as a solid state drive (SSD), data corresponding to a write request together with an error correction code for the data are stored into a nonvolatile memory, thereafter in response to a read request, the data and the error correction code are read from the nonvolatile memory, and the data are reconstituted by error-correction decoding using the error correction code. At this time, it is desired to reconstitute the data appropriately. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a memory system according to an embodiment; 
         FIG. 2  is a diagram illustrating a configuration of a logical block in the embodiment; 
         FIG. 3  is a diagram illustrating a configuration of a logical page in the embodiment; 
         FIG. 4  is a diagram illustrating a configuration of a physical block in the embodiment; 
         FIG. 5  is a flow chart illustrating an operation of the memory system at a time of power supply disconnection according to the embodiment; 
         FIGS. 6A to 6D  are diagrams illustrating a data structure of discarded-host write information in the embodiment; 
         FIG. 7  is a flow chart illustrating an operation of the memory system at a time of power return according to the embodiment; 
         FIG. 8  is a diagram illustrating write status information (a write status map) in the embodiment; 
         FIG. 9  is a diagram illustrating address conversion information logical-physical conversion table) in the embodiment; 
         FIG. 10  is a diagram illustrating symbol read information (a symbol read table) in the embodiment; 
         FIG. 11  is a diagram illustrating block management information (a logical block management table) in a modified example of the embodiment; 
         FIG. 12  is a diagram illustrating a defect avoiding process in the modified example of the embodiment; 
         FIG. 13  is a diagram illustrating symbol read information (a symbol read table) in a case where the defect avoiding process is performed in the modified example of the embodiment; and 
         FIG. 14  is a diagram illustrating a fill-in-the-blank read process in another modified example of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided a memory system including a nonvolatile memory and a controller. The nonvolatile memory has a first memory area including multiple chips and a second memory area. The controller is configured to write a first data among write data to be written across the multiple chips of the first memory area into part of the first memory area and write, in response to a power supply disconnection being detected before writing a second data among the write data into the first memory area, a first information about a storage location where the second data has been stored and the second data into the second memory area. The controller is configured to read, in response to power return being detected, the first data from the part of the first memory area, and read the first information from the second memory area. The controller is configured to generate a second information about a reference location to access the second data based on the read first information. The controller is configured to read the second data from the second memory area to store the read second data based on the generated second information. The controller is configured to write the read first data and the stored second data into another part of the first memory area. 
     Exemplary embodiments of a memory system will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the following embodiments. 
     EMBODIMENT 
     In a memory system such as a solid state drive (SSD), data corresponding to a write request together with an error correction code for the data are stored into a nonvolatile memory, thereafter in response to a read request, the data and the error correction code are read from the nonvolatile memory, and the data are reconstituted by error-correction decoding using the error correction code. For example, a memory system  1  is configured as shown in  FIG. 1 .  FIG. 1  is a diagram illustrating a configuration of the memory system  1 . 
     The memory system  1  is connected externally to a host  2  via a communication medium and functions as an external storage medium for the host  2  The host  2  includes, for example, a personal computer or a CPU core. The memory system  1  includes, for example, a solid state drive (SSD). 
     The memory system  1  includes a controller a, a power supply circuit  6 , and a nonvolatile memory  7 . The controller  5  includes a front end circuit (FE)  10  and a back end circuit (BE)  20 . The FE  10  includes a host interface (host I/F)  11 . The BE  20  includes a central processing unit (CPU)  21 , a volatile memory  22 , an error correction (FCC) circuit  23 , and a memory interface (memory I/F)  24 . The power supply circuit  6  is connected to an external power supply  4  and includes a backup battery  40 . The memory I/F  24  and the nonvolatile memory  7  are connected via multiple channels (e.g., 16 channels CH 0  to CH 15  in the case of  FIG. 1 ). 
     The memory system  1  stores data in a nonvolatile manner using the nonvolatile memory  7 . The nonvolatile memory  7  includes a NAND flash memory, a three-dimensionally structured flash memory, a resistive random access memory (ReRAM), a ferroelectric random access memory (FeRAM), a phase change memory (PCM), a magnetoresistive random access memory (PRAM), or the like.  FIG. 1  illustrates a case where the nonvolatile memory  7  is a NAND flash memory. 
     The nonvolatile memory  7  has multiple logical blocks. Some of the multiple logical blocks are assigned as an emergency save area  30 , and the remaining (most) logical blocks are assigned as a storage area  31 . Each logical block includes multiple logical pages. The storage area  31  is an area to store data which the host  2  made a request to write and management data in the memory system  1 , and the emergency save area  30  is an area in which to save data to be saved when a disconnection of power supply to the power supply circuit  6  occurs. 
     For example, a logical block BL 0  includes multiple logical pages PG- 0  to PG-(N+3) (when logical pages are not distinguished, they are referred to as PG) as shown in  FIG. 2 .  FIG. 2  is a diagram illustrating a configuration of the logical block BL 0 . N denotes any integer greater than or equal to one. Each logical page PG includes multiple banks for which bank interleaving is possible. In  FIG. 2 , for each channel CH 0  to CH 15 , two arrows indicate that bank interleaving across two banks BK 0 , BK 1  is possible. The tip of each arrow indicates the degree of progress of processing by bank interleaving. Each bank includes multiple memory chips. Each memory chip can include multiple planes accessible in parallel. 
     For example, the logical page PG-(N+2) includes two banks BK 0 , BK 1  as shown in  FIG. 3 .  FIG. 3  is a diagram illustrating a configuration of the logical page PG-(N+2), and the other logical pages PG- 0  to PG-(N+1) and PG-(N+3) are the same in configuration as the logical page PG-(N+2). 
