Patent Publication Number: US-7594064-B2

Title: Free sector manager for data stored in flash memory devices

Description:
RELATED APPLICATION 
     This application is a continuation of and claims priority from the co-pending U.S. patent application Ser. No. 10/087,590, filed Feb. 27, 2002, entitled “System and Method for Tracking Data Stored in a Flash Memory Device.” 
    
    
     TECHNICAL FIELD 
     This invention relates to flash memory devices, and flash memory controllers. 
     BACKGROUND 
     Flash memory devices have many advantages for a large number of applications. These advantages include their non-volatility, speed, ease of erasure and reprogramming, small physical size and related factors. There are no mechanical moving parts and as a result such systems are not subject to failures of the type most often encountered with hard disk storage systems. As a result many portable computer devices, such as laptops, portable digital assistants, portable communication devices, and many other related devices are using flash memory as the primary medium for storage of information. 
     Flash memory devices are generally operated by first setting all bits in a block to a common state, and then reprogramming them to a desired new state. Blocks of data need to be shuffled during the reprogramming process, which can slow the completion of the operation. Besides being time consuming, reprogramming a block of data can subject the entire block to accidental loss, in the event there is a power failure during the reprogramming process. Normally, as the block is shuffled, it is temporarily stored in a volatile memory device, such as Random Access Memory (RAM). The entire block of data (not just newly entered data) is susceptible to permanent loss if the reprogramming process has not completed prior to the power failure. In these circumstances, an entire block of data may need to be reentered by a user anew. 
     SUMMARY 
     A system and method for tracking data stored in a flash memory device is described. The system and method allows write operations to complete without interruption, because there is no requirement to perform an erase operation in order to perform a write operation to the flash memory medium. 
     In one described implementation, a request to write data to a logical sector address of a flash memory medium is received from a file system. A free physical sector address is assigned to the logical sector address, which forms a corresponding relationship between these two addresses. This corresponding relationship is stored in a table. The data is then written into a physical sector of the flash memory medium at a location indicated by the physical sector address. 
     Data loss, due to power interruption during a write operation, is also minimized in a described implementation. The logical-to-physical sector mapping stored in the table is backed-up on the flash memory medium. In the event there is a catastrophic power interruption, logical-to-physical sector mapping can easily be reestablished by scanning the backed-up mapping stored on the flash memory medium. For example, a logical address sector corresponding to a physical address sector can be stored in the error code correction portion of a NAND flash memory medium or within the physical sector of a NOR flash memory medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. 
         FIG. 1  illustrates a logical representation of a NAND flash memory medium. 
         FIG. 2  illustrates a logical representation of a NOR flash memory medium. 
         FIG. 3  illustrates pertinent components of a computer device, which uses one or more flash memory devices to store information. 
         FIG. 4  illustrates a block diagram of flash abstraction logic. 
         FIG. 5  illustrates an exemplary block diagram of a flash medium logic. 
         FIG. 6A  shows a data structure used to store a corresponding relationship between logical sector addresses and physical sector addresses. 
         FIG. 6B  shows a data structure which is the same as the data structure in  FIG. 6B , except its contents have been updated. 
         FIG. 7  illustrates a process used to track data on the flash memory medium when the file system issues write requests to the flash driver. 
         FIG. 8  illustrates a process for safeguarding mapping of logical-to-physical sector address information stored in volatile data structures, such as the data structures shown in  FIGS. 6A and 6B . 
         FIG. 9  illustrates a location within the flash memory medium in which the logical sector address can be stored for safeguarding in the event of a power failure. 
         FIG. 10  illustrates a dynamic look-up data structure to track data stored in the flash memory medium. 
         FIG. 11  illustrates a process for dynamically allocating look-up data structures for tracking data on the flash memory medium. 
         FIG. 12  is a diagram of the flash memory medium viewed and/or treated as a continuous circle by the flash driver. 
         FIG. 13  depicts another illustration of the media viewed as a continuous circle. 
