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Method and managing bad memory blocks in a nonvolatile memory device, and nonvolatile-memory device implementing the management methodMethod and managing bad memory blocks in a nonvolatile memory device, and nonvolatile-memory device implementing the management method description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060018166, Method and managing bad memory blocks in a nonvolatile memory device, and nonvolatile-memory device implementing the management method. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a method for managing bad memory blocks in a nonvolatile-memory device, and to a nonvolatile-memory device implementing the management method. [0003] 2. Discussion of Related Art [0004] As is known, nonvolatile-memory devices having an architecture of a NAND type have a memory array divided into memory blocks, each of which is in turn divided into sub-blocks referred to as pages. Each page is formed by a given number of bytes, the majority of which, approximately 95%, define the so-called data area, while the remaining bytes define an area reserved to the storage of given information ("page spare area"). [0005] Nonvolatile-memory devices having an architecture of a NAND type can be divided into two major categories according to the number of bad memory blocks that they contain: the first category comprises so-called good nonvolatile-memory devices, which do not contain bad memory blocks, while the second category includes so-called "mostly good" storage devices, which contain a number of bad memory blocks lower than a given threshold. [0006] For the purpose of guaranteeing the execution of a minimum number of write and erase cycles during the life of the storage device, typically one hundred thousand, the nonvolatile-memory devices having an architecture of a NAND type shall not intrinsically contain, when they have just been produced, a number of bad memory blocks higher than a given threshold, typically in the region of 2% of the available memory blocks. [0007] In response to the continuous market requirements, some manufacturers of nonvolatile-memory devices guarantee the execution of a number of write and erase cycles that is decidedly greater than one hundred thousand during the life of the storage device, typically three hundred thousand, so that it is necessary for the number of bad memory blocks that will inevitably be created during the further two hundred thousand write and erase cycles not to be higher than another threshold, typically in the region of 1.8% of the non-bad memory blocks after the first one hundred thousand cycles. [0008] To manage the presence of bad memory blocks, it is known practice to provide, in nonvolatile-memory devices, a certain number of spare memory blocks, in which the bad memory blocks are re-mapped. [0009] For example, in a 512-Mbit flash memory device, with 4096 memory blocks, each formed by 32 pages of 528 bytes each, of which 512 defining the data area and the remaining 16 defining the reserved area, it is necessary to provide a number of spare memory blocks equal to 153, of which 82 have the function of guaranteeing reaching the first one hundred thousand write and erase cycles, and the other 71 have the function of guaranteeing reaching three hundred thousand write and erase cycles. [0010] The management of the bad memory blocks is performed by some manufacturers of nonvolatile-memory devices by dividing the memory array into two physically distinct areas, one reserved to the users (user-block area--UBA), and one reserved to re-mapping of bad memory blocks present in the UBA (reserved-block area--RBA). [0011] In particular, re-mapping of bad memory blocks into spare memory blocks is performed by first constructing a re-directing table, which, for each bad memory block, stores the address of the bad memory block and the address of the corresponding spare memory block, and then re-directing any request for access to said bad memory blocks to the corresponding spare memory blocks. [0012] The re-directing table is then stored permanently in the area of memory reserved to the users and is read whenever the user requests access to a memory block. It is then verified whether the address of the memory block to which access is requested is contained in the re-directing table and, if it is, the address of the spare memory block in which said bad memory block has been re-mapped is read, and the access is then re-directed towards said spare memory block. [0013] Generally, however, the re-directing table involves a rather high occupation of memory, so that its storage typically entails a considerable reduction in the amount of memory actually available to users. SUMMARY OF THE INVENTION [0014] One aim of the present invention is to provide a method for managing bad memory blocks in a nonvolatile-memory device, implementation of which will call for a reduced occupation of memory. [0015] According to the present invention a method is provided for managing bad memory blocks in a nonvolatile-memory device, comprising a plurality of available memory blocks, each of which associated to a respective logic address, said management method comprising the steps of: dividing the available memory blocks into a first set and a second set, the first set being formed by addressable memory blocks that are to be used by a user, and the second set being formed by spare memory blocks reserved to the replacement of bad addressable memory blocks; each memory block assuming, in the respective set, a respective position; and re-mapping the bad addressable memory blocks in corresponding spare memory blocks; said step of re-mapping comprising the step of: constructing a data base containing, for each bad addressable memory block, information indicating the logic address of the corresponding spare memory block; said management method being characterized in that said step of creating a data base comprises the steps of: seeking bad spare memory blocks; storing the logic address of each bad spare memory block in a re-directing vector in a position corresponding to that of the bad spare memory block in the respective set; seeking bad addressable memory blocks; and storing the logic address of each bad addressable memory block in a free position in the re-directing vector. [0016] Moreover, according to the present invention, there is provided a nonvolatile-memory device, comprising a plurality of memory blocks, and means for managing bad memory blocks, wherein said management means are configured in such a way as to implement a management method comprising the steps of: dividing the available memory blocks into a first set and a second set, the first set being formed by addressable memory blocks that are to be used by a user, and the second set being formed by spare memory blocks reserved to the replacement of bad addressable memory blocks; each memory block assuming, in the respective set, a respective position; and re-mapping the bad addressable memory blocks in corresponding spare memory blocks; said step of re-mapping comprising the step of: constructing a data base containing, for each bad addressable memory block, information indicating the logic address of the corresponding spare memory block; said step of creating a data base comprises the steps of: seeking bad spare memory blocks; storing the logic address of each bad spare memory block in a re-directing vector in a position corresponding to that of the bad spare memory block in the respective set; seeking bad addressable memory blocks; and storing the logic address of each bad addressable memory block in a free position in the re-directing vector. BRIEF DESCRIPTION OF THE DRAWINGS [0017] For a better understanding of the present invention, a preferred embodiment thereof is described hereinafter, purely as a non-limiting example, with reference to the attached drawings, wherein: [0018] FIG. 1 shows a flowchart of the management of the re-mapping of bad memory blocks according to the present invention; [0019] FIG. 2 is a schematic illustration of the memory blocks of a nonvolatile-memory device and a corresponding re-directing vector according to the invention; [0020] FIGS. 3 and 4 show how the re-directing vector of FIG. 1 is modified when a new memory block becomes bad during operation of the storage device; and [0021] FIG. 5 shows a flowchart of the management of a request for access to a bad memory block. Continue reading about Method and managing bad memory blocks in a nonvolatile memory device, and nonvolatile-memory device implementing the management method... 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