     The banks BK 0 , BK 1  are configured such that parallel operation by bank interleaving is possible. The bank BK 0  includes memory chips CP 0 , CP 2 , CP 4 , CP 6 , CP 5 , CP 10 , CP 12 , CP 14 , CP 16 , CP 18 , CP 20 , CP 22 , CP 21 , CP 26 , CP 28 , and CP 30  (hereinafter referred to as CP 0  to CP 30 ). The memory chips CP 0  to CP 30  in the bank BK 0  can be accessed in parallel by the memory I/F  24  via the multiple channels CH 0  to CH 15 . The bank BK 1  includes memory chips CP 1 , CP 3 , CP 5 , CP 7 , CP 9 , CP 11 , CP 13 , CP 15 , CP 17 , CP 19 , CP 21 , CP 23 , CP 25 , CP 27 , CP 29 , and CP 51  (hereinafter referred to as CP 1  to CP 31 ). The memory chips CP 1  to CP 31  in the bank BK 1  can be accessed in parallel by the memory I/F  24  via the multiple channels CH 0  to CH 15 . Each memory chip CP 0  to CP 31  can include multiple planes Plane 0 , Plane 1  that can operate in parallel in the memory chip CP 0  to CP 31 . Each memory chip CP 0  to CP 31  includes multiple memory cell arrays, some of which form one or multiple physical blocks. 
     Each physical block in the memory cell arrays of each memory chip CP 0  to CP 31  is configured as shown in, e.g.,  FIG. 4 .  FIG. 4  is a diagram illustrating a configuration of the physical block. 
     Each physical block comprises (p+1) number of NAND strings arranged along the X direction, where p≥0. A selection transistor ST 1  included in each of the (p+1) number of NAND strings, has its drain connected to a bit line BL 0  to BLp and its gate connected in common to a selection gate line SGB. A selection transistor ST 2  has its source connected in common to a source line SL and its gate connected in common to a selection gate line SGS. 
     Each memory cell transistor MT is formed of a metal oxide semiconductor field effect transistor (MOSFET) having a stacked gate structure formed on a semiconductor substrate. The stacked gate structure includes a floating gate formed on the semiconductor substrate with a tunnel oxide film in between and a control gate electrode formed on the floating gate with an inter-gate insulating film in between. The threshold voltage changes according to the number of electrons stored in the floating gate. The memory cell transistor MT stores data according to the difference in threshold voltage. That is, the memory cell transistor MT holds a corresponding amount of charge to data in the floating gate. 
     In the NAND string, a (q+1) number of memory cell transistors MT are arranged between the source of the selection transistor ST 1  and the drain of the selection transistor ST 2  such that their current paths are connected in series, where q≥0. The control gate electrodes are connected to word lines WL 0  to WLq respectively in the order of from the memory cell transistor MT located closest to the selection transistor ST 1 . Thus, the drain of the memory cell transistor MT connected to the word line WL 0  is connected to the source of the selection transistor ST 1 , and the source of the memory cell transistor MT connected to the word line WLq is connected to the drain of the selection transistor ST 2 . 
     Each of the word lines WL 0  to WLq (when the word lines are not distinguished, they are referred to as WL) connects in common to the control gate electrode of memory cell transistor MT of each NAND string in the physical block. In other words, the control gate electrodes of the memory cell transistors MT in the same row in the physical block are connected to the same word line WL. That is, the physical block includes multiple memory cell groups MG corresponding to multiple word lines WL, and each memory cell group MG includes a (p+1) number of memory cell transistors MT connected to the same word line WL. When each memory cell transistor MT is configured to be able to hold a one-bit value (when operating in a single level cell (SLC) mode), the (p+1) memory cell transistors MT (i.e., the memory cell group MG) connected to the same word line WL are dealt with as one physical page, and data programming and data read are performed for each such physical page. 
     Each memory cell transistor MT may be configured to be able to hold a multiple-bit value. For example, when each memory cell transistor MT can store an n-bit value, where n≥2, the storage capacity per word line WL is equal to the size of n number of physical pages. That is, each memory cell group MG is dealt with as n number of physical pages. For example, in a multi-level cell (PLC) mode where each memory cell transistor MT stores a two-bit value, in each word line ML, two physical pages worth of data is held. In a triple-level cell (TLC) mode where each memory cell transistor MT stores a three-bit value, in each word line WL, three physical pages worth of data is held. 
     Referring back to  FIG. 1 , the emergency save area  30  in the nonvolatile memory  7  is an area into which to write data in the volatile memory  22  (at least a write buffer  22   a ) using power from the backup battery  40  when a sudden disconnection of power supply to the power supply circuit  6  occurs. The emergency save area  30  can be configured to operate in the SLC mode because high-speed write operation is required of it. The storage area  31  is an area to store data. The storage area  31  can be configured to operate in the MLC or TLC mode because the data storage capacity is required to be secured. 
     The controller  5  is formed of, e.g., a semiconductor chip (SoC: System on a chip) such as a large-scale integrated circuit (LSI). The controller  5  controls data transfer between the host  2  and the nonvolatile memory  7  and so on. 
     In the controller  5 , when receiving a write request and data to be written from the host  2 , the host I/F  11  of the FE  10  transfers the write request and data to the BE  20 . In the BE  20 , the CPU  21  includes a write controller  21   a , a block managing unit  21   b , and a garbage collection (GC) manager  21   c.    