         FIG. 14  illustrates a process used by the sector manager to determine the next available free sector location for the flash driver to store data on the medium. 
         FIG. 15  illustrates another view of media treated as a continuous circle. 
         FIG. 16  is a flow chart illustrating a process used by the compactor to recycle sectors. 
         FIG. 17  shows one exemplary result from the process illustrated in  FIG. 16 . 
         FIG. 18  illustrates a logical representation of a NOR flash memory medium divided in way to better support the processes and techniques implemented by the flash driver. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to flash drivers. The subject matter is described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different elements or combinations of elements similar to the ones described in this document, in conjunction with other present or future technologies. 
     Overview 
     This discussion assumes that the reader is familiar with basic operating principles of flash memory media. Nevertheless, a general introduction to two common types of nonvolatile random access memory, NAND and NOR Flash memory media, is provided to better understand the exemplary implementations described herein. These two example flash memory media were selected for their current popularity, but their description is not intended to limit the described implementations to these types of flash media. Other electrically erasable and programmable read-only memories (EEPROMs) would work too. In most examples used throughout this Detailed Description numbers shown in data structures are in decimal format for illustrative purposes. 
     Universal Flash Medium Operating Characteristics 
       FIG. 1  and  FIG. 2  illustrate logical representations of example NAND and NOR flash memory media  100 ,  200 , respectively. Both media have universal operating characteristics that are common to each, respectively, regardless of the manufacturer. For example referring to  FIG. 1 , a NAND flash memory medium is generally split into contiguous blocks ( 0 ,  1 , through N). Each block  0 ,  1 ,  2 , etc. is further subdivided into K sectors  102 ; standard commercial NAND flash media commonly contain 8, 16, or 32 sectors per block. The amount of blocks and sectors can vary, however, depending on the manufacturer. Some manufacturers refer to “sectors” as “pages.” Both terms as used herein are equivalent and interchangeable. 
     Each sector  102  is further divided into two distinct sections, a data area  103  used to store information and a spare area  104  which is used to store extra information such as error correction code (ECC). The data area  103  size is commonly implemented as 512 bytes, but again could be more or less depending on the manufacturer. At 512 bytes, the flash memory medium allows most file systems to treat the medium as a nonvolatile memory device, such as a fixed disk (hard drive). As used herein RAM refers generally to the random access memory family of memory devices such as DRAM, SRAM, VRAM, VDO, and so forth. Commonly, the size of the area spare  104  is implemented as 16 bytes of extra storage for NAND flash media devices. Again, other sizes, greater or smaller can be selected. In most instances, the spare area  104  is used for error correcting codes, and status information. 
     A NOR memory medium  200  is different than NAND memory medium in that blocks are not subdivided into physical sectors. Similar to RAM, each byte stored within a block of NOR memory medium is individually addressable. Practically, however, blocks on NOR memory medium can logically be subdivided into physical sectors with the accompanying spare area. 
     Aside from the overall layout and operational comparisons, some universal electrical characteristics (also referred to herein as “memory requirements” or “rules”) of flash devices can be summarized as follows:
         1. Write operations to a sector can change an individual bit from a logical ‘1’ to a logical ‘0’, but not from a logical ‘0’ to logical ‘1’ (except for case No. 2 below);   2. Erasing a block sets all of the bits in the block to a logical ‘1’;   3. It is not generally possible to erase individual sectors/bytes/bits in a block without erasing all sectors/bytes within the same block;   4. Blocks have a limited erase lifetime of between approximately 100,000 to 1,000,000 cycles;   5. NAND flash memory devices use ECC to safeguard against data corruption due to leakage currents; and   6. Read operations do not count against the write/erase lifetime.       

     Flash Driver Architecture 
       FIG. 3  illustrates pertinent components of a computer device  300 , which uses one or more flash memory devices to store information. Generally, various different general purpose or special purpose computing system configurations can be used for computer device  300 , including but not limited to personal computers, server computers, hand-held or laptop devices, portable communication devices, multiprocessor systems, microprocessor systems, microprocessor-based systems, programmable consumer electronics, gaming systems, multimedia systems, the combination of any of the above example devices and/or systems, and the like. 