     The write controller a has the ECC circuit  23  perform error correction code processing on the data to generate symbols including a data part corresponding to the data and an error correction code and to store in the volatile memory  22  temporarily. 
     In this case, the ECC circuit  23  can perform, for example, four tiers of error correction code processing, that is, a first ECC code (a code generated by L1 error correction code processing or an L1 code), a second ECC code (a code generated by L2 error correction code processing or an L2 code), a third ECC code (a code generated by L3 error correction code processing or an L3 code), and a fourth ECC code (a code generated by L4 error correction code processing or an L4 code). The degree of dispersion in encoding is higher in the order of the L1 code, L2 code, L3 code, and L4 code. For the L1 code and L2 code, data to be encoded is in one memory chip, whereas for the L3 code and L4 code, data to be encoded is spread across multiple memory chips. 
     The write controller  21   a  writes the symbols temporarily stored in the volatile memory  22  into the nonvolatile memory  7  via the memory I/F  24 . 
     The volatile memory  22  is a volatile semiconductor memory capable of higher access than the nonvolatile memory  7 , and a static random access memory (SRAM) or dynamic random access memory (DRAM) is used as that memory.  FIG. 1  illustrates a case where the volatile memory  22  is a DRAM. 
     The volatile memory  22  includes the write buffer  22   a  and a garbage collection (GC) buffer  22   b . The write buffer  22   a  temporarily stores write data that the host  2  has made a request to write. When GC is performed in the memory system  1 , the GC buffer  22   b  temporarily stores data on which to perform GC. 
     The GC manager  21   c  controls the operation of each component associated with GC using the GC buffer  22   b . The GC is a process of sorting not-used logical pages in logical blocks. By performing the GC, logical pages spread and being used are gathered together in one logical block so as to secure free blocks. 
     For example, under control associated with the GC by the GC manager  21   c , the memory I/F  24  reads symbols (=a data part+an error correction code) from the nonvolatile memory  7 , and the ECC circuit  23  performs error-correction decoding on the data part of the read symbols using the error correction code to reconstitute into original data. 
     At this time, the ECC circuit  23 , for example, performs L1 error-correction decoding and, if an L1 error occurs, performs L2 error-correction decoding and, if an L2 error occurs, performs L3 error-correction decoding and, if an L3 error occurs, performs L4 error-correction decoding. As such, the ECC circuit  23  can perform error-correction decoding while enlarging the scale stepwise in the order of L1, L2, L3, and L4 until succeeding in error-correction decoding. 
     The block managing unit  21   b  manages the logical blocks in the nonvolatile memory  7 . The block managing unit  21   h  assigns some of the multiple logical blocks as the emergency save area  30  and the remaining (most) logical blocks as the storage area  31 . Further, the block managing unit  21   b  manages the number of free blocks that are logical blocks into which to be able to write data, the rates of valid data in logicals, and so on. When short of free blocks in number, the block managing unit  21   b  has the GC manager  21   c  perform ttie GC, thereby increasing the number of free blocks. 
     The memory I/F  24  controls the operation of the nonvolatile memory  7 . The memory I/F  24  issues a write command in response to a write request from the host  2  to supply (output) the write command and data to the nonvolatile memory  7 . The nonvolatile memory  7  writes the data into memory cells according to the write command. The memory I/F  24  issues a read command in response to a read request from the host  2  to supply (output) the read command to the nonvolatile memory  7 . The nonvolatile memory  7  reads data from memory cells according to the read command to supply tc the memory I/F  24 . 
     The volatile memory  22  stores address conversion information  22   c  and block management information  22   d . Write requests from the host  2  received by the host I/F  11  include a logical address (LBA) to designate a write destination. Read requests from the host  2  received by the host I/F  11  include a logical address (LBA) to designate a target to read from. 
     For example, in the memory system  1 , internal data management is performed by the controller  5  on a cluster basis, and the updating of data from the host  2  is performed on a sector basis. It is assumed that a logical page PG as shown in  FIG. 3  is a unit of multiple clusters gathered together and that the cluster is a unit of multiple sectors gathered together. The sector is a minimum access unit of data from the host  2 . The sector has a size of, e.g., 512 B, and the cluster has a size of, e.g., 4 KB. The host  2  can designate data to access by logical block addressing (LBA) on a sector basis. 
     Referring back to  FIG. 1 , the address conversion information  22   c  is used in performing address conversion between a logical address specified by the host  2  and a physical address in the nonvolatile memory  7 . The address conversion information  22   c  can be, for example, in the form of a table (as a logical-physical conversion table). In the address conversion information  22   c , logical addresses are associated with physical addresses. In writing symbols temporarily stored in the volatile memory  22  into the nonvolatile memory  7  via the memory I/F  24 , the write controller  21   a  can update the address conversion information  22   c  such that logical addresses corresponding to the symbols are associated with physical addresses in the nonvolatile memory  7 . 
     The block management information  22   d  is used in managing the logical blocks in the nonvolatile memory  7  and can be updated by the block managing unit  21   b  as needed. The block management information  22   d  can be, for example, in the form of a table (as a logical block management table). The block management information  22   d  can include information about logical blocks (bad blocks) including a memory chip (defective memory chip) containing a memory cell regarded as having a defect. That is, having received the result of error-correction decoding from the ECC circuit  23 , the block managing unit  21   b  regards memory chips for which error correction failed in error-correction decoding (even with enlarging the scale of error correction in the order of L1, L2, L3, and L4) as defective memory chips to register logical blocks including the defective memory chips into the block management information  22   d . Thus, the block management information  22   d  can be updated. The defects may be managed on the basis of physical blocks or physical pages in memory chips. 