     Computer device  300  generally includes a processor  302 , memory  304 , and a flash memory media  100 / 200 . The computer device  300  can include more than one of any of the aforementioned elements. Other elements such as power supplies, keyboards, touch pads, I/O interfaces, displays, LEDs, audio generators, vibrating devices, and so forth are not shown, but could easily be a part of the exemplary computer device  300 . 
     Memory  304  generally includes both volatile memory (e.g., RAM) and non-volatile memory (e.g., ROM, PCMCIA cards, etc.). In most implementations described below, memory  304  is used as part of computer device&#39;s 302 cache, permitting application data to be accessed quickly without having to permanently store data on a non-volatile memory such as flash medium  100 / 200 . 
     An operating system  309  is resident in the memory  304  and executes on the processor  302 . An example operating system implementation includes the Windows®CE operating system from Microsoft Corporation, but other operation systems can be selected from one of many operating systems, such as DOS, UNIX, etc. For purposes of illustration, programs and other executable program components such as the operating system are illustrated herein as discrete blocks, although it is recognized that such programs and components reside at various times in different storage components of the computer, and are executed by the processor(s) of the computer device  300 . 
     One or more application programs  307  are loaded into memory  304  and run on the operating system  309 . Examples of applications include, but are not limited to, email programs, word processing programs, spreadsheets programs, Internet browser programs, as so forth. 
     Also loaded into memory  304  is a file system  305  that also runs on the operating system  309 . The file system  305  is generally responsible for managing the storage and retrieval of data to memory devices, such as magnetic hard drives, and this exemplary implementation flash memory media  100 / 200 . Most file systems  305  access and store information at a logical level in accordance with the conventions of the operating system the file system  305  is running. It is possible for the file system  305  to be part of the operating system  309  or embedded as code as a separate logical module. 
     Flash driver  306  is implemented to function as a direct interface between the file system  305  and flash medium  100 / 200 . Flash driver  306  enables computer device  300  through the file system  305  to control flash medium  100 / 200  and ultimately send/retrieve data. As shall be described in more detail, however, flash driver  306  is responsible for more than read/write operations. Flash driver  306  is implemented to maintain data integrity, perform wear-leveling of the flash medium, minimize data loss during a power interruption to computer device  300  and permit OEMs of computer devices  300  to support their respective flash memory devices regardless of the manufacturer. The flash driver  306  is file system agnostic. That means that the flash driver  306  supports many different types of files systems, such as File Allocation Data structure File System (FAT16), (FAT32), and other file systems. Additionally, flash driver  306  is flash memory medium agnostic, which likewise means driver  306  supports flash memory devices regardless of the manufacturer of the flash memory device. That is, the flash driver  306  has the ability to read/write/erase data on a flash medium and can support most, if not all, flash devices. 
     In the exemplary implementation, flash driver  306  resides as a component within operating system  309 , that when executed serves as a logical interface module between the file system  305  and flash medium  100 / 200 . The flash driver  306  is illustrated as a separate box  306  for purposes of demonstrating that the flash driver when implemented serves as an interface. Nevertheless, flash driver  306  can reside in other applications, part of the file system  305  or independently as separate code on a computer-readable medium that executes in conjunction with a hardware/firmware device. 
     In one implementation, flash driver  306  includes: a flash abstraction logic  308  and a programmable flash medium logic  310 . Flash abstraction logic  308  and programmable medium logic  310  are coded instructions that support various features performed by the flash driver  306 . Although the exemplary implementation is shown to include these two elements, various features from each of the flash abstraction logic  308  and flash medium logic  310  may be selected to carry out some of the more specific implementations described below. So while the described implementation shows two distinct layers of logic  308 / 310 , many of the techniques described below can be implemented without necessarily requiring all or a portion of the features from either layer of logic. Furthermore, the techniques may be implemented without having the exact division of responsibilities as described below. 