     The power supply circuit  6  supplies power sent from the external power supply  4  to the controller  5  and the nonvolatile memory  7 . Further, the power supply circuit  6  supplies power sent from the external power supply  4  to the backup battery  40  to charge the backup battery  40 . The backup battery  40  is, for example, an electric double layer capacitor, an electrolytic capacitor, a ceramic capacitor, a secondary battery, or the like. 
     The backup battery  40  stores power sent from the external power supply  4 . When power sent from the external power supply  4  stops suddenly (when a sudden power supply disconnection occurs), the backup battery  40  supplies saved power to the controller  5  and the nonvolatile memory  7 . The case when a sudden power supply disconnection occurs refers to, for example, the case when power sent from the external power supply  4  to the power supply circuit  6  is cut off at a timing which the user does not intend for happening at, or the like. 
     The power supply circuit  6  monitors power sent from the external power supply  4  to the power supply circuit  6  and, if the power is at or above a predetermined threshold level, determines that power is being normally supplied and can supply the power from the external power supply  4  to each component. The power supply circuit  6 , if the power from the external power supply  4  is below the predetermined threshold level, determines that a power supply disconnection has occurred and switches to drive by the backup battery  40  and can supply the power from the backup battery  40  to each component. 
     The memory system  1  has a power loss protection (PLP) function of protecting data corresponding to a write request. The PLP function is for ensuring the permanence of data for which a write-completion response has been made even if a sudden power supply disconnection occurs. In other words, the PLP function is for storing the data so that data which seems already written to the host  2  is not lost even when a sudden power supply disconnection occurs. 
     Next, a function (multi-write processing function) of the memory system  1  corresponding to a multi-stream function of the host  2  will be described. 
     In the memory system  1 , an equal number of logical pages to the number of streams may need to be simultaneously opened in order to support a write operation corresponding to the multi-stream function of the host  2 . For example, as shown in  FIG. 2 , because the number of channels×the number of banks=16×2=32, an equal number of parallel operations (multi-write operation process) to the number (=32) of streams are possible. In the multi-write operation process, writing in parallel into multiple regions in a logical page PG via the channels CH 0  to CH 15  is performed sequentially for multiple logical pages PG- 0  to PG-(N+3) (in page-number order). 
     Thus, the number of logical pages PG which can be being written into simultaneously can become large (e.g., a maximum of 32).  FIG. 2  illustrates the case where the logical pages PG being written into are three logical pages PG-N, PG-(N+1), and PG-(N+2). In the leading page PG-(N+2) being written into (=the most progressing logical page in writing=the highest numbered logical page of the logical pages PG being written into), for example, memory chips CP 1 , CP 7 , CP 15 , CP 29 , and CP 31  are being written into as indicated by oblique hatching in  FIG. 3 . 
     As shown in  FIG. 3 , in the logical page PG-(N+2), the data group DG 1  enclosed by dot-dashed lines is a data group corresponding to an L3 error correction code (also called an L3 error correction group). The data group DG 1  includes the leading cluster of data in each plane Plane 0 , Plane 1  of each memory chip CP 0  to CP 31 . An L3 number from 0 to 63 (a number to identify one of the clusters of data belonging to the data group DG 1 ) is assigned to each cluster in the data group DG 1 . The L3 error correction code processing can generate a third ECC code (L3 code) for the clusters of data designated by L3 numbers 0 to 61. In the L3 error correction code processing, the data part of symbols is stored as the clusters of data designated by L3 numbers 0 to 61 into the volatile memory  22 , and the L3 code is stored as the clusters of data designated by L3 numbers 62, 63. The L3 code is generated with being updated sequentially for already written data, data currently being written, and not-yet written data (duty data) while the clusters of data designated by L3 numbers 0 to 61 are being written into the storage area  31  and is held in the volatile memory  22  until the completion of writing all the clusters of data in the data group (L3 error correction group) DG 1 . The L3 code is written into the storage area  31  after the completion of writing all data in the data group (L3 error correction group) DG 1  into the storage area  31 . 
     When a sudden power supply disconnection occurs, to finish writing logical pages PG being written into the storage area  31  may increase the number of logical pages PG to finish writing to a large number, and thus it may be difficult to finish writing logical pages PG being written using the power of the backup battery  40 . 
     If the capacity (e.g., the number of capacitors) of the backup battery  40  to be mounted in the memory system  1  is increased to deal with this, component cost may increase, so that the production cost of the memory system  1  may increase, and since the startup time for charging the backup battery (e.g., capacitors)  40  may increase, the performance of the memory system  1  may decrease. 
     Hence, in order to suppress an increase in the capacity of the backup battery  40 , it can be thought of that when a sudden power supply disconnection occurs, an error correction code is generated and saved into the emergency save area  30  while not finishing writing logical pages PG being written into the storage area  31 , that after power return, already written data is read from the storage area  31  while the error correction code is read from the emergency save area  30 , and that the missing portions of logical pages PG being written are filled with dummy values (e.g., bit values of 0) to generate symbols. In this case, if an L1/L2 error occurs in performing error-correction decoding on data of logical pages PG being written during operation, then L3/L4 error-correction decoding is performed, but symbols used in the L3/L4 error-correction decoding are different from those in error-correction code processing. That is, in the error-correction code processing, the error correction code is generated using both already written data and not-yet written data of logical pages PG being written, whereas, in the error-correction decoding, the error-correction decoding using the error correction code is performed on already written data and the dummy values of logical pages PG being written. Thus, the L3/L4 error-correction decoding may not be able to reconstitute data appropriately. 