     In one implementation, the Flash abstraction logic  308  manages those operating characteristics that are universally common to flash memory media. These universal memory requirements include wear-leveling, maintaining data integrity, and handling recovery of data after a power failure. Additionally, the flash abstraction logic  308  is responsible for mapping information stored at a physical sector domain on the flash memory medium  100 / 200  to a logical sector domain associated with the file system  305 . That is, the flash abstraction logic  308  tracks data going from a logical-to-physical sector addresses and/or from a physical-to-logical sector addresses. Driver  306  uses logical-to-physical sector addresses for both read/write operations. Driver  306  goes from physical-to-logical sector addresses when creating a look-up table (to be described below) during driver initialization. Some of the more specific commands issued by the file system that are dependent upon a certain type of flash memory media are sent directly to the flash medium logic  310  for execution and translation. Thus, the flash abstraction logic  308  serves as a manager to those universal operations, which are common to flash memory media regardless of the manufacturer for the media, such as wear-leveling, maintaining data integrity, handling data recovery after a power failure and so forth. 
       FIG. 4  illustrates an exemplary block diagram of the flash abstraction logic  308 . Flash abstraction logic  308  includes a sector manager  402 , a logical-to-physical sector mapping module  404 , and a compactor  406 . Briefly, the sector manager  402  provides a pointer to a sector available, i.e., “free” to receive new data. The logical-to-physical sector mapping module  404  manages data as it goes from a file system domain of logical sector addressing to a flash medium domain of physical sector addressing. The compactor  406  provides a mechanism for clearing blocks of data (also commonly referred to in the industry as “erasing”) to ensure that enough free sectors are available for writing data. Additionally, the compactor  406  helps the driver  306  system perform uniform and even wear leveling. All these elements shall be described in more detail below. 
     Referring back to  FIG. 3 , the flash medium logic  310  is used to translate logical commands, received from either the flash abstraction logic  308  or file system  305 , to physical sector commands for issuance to the flash memory medium  100 / 200 . For instance, the flash medium logic  310  reads, writes, and erases data to and/or from the flash memory medium. The flash medium logic  310  is also responsible for performing ECC (if necessary). In one implementation, the flash medium logic  310  is programmable to permit users to match particular flash medium requirements of a specific manufacturer. Thus, the flash medium logic  310  is configured to handle specific nuances, ECC, and specific commands associated with controlling physical aspects of flash medium  100 / 200 . 
       FIG. 5  illustrates an exemplary block diagram of the flash medium logic  310 . As shown, the flash medium logic  310  includes a programmable entry point module  502 , I/O module  504  and an ECC module  506 . The programmable entry point module  502  defines a set of programming interfaces to communicate between flash abstraction logic  308  and flash medium  100 / 200 . In other words, the programmable entry points permit manufacturers of computer devices  300  to program the flash media logic  310  to interface with the actual flash memory medium  100 / 200  used in the computer device  300 . The I/O module  504  contains specific code necessary for read/write/erase commands that are sent to the Flash memory medium  100 / 200 . The user can program the ECC module  506  to function in accordance with any particular ECC algorithm selected by the user. 
     Tracking Data 
     File system  305  uses logical sector addressing to read and store information on flash memory medium  100 / 200 . Logical sector addresses are address locations that the file system reads and writes data to. They are “logical” because they are relative to the file system. In actuality, data may be stored in completely different physical locations on the flash memory medium  100 / 200 . These physical locations are referred to as physical sector addresses. 
     The flash driver  306  is responsible for linking all logical sector address requests (i.e., read &amp; write) to physical sector address requests. The process of linking logical-to-physical sector addresses is also referred to herein as mapping. Going from logical to physical sector addresses permits flash driver  306  to have maximum flexibility when deciding where to store data on the flash memory medium  100 / 200 . The logical-to-physical sector mapping module  404  permits data to be flexibly assigned to any physical location on the flash memory medium, which provides efficiency for other tasks, such as wear-leveling and recovering from a power failure. It also permits the file system  305  to store data in the fashion it is designed to do so, without needing intelligence to know that the data is actually being stored on a flash medium in a different fashion. 
       FIG. 6A  shows an exemplary implementation of a data structure (i.e., a table)  600 A generated by the flash driver  306 . The data structure  600 A is stored in a volatile portion of memory  304 , e.g. RAM. The data structure  600 A includes physical sector addresses  602  that have a corresponding logical sector address  604 . An exemplary description of how table  600 A is generated is described with reference to  FIG. 7 . 
       FIG. 7  illustrates a process  700  used to track data on the flash memory medium  100 / 200  when the file system  305  issues write requests to the flash driver  306 . Process  700  includes steps  702 - 718 . Referring to  FIGS. 6A and 7 , in step  702 , flash abstraction logic  308  receives a request to write data to a specified logical sector address  604 . 
     In step  704 , the sector manager  402  ascertains a free physical sector address location on the flash medium  100 / 200  that can accept data associated with the write request (how the sector manager  402  chooses physical sector addresses will be explained in more detail below). A free physical sector is any sector that can accept data without the need to be erased first. Once the sector manager  402  receives the physical sector address associated with a free physical sector location, the logical-to-physical sector mapping module  404  assigns the physical sector address to the logical sector address  604  specified by write request forming a corresponding relationship. For example, a physical sector address of  0  through N can be assigned to any arbitrary logical sector address  0  through N. 
     Next, in step  706 , the logical-to-physical sector mapping module  404  stores the corresponding relationship of the physical sector address to the logical sector address in a data structure, such as the exemplary table  600 A in memory  305 . As shown in the exemplary data structure  600 A, three logical sector addresses  604  are assigned to corresponding physical sector addresses  602 . 
     Next, in step  708  data associated with the logical sector address write request is stored on the flash medium  100 / 200  at the physical sector address location assigned in step  704 . For example, data would be stored in physical sector address location of zero on the medium  100 / 200 , which corresponds to the logical sector address of  11 . 
     Now, in step  710 , suppose for example purposes the file system  305  issues another write request, but in this case, to modify data associated with a logical sector address previously issued in step  702 . Then, flash driver  306  performs steps  712  through  714 , which are identical to steps  704  through  708 , respectively, which are described above. 
     In step  718 , however, after the updated data associated with step  710  is successfully stored on the flash medium  100 / 200 , the logical-to-physical sector mapping module  404  marks the old physical sector address assigned in step  704  as “dirty.” Old data is marked dirty after new data is written to the medium  100 / 200 , so in the event there is a power failure in the middle of the write operation, the logical-to-physical sector mapping module  404  will not lose old data. It is possible to lose new or updated data from steps  702  or  710 , but since there is no need to perform an erase operation only one item of new or modified data is lost in the event of a power failure. 
       FIG. 6B  shows a data structure  600 B which is the same as data structure  600 A, except its contents have been updated. In this example the file system  305  has updated data associated with logical sector address  11 . Accordingly, the flash driver  306  reassigns logical sector address  11  to physical sector address  3  and stores the reassigned corresponding relationship between the these two addresses in data structure  600 B. As illustrated in data structure  600 B, the contents of logical sector  11  are actually written to physical sector address  3  and the contents of sector  0  are marked “dirty” after the data contents are successfully written into physical sector address  3  as was described with reference to steps  710 - 718 . 
     This process of reassigning logical-to-physical sector address when previously stored data is updated by the file system  305 , permits write operations to take place without having to wait to move an entire block of data and perform an erase operation. So, process  700  permits the data structure to be quickly updated and then the physical write operation can occur on the actual physical medium  100 / 200 . Flash abstraction logic  308  uses the data structures, such as  600 A/ 600 B, to correctly maintain logical-to-physical mapping relationships. 