     Hence, the memory system  1  according to the present embodiment saves not-yet written data and information about save locations for that data into the emergency save area  30  at the time of power supply disconnection and, on power return, reads those to reproduce symbols of logical pages PG used in generating an error correction code and including not-yet written portions and performs the error-correction decoding. By this means, with the memory system  1  according to the present embodiment, both suppressing an increase in the capacity of the backup battery  40  and improving the accuracy of the L3/L4 error-correction processing can be achieved. 
     Specifically, the memory system  1  performs an operation as shown in  FIG. 5  relating to the time when a sudden power supply disconnection occurs.  FIG. 5  is a flow chart illustrating the operation of the memory system  1  at a time of power supply disconnection. 
     The memory system  1  performs normal operation until detecting an occurrence of power supply disconnection (No at S 1 ) and, when detecting an occurrence of power supply disconnection (Yes at S 1 ), stops accepting a write request from the host  2  (S 2 ). The memory system  1  stops writing into the storage area  31  of the nonvolatile memory  7  (S 3 ). That is, the memory system  1  causes the memory I/F  24  to discard a write command to write into the storage area  31  and the memory I/F  24  itself to stop. The memory system  1  saves not-yet written data (write data requested by the host to write and held in the write buffer  22   a  in the volatile memory  22 ) and cluster data of a log and cluster data of an L3 code into the emergency save area  30  to make those nonvolatile (S 4 ). The log is information about an update (difference) of the entry when a change is made to a certain entry of management information in the nonvolatile memory  7  and is, for example, an update difference of the address conversion information  22   c.    
     The memory system  1  identifies the number of the leading page being written into (=the number of the most progressing logical page in writing) and generates and causes leading page information  22   f  to be held in the volatile memory  22  (S 5 ). In the case shown in  FIG. 2 , the memory system  1  identifies PG-(N+2) as the leading page number and generates leading page information  22   f  indicating the leading page number PG-(N+2). 
     The memory system  1  extracts the write command discarded in the memory I/F  24  and generates and causes discarded-host write information  22   e  to be held in the volatile memory  22  (S 6 ). At this time, the memory system  1  extracts a write command discarded for each memory chip of each logical page PG and generates, in the case of  FIG. 2 , discarded-host write information  22   e  for each memory chip being written into of logical pages PG-N to PG-(N+2) being written into. 
     The discarded-host write information  22   e  has, for example, a data structure as shown in  FIGS. 6A to 6D .  FIGS. 6A to 6D  are diagrams illustrating the data structure of the discarded-host write information  22   e.    
     If the discarded write command is a command from the host  2  to write data, as shown in  FIG. 6A , the discarded-host write information  22   e  contains the storage location of data (e.g., logical block number BL 0 /logical page number PG-(N+1)/channel number CH 3 /bank number BK 1 ), the type of information (Data), and an address in the volatile memory  22  (e.g., VADD-35). 
     If the discarded write command is a command instructing to do an empty write or invalid write, as shown in  FIG. 6B , the discarded-host write information  22   e  contains the storage location of data (e.g., logical block number BL 0 /logical page number PG-(N+3)/channel number CH 0 /bank number BK 0 ) and the type of information (Null). 
     For example, if the command sequence of a write command includes a write instruction (Cmd80h-Adr-DataIn-Cmd10h), access processing (tProg), and a status read instruction (Cmd70-StatusOut), the write command being an empty write means that data designated by the data designating part “Datain” of the write instruction is empty (not designated). The write command being an invalid write means that the write destination designated by the address specifying part “Adr” of the write instruction is a logical block (bad block) including a defect memory chip in the storage area  31  and that a write error occurs when the command is executed. 
     If the discarded write command is a command to write a log, as shown in  FIG. 6C , the discarded-host write information  22   e  contains the storage location of data (e.g., logical block number BL 0 /logical page number PG-(N+3)/channel number CH 7 /bank number BK 1 ), the type of information (Log), and an address in the volatile memory  22  (e.g., VADD-46). 
     If the discarded write command is a command to write an L3 code, as shown in  FIG. 6D , the discarded-host write information  22   e  contains the storage location of data (e.g., logical block number BL 0 /logical page number PG-(N+2)/channel number CH 14 /bank number BK 1 ), the type of information (L3), and an address the volatile memory  22  (e.g., VADD-62). 
     These discarded-host write information  22   e  may be provided for each logical block or be in the form of table with all together or be gathered together for each type of data. 
     Referring back to  FIG. 5 , the memory system  1  saves not-yet written data (WB dirty) held in the write buffer  22   a , the leading page information  22   f  generated at S 5 , the discarded-host write information  22   e  generated at S 6 , and other for-reconstitution management data into the emergency save area  30  to make those nonvolatile (S 7 ). The WB dirty (not-yet written data) is write request data designated by a write request from the host  2  and is temporarily stored in the write buffer  22   a.    
     The memory system  1  performs an operation as shown in  FIG. 7  relating to the time of power return.  FIG. 7  is a flow chart illustrating the operation of the memory system  1  at a time of power return. 
     The memory system  1  waits until detecting power return (No at S 11 ) and, when detecting power return (Yes at S 11 ), loads the log, L3 code, WB dirty (not-yet written data), leading page information  22   f , and discarded-host write information  22   e  saved in the emergency save area  30  of the nonvolatile memory  7  into the volatile memory  22  (S 12 ). At this time, the log, L3 code, and WB dirty (data) are loaded into the volatile memory  22  at address locations (see  FIG. 6 ) specified by the discarded-host write information  22   e.    