     When there is a read request issued by the files system  305 , the flash abstraction logic  308 , through the logical-to-physical mapping module  404 , searches the data structure  600 A/ 600 B to obtain the physical sector address which has a corresponding relationship with the logical sector address associated with read request. The flash medium logic  310  then uses that physical sector address as a basis to send data associated with the read request back to the file system  305 . The file system  305  does not need intelligence to know that its requests to logical sector addresses are actually mapped to physical sector addresses. 
     Power-Interruption Protection 
     Write operations are performed at the sector-level as opposed to the block-level, which minimizes the potential for data loss during a power-failure situation. A sector worth of data is the finest level of granularity that is used with respect to most file systems  305 . Therefore, if the flash driver  306  is implemented to operate at a per sector basis, the potential for data loss during a power failure is reduced. 
     As mentioned above, data structures  600 A,  600 B are stored in memory  304 , which in one exemplary implementation is typically a volatile memory device subject to complete erasure in the event of a power failure. To safeguard data integrity on the flash medium  100 / 200 , logical-to-physical mapping information stored in the data structures  600 A/ 600 B is backed-up on the flash memory medium. 
     In one exemplary implementation, to reduce the cost associated with storing the entire data structure on the flash memory medium  100 / 200 , the logical sector address is stored in the spare  104  area of the medium with each physical sector in which the logical sector address has a corresponding relationship. 
       FIG. 8  illustrates a process  800  for safeguarding mapping of logical-to-physical sector address information stored in volatile data structures, such as exemplary data structures  600 A and  600 B. Process  800  includes steps  802 - 814 . The order in which the process is described is not intended to be construed as a limitation. Furthermore, the process can be implemented in any suitable hardware, software, firmware, or combination thereof. In step  802 , the logical sector address associated with the actual data is stored in the physical sector of the flash memory medium  100 / 200  at the physical sector address assigned to the logical sector address. In the case of a NAND flash memory medium  100 , the logical sector address is stored in the spare area  104  of the medium. Using this scheme, the logical-to-physical sector mapping information is stored in a reverse lookup format. Thus, after a power failure situation, it is necessary to scan the spare area for each physical sector on the media, determine the corresponding logical sector address, and then update the in-memory lookup table accordingly.  FIG. 9  illustrates a location with in media  100 / 200  in which the logical sector address can be stored. As previously mentioned, blocks of NOR flash memory can be logically subdivided into physical sectors each with a spare area (similar to NAND). Using this technique, the logical sector address is stored in the spare area for each the physical sector similar to the process used with NAND flash memory (shown in  FIG. 15  as space  1504  to be described with reference to  FIG. 15 ). 
     In the event there is a power interruption and the data structures  600 A,  600 B are lost, as indicated by the YES branch of decisional step  804  of  FIG. 8 , then flash abstraction logic  308  uses the flash medium logic  310  to scan the flash memory medium to locate the logical sector address stored with data in each physical address (see  FIG. 9 ), as indicated in step  806 . In step  808 , the physical sector address in which data is contained is reassigned to the logical sector address located with the data on the medium. As the physical and logical sector address are reestablished they are stored back in the data structures  600 A,  600 B and the flash medium logic  310  goes to the next sector containing data as indicated in step  812 . Steps  806 - 812  repeat until all sectors containing data have been are scanned and the data structure is reestablished. Normally, this occurs at initialization of the computer device  300 . 
     Accordingly, when a power failure occurs, process  800  enables the flash abstraction logic  308  to scan the medium  100 / 200  and rebuild the logical-to-physical mapping in a data structure such as the exemplary data structure  600 . Process  800  ensures that mapping information is not lost during a power failure and that integrity of the data is retained. 
     Dynamic Look-Up Data Structure for Tracking Data 
       FIG. 10  illustrates a dynamic look-up data structure  1000  to track data stored in the flash memory medium  100 / 200 . Data structure  1000  includes a master data structure  1002  and one or more secondary data structures  1004 ,  1006 . The data structures are generated and maintained by the flash driver  306 . The data structures are stored in a volatile portion of memory  304 . The one or more secondary tables  1004 ,  1006  contain mappings of logical-to-physical sector addresses. Each of the secondary data structures  1004 ,  1006 , as will be explained, has a predetermined capacity of mappings. The master data structure  1002  contains a pointer to each of the one or more secondary data structures  1004 ,  1006 . Each secondary data structure is allocated on as needed basis for mapping those logical-to-physical addresses that are used to store data. Once the capacity of a secondary data structure  1004 ,  1006 , etc., is exceeded, another secondary data structure is allocated, and another, etc., until eventually all possible physical sector addresses on the flash medium  100 / 200  are mapped to logical sector addresses. Each time a secondary table is allocated, a pointer contained in the master data structure  1002  is enabled by the flash driver  306  to point to it. 
     Accordingly, the flash driver  306  dynamically allocates one or more secondary data structures  1004 ,  1006  based on the amount of permanent data stored on the flash medium itself. The size characteristics of the secondary data structures are computed at run-time using the specific attributes of the flash memory medium  100 / 200 . Secondary data structures are not allocated unless the secondary data structure previously allocated is full or insufficient to handle the amount of logical address space required by the file system  305 . Dynamic look-up data structure  1000 , therefore, minimizes usage of memory  304 . Dynamic look-up data structure  1000  lends itself to computer devices  300  that use calendars, inboxes, documents, etc. where most of the logical sector address space will not need to be mapped to a physical sector address. In these applications, only a finite range of logical sectors are repeatedly accessed and new logical sectors are only written when the application requires more storage area. 
     The master data structure  1002  contains an array of pointers,  0  through N that point to those secondary data structures that are allocated. In the example of  FIG. 10 , the pointers at location  0  and  1  point to secondary data structures  1004  and  1006 , respectively. Also, in the example illustration of  FIG. 10 , pointers  2  through N do not point to any secondary data structures and would contain a default setting, “NULL”, such that the logical-to-physical sector mapping module  404  knows that there are no further secondary data structures allocated. 
     Each secondary data structure  1004 ,  1006  is similar to data structures  600 , but only a portion of the total possible medium is mapped in the secondary data structures. The secondary data structures permit the flash abstraction logic  308  to reduce the amount space needed in memory  304 , to only those portions of logical sectors addresses issued by the file system. Each secondary data structure is (b*k) bytes in size, where k is the number of physical sector addresses contained in the data structure and b is the number of bytes used to store each physical sector address. 
       FIG. 11  illustrates a process  1100  for dynamically allocating look-up data structures for tracking data on the flash memory medium  100 / 200 . Process  1100  includes steps  1102  through  1106 . The order in which the process is described is not intended to be construed as a limitation. Furthermore, the process can be implemented in any suitable hardware, software, firmware, or combination thereof. 
     In step  1102 , a master data structure  1002  containing the pointers to one or more secondary data structures  1004 ,  1006  is generated. The master data structure  1002  in this exemplary implementation is fixed in size. At the time the computer device  300  boots-up, the flash medium logic  310  determines the size of the flash memory medium  100 / 200  and relays this information to the flash abstraction logic  308 . Based on the size of the flash medium, the flash abstraction logic  308  calculates a range of physical addresses. That is, suppose the size of the flash medium is 16 MB, then a NAND flash medium  100  will typically contain 32768 sectors each 512 bytes in size. This means that the flash abstraction logic  308  may need to map a total of 0 through 32768 logical sectors in a worse case scenario, assuming all the memory space is used on the flash medium. Knowing that there are 2 15  sectors on the medium, the flash abstraction logic  308  can use 2 bytes to store the physical sector address for each logical sector address. So the master data structure is implemented as an array of 256 DWORDs (N=256), which covers the maximum quantity of logical sector addresses (e.g., 32768) to be issued by the files system. So, there are a total of 256 potential secondary data structures. 