     The memory system  1  generates write status information  22   g  indicating the write status a logical block based on the leading page information  22   f  and discarded-host write information  22   e  loaded, to store into the volatile memory  22  (S 13 ). The discarded-host write information  22   e  has been generated for each memory chip being written into of logical pages PG-N to PG-(N+2) being written into. Thus, the memory system  1  can generate the write status information  22   g  indicating the write status of the logical block containing logical pages PG being written into for the logical block containing logical pages PG being written into. 
     The memory system  1  can generate the write status information  22   g , for example, in the form of a map (as a write status map) as shown in  FIG. 8 .  FIG. 8  is a diagram illustrating the write status information (write status map)  22   g  for logical block BL 0 . 
     In  FIG. 8 , a term “slot” is used as an index of each of multiple memory chips forming the logical block BL 0 . In this embodiment, the slot corresponds to a combination of a channel number and a Lank number. That is, slot SL 0  corresponds to channel CH 0  and bank BK 0 ; slot SL 1  corresponds to channel CH 1  and bank BK 0 ; . . . ; and slot SL 31  corresponds to channel CH 15  and bank BK 1 . Banks  0 ,  1  of the same channel may be in the same memory chip. 
     The write status information  22   g  indicates distinguishably either “already written” or “discarded” for each memory chip of each logical page PG. The discarded-host write information  22   e  as shown in  FIGS. 6A to 6D  can be associated with places labelled “discarded”. For example, the write status information  22   g  is configured such that its corresponding discarded-host write information  22   e  can be referred to when a place labelled “discarded” is designated. 
     Referring to the write status information  22   g  shown in  FIG. 8 , it is seen that, although writing into the storage area  31  has been finished for all the slots SL 0  to SL 31  in each of logical pages PG- 0  to PG-(N−1), logical pages PG-N to PG-(N+2) are being written into. Slots for which a write command has been discarded exist in each of logical pages PG-N to PG-(N+2). In logical page PG-N, a write command has been discarded for memory chips corresponding to slots SL 16 , SL 30 , and SL 31 . In logical page PG-(N+1), a write command has been discarded for memory chips corresponding to slots SL 16 , SL 19 , SL 30 , and SL 31 . In logical page PG-(N+2), a write command has been discarded for memory chips corresponding to slots SL 16 , SL 19 , SL 23 , SL 30 , and SL 31 . 
     Referring to the write status information  2   g  shown in  FIG. 8 , regions indicated by oblique hatching are unused (not-yet written) memory chips and, if the nonvolatile memory  7  is a NAND flash memory, may not be able to be overwritten. In order to store data of each memory chip of a logical page into the storage area  31  at consecutive addresses, a data move process (process of writing data of a logical page being written into a new logical block (free block)) can be used. In  FIG. 8 , logical page PG-(N+3), for which operations of writing into the nonvolatile memory  7  according to write commands for all the slots SL 0  to SL 31  have been discarded, can be said to be a logical page practically not written. Thus, by referring to the write status information  22   g , it can be found out that the logical pages PG-N to PG-(N+2) are a range of pages subject to a move (a range needing the data move process). 
     Referring back to  FIG. 7 , the memory system  1  reads already-written data of the logical pages PG-N to PG-(N+2) being written into from the storage area  31  and causes to be held in the volatile memory  22  (S 14 ). 
     The memory system  1  performs L1/L2 error-correction decoding on the data held in the volatile memory  22  and, if an L1/L2 error does not occur (No at S 15 ), stores data reconstituted by error-correction decoding into the nonvolatile memory  7  ( 320 ). 
     On the other hand, the memory system  1  performs L1/L2 error-correction decoding on the data held in the volatile memory  22  and, if an L1/L2 error occurs (Yes at S 15 ), generates symbol read information  22   h  based on the discarded-host write information  22   e  to store into the volatile memory  22  (S 16 ). 
     For example, in the write status information  22   g  shown in  FIG. 8 , when a place labelled “discarded” is designated, its corresponding discarded-host write information  22   e  is referred to, so that, if the data type is “Data”, “Log”, or “L3”, a reference location (address) in the volatile memory  22  can be acquired (see  FIGS. 6A, 6C, 6D ). 
     Thus, the memory system  1  can generate the symbol read information  22   h  as shown in  FIG. 10  about the logical page PG-(N+2) shown in  FIG. 3 . The symbol read information  22   h  can be generated, for example, in the form of a table as shown in  FIG. 10  (as a symbol lead table).  FIG. 10  is a diagram illustrating the symbol read information (symbol read table)  22   h . In the symbol read information  22   h , an L3 number  22   h   1  is associated with location information  22   h   2  for multiple L3 numbers. For example, because L3 number 0 is “already written” (see  FIG. 8 ), a reference location (physical address NVADD-0) in the storage area  31  is recorded. Because L3 number 35 is a log cluster for which a write command has been discarded (see  FIG. 8 ), a reference location (address VADD-35) in the volatile memory  22  is recorded (see  FIG. 6C ). Because L3 number 39 is a cluster in which empty information (Null) is already written, a value indicating being invalid (ALL FF: A bit pattern where all the bits are 1) is recorded (see  FIG. 6B ). Because L3 number 62 is an L3 cluster for which a write command has been discarded (see  FIG. 8 ), a reference location (address VADD-62) in the volatile memory  22  is recorded (see  FIG. 6D ). Likewise, because L3 number 63 is an L3 cluster for which a write command has been discarded (see  FIG. 8 ), a reference location (address VADD-63) in the volatile memory  22  is recorded. That is, by referring to the symbol read information  22   h , the reference location of each cluster in the data group (L3 error correction group) corresponding to the error correction code can be identified. 
     Referring back to  FIG. 7 , the memory system  1  performs error-correction decoding based on the L3 error correction code and the symbol read information  22   h  generated at S 16 . The memory system  1  reads already written data from reference locations (addresses) in the storage area  31  specified by the symbol read information  22   h  and not-yet written data from reference locations (addresses) in the volatile memory  22  specified by the symbol read information  22   h  (S 17 ) and reconstructs those as symbols of the data group (L3 error correction group) for L3 correction in the GC buffer  22   b . The memory system  1  performs error-correction decoding using the L3 code (designated by L3 numbers 62 and 63 in the case of  FIG. 10 ) on the data part (designated by L3 numbers 0 to 61 in the case of  FIG. 10 ) of the reconstructed symbols (S 18 ). 
     If an L3 error does not occur (No at S 19 ), the memory system  1  stores ata reconstituted by error-correction decoding into the nonvolatile memory  7  (S 20 ). 
     If an L3 error occurs (Yes at S 19 ), the memory system  1  notifies the occurrence of an L3 error to the host  2  and ends the process. 
     As such, the memory system  1  according to the embodiment saves not-yet written data and the discarded-host write information  22   e  relating to save locations for that data into the emergency save area  30  at the time of power supply disconnection and, on power return, reads those to reproduce symbols of logical pages PG used in generating an error correction code and including portions not yet written into the storage area  31  and performs error-correction decoding (e.g., L3 error-correction decoding) across multiple memory chips. By this means, at the time of power supply disconnection, information necessary for error correction across multiple memory chips can be saved into the emergency save area  30  within the range of power that can be supplied by the backup battery  40 , and after power return, symbols to be used in error-correction decoding across multiple memory chips can be made to almost coincide with symbols used in error-correction code processing across the multiple memory chips immediately before the power supply disconnection. As a result, while suppressing an increase in the capacity of the backup battery  40 , the accuracy of the error-correction processing across multiple memory chips can be improved, so that data can be reconstituted appropriately. Thus, an increase in the production cost of the memory system  1  can be suppressed, and the reliability of data of the memory system  1  can be improved. 
     The concept of the present embodiment can be applied to operation of performing the power loss protection (PLP) while not finishing writing logical pages PG being written at the time of power supply disconnection even if the memory system  1  does not have or use a multi-write processing function, not being limited to the case where the memory system  1  has a function (multi-write processing function) supporting the multi-stream function of the host  2 . 
     Or performing L3 error-correction decoding using the symbol read information  22   h  (S 15  to  318  of  FIG. 7 ) may be executed not only at power-on but also in normal operation (for example, in a read operation according to a read request from the host  2 ). 
     Or although the embodiment illustrates the case where logical pages PG being written into do not include a defective memory chip, if physical address locations assigned to cluster data of an L3 code and cluster data of a log (correction information and management information necessary in a page) are in a defective memory chip, a defect avoiding process may be performed. The defect avoiding process is a process of making storage locations registered as locations in a defective memory chip by the block managing unit  21   b  be not accessed by the write controller  21   a  (unusable). With defective memory chips, there are cases where, although a defect exists innately, it is not exposed because of being unused and where a defect occurs due to degradation in a posterior manner. In either case, in the defect avoiding process, in response to the revelation that a defect exists in storage locations into which to write cluster data of an L3 and cluster data of a log within the data group (L3 error correction group) corresponding to the L3 code, instead of these storage locations, the controller  5  (CPU  21 ) can assign storage locations (e.g., the preceding cluster in the same correction group) for other cluster data in the data group (e.g., in the data group DG 1  shown in  FIG. 3 ) that is an L3 error correction group. The write controller  21   a  can control the memory I/F  24  to issue write commands according to this assignment. Note that, in this process, the controller  5  (CPU  21 ) can change storage locations in such a way as to make another logical page include storage locations for cluster data that were assigned to the destinations newly assigned. 
     For example, it is assumed that the memory system  1  refers to the block management information  22   d  as shown in  FIG. 11  to find out that slots SL 29 , SL 31  in the logical page PG-(N+2) of the logical block PL 0  include defective memory chips.  FIG. 11  is a diagram illustrating the block management information (logical block management table)  22   d . In this case, as shown in  FIG. 12 , the memory system  1  finds out that the leading cluster (cluster L3-numbered in an L3 error correction group) of Plane 1  of a memory chip CP 29  and the leading cluster (cluster L1-numbered  63  in the same group) of Plane 1  of a memory chip CP 31  in the logical page PG-(N+2) are unusable.  FIG. 12  is a diagram illustrating the defect avoiding process. The memory system  1  changes the write destination for an L3 code that has been scheduled to be written into clusters L3-numbered  62 ,  63  to clusters L3-numbered  60 ,  61  and registers clusters L3-numbered  62 ,  63  as being not for use in the block management information  22   d . At this time, cluster data that was scheduled to be written into clusters numbered  60 ,  61  can be driven out and assigned as cluster data of another logical page PG. 
     In this case, the memory system  1  refers to the block management information  22   d  as well as the write status information  22   g  at  316  of  FIG. 7  to generate symbol read information  22   h  as shown in  FIG. 13 . In comparison with the symbol read information  22   h  shown in  FIG. 10 , L3 numbers corresponding to reference locations (address VADD-62, address VADD-63 in the volatile memory  22  for cluster data of L3 are changed from 62, 63 to 60, 61. In numbers 62, 63, as storage locations (Null) in an invalid defective memory chip, a value indicating being invalid (ALL FF: A bit pattern where all the bits are 1) is recorded (see  FIG. 6B ). Thus, the memory system  1  can appropriately perform error-correction decoding on the data part of the data group corresponding to an L3 code while avoiding storage locations contained in the defective memory chip at S 17  shown in  FIG. 7 . In the case of  FIG. 13 , although the number of clusters in the L3 error correction group decreases, the controller  5  (CPU  21 ) can drive out cluster data that was scheduled to be written into clusters L3-numbered  60 ,  61  and assign as cluster data of another logical page PG. 
     Or the write buffer  22   a  may be configured taking into account a fill-in-the-blank read process. For example, a cluster that is a data management unit in the nonvolatile memory  7  is larger than a sector that is a management unit for data designated by a write request from the host  2 . Hence, in writing data into the nonvolatile memory  7 , when sector data (other sector data) of the same cluster address as to-be-written sector data corresponding to the write request exists in the nonvolatile memory  7 , the memory system  1  performs the fill-in-the-blank read process of reading the other sector data from the nonvolatile memory  7  to merge with the to-be-written sector data so as to generate data in a cluster unit (a cluster of data). If a sudden power supply disconnection occurs during or before the fill-in-the-blank read process, the memory system  1  cannot finish the fill-in-the-blank read process for the other sector but performs L3 error-correction code processing using cluster data merged with not-guaranteed data (e.g., data of 0 s and 1 s mixed) present in the volatile memory (DRAM) instead of the other sector data to generate an L3 code. Then the memory system  1  saves cluster data including the to-be-written sector data and the not-guaranteed data and an L3 code generated based on this cluster data into the nonvolatile memory  7  (the emergency save area  30 ). Then on power return, the saved cluster data (the to-be-written sector data and the not-guaranteed data) and L3 correction code are restored from the emergency save area  30  into the volatile memory  22  (the write buffer  22   a ) and the fill-in-the-blank read process is performed for the restored cluster data. Thus, the not-guaranteed data is replaced with data whose values are guaranteed (correct other sector data). When the cluster data having the blank filled in cannot be corrected by L1 error-correction decoding, error-correction decoding is performed using the restored L3 correction code, but the correction fails (cannot be done) because the contents of the cluster data are different between when encoding and when decoding. Thus, also at the time of L3 error-correction decoding, the not-guaranteed data when the L3 correction code was generated is necessary. However, when the fill-in-the-blank read process is performed for the restored cluster data, sector data other than the to-be-written sector data out of the cluster data is filled with the correct other sector data to perform the L3 error-correction decoding. That is, because of the fill-in-the-blank reading, the not-guaranteed data is overwritten with the data whose values are guaranteed so as to disappear. 
     Hence, the memory system  1  includes a temporary buffer region  22   a   1  and a main buffer region  22   a   2  in the write buffer  22   a  so that the not-guaranteed data necessary for L3 error-correction decoding is not overwritten but certainly remains in the write buffer  22   a  until L3 error-correction decoding. The temporary buffer region  22   a   1  temporarily holds data to be stored in the main buffer region  22   a   2  and holds, for example, data necessary for computation in L3 error-correction decoding. The main buffer region  22   a   2  temporarily holds data to be stored in the nonvolatile memory  7  and holds, for example, part of data on which L3 error-correction decoding was performed. 
     For example, at S 12  of  FIG. 7 , the memory system  1  reads not-yet written data (WB dirty) of a specified logical page PG from the emergency save area  3 C into the temporary buffer region  22   a   1  as indicated by a solid arrow in  FIG. 14 . At S 14  of  FIG. 7 , the memory system  1  reads already written data of the specified logical page PG from the storage area  31  into the temporary buffer region  22   a   1  as indicated by a solid arrow in  FIG. 14 . Thus, data of the specified logical page PG including already written data and not-yet written data (WB dirty) is reconstructed in the temporary buffer region  22   a   1 , so that L3 error-correction decoding can be performed appropriately. 
     The memory system  1  executes S 16  to S 18  of  FIG. 7  and, if an L3 error does not occur (No at S 19 ), transfers (copies or moves) selectively data of at least valid part (indicated by oblique lines in  FIG. 14 ) of the specified logical page PG from the temporary buffer region  22   a   1  into the main buffer region  22   a   2  as indicated by a broken-line arrow in  FIG. 14 . 
     The size of each cluster of data transferred into the main buffer region  22   a   2  may be smaller than the data size (e.g., 4 KB) of a unit in writing into the nonvolatile memory  7 . Hence, the memory system  1  performs the fill-in-the-blank read process at S 20  of  FIG. 7 . For example, the memory system  1  reads other data corresponding in size to the missing portions (other sector data of the same cluster address) from the storage area  31  into the main buffer region  22   a   2  as indicated by a dot-dashed arrow in  FIG. 14  to add to the data transferred from the temporary buffer region  22   a   1 . Thus, each cluster of data of the data size of the unit in writing into the nonvolatile memory can be formed, so that the memory system  1  stores, for example, one logical page worth of data formed by the fill-in-the-blank read process into a region corresponding in size to one logical page of another logical block in the storage area  31  as indicated by a two-dot-dashed arrow in  FIG. 14 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.