     In step  1104  the secondary data structure(s) are allocated. First, the flash abstraction logic determines the smallest possible size for each potential secondary data structure. Using simple division, 32768/256=128 logical sector addresses supported by each data structure. As mentioned above, the entire physical space can be mapped using 2 bytes, b=2, therefore, each secondary data structure will by 256 bytes in size or (b=2*k=128). 
     Now, knowing the size of each secondary data structure, suppose that the file system  305  requests to write to logical sector addresses  50 - 79 , also known as LS 50 -LS 79 . To satisfy the write requests from the files system  305 , the flash abstraction logic  308  calculates that the first pointer in master data structure  1002  is used for logical sector addresses LS 0 -LS 127  or data structure  1004 . Assuming the first pointer is NULL, the flash abstraction logic  308  allocates data structure  1004  (which is 256 bytes in size) in memory  304 . As indicated in step  1106 , the flash abstraction logic  308  enables the pointer in position  0  of the master data structure to point to data structure  1004 . So, in this example, data structure  1004  is used to store the mapping information for logical sectors LS 50 -LS 79 . 
     The flash abstraction logic  308  allocates a secondary data structure, if the file system  305  writes to the corresponding area in the flash medium  100 / 200 . Typically, only the logical sector addresses that are used are mapped by the flash abstraction logic  308 . So, in the worst case scenario, when the file system  305  accesses the entire logical address space, then all 256 secondary data structures (only two,  1004 ,  1006  are shown to be allocated in the example of  FIG. 10 ), each 256 bytes in size will be allocated requiring a total of 64 KB of space in memory  304 . 
     When an allocated data structure  1004 , for instance, becomes insufficient to store the logical sector address space issued by the file system  305 , then the flash abstraction logic  308  allocates another data structure, like data structure  1006 . This process of dynamically allocating secondary data structures also applies if data structure  1004  becomes sufficient at a later time to again handle all the logical sector address requests made by the file system. In this example, the pointer to data structure  1006  would be disabled by the flash abstraction logic  308 ; and data structure  1006  would become free space in memory  304 . 
     Uniform Wear Leveling and Recycling of Sectors 
       FIG. 12  is a diagram of flash memory medium  100 / 200  viewed and/or treated as a continuous circle  1200  by the flash driver  306 . Physically the flash memory media is the same as either media  100 / 200  shown in  FIGS. 1 and 2 , except the flash abstraction logic  308 , organizes the flash memory medium as if it is a continuous circle  1200 , containing  0 -to-N blocks. Accordingly, the highest physical sector address (individual sectors are not shown in  FIG. 12  to simplify the illustration, but may be seen in  FIGS. 1 and 2 ) within block N and the lowest physical sector address within block  0  are viewed as being contiguous. 
       FIG. 13  illustrates another view of media  100 / 200  viewed as a continuous circle  1200 . In this exemplary illustration, the sector manager  402  maintains a write pointer  1302 , which indicates a next available free sector to receive data on the medium. The next available free sector is a sector that can accept data without the need to be erased first in a prescribed order. The write pointer  1102  is implemented as a combination of two counters: a sector counter  1306  that counts sectors and a block counter  1304  that counts blocks. Both counters combined indicate the next available free sector to receive data. 
     In an alternative implementation, the write pointer  1302  can be implemented as a single counter and indicate the next physical sector that is free to accept data during a write operation. According to this implementation, the sector manager  402  maintains a list of all physical sector addresses free to receive data on the medium. The sector manager  402  stores the first and last physical sector addresses (the contiguous addresses) on the medium and subtracts the two addresses to determine an entire list of free sectors. The write pointer  1302  then advances through the list in a circular and continuous fashion. This reduces the amount of information needed to be stored by the sector manager  402 . 
       FIG. 14  illustrates a process  1400  used by the sector manager  402  to determine the next available free sector location for the flash driver  306  to store data on the medium  100 / 200 . Process  1400  also enables the sector manager  402  to provide each physical sector address (for the next free sector) for assignment to each logical sector address write request by the file system  305  as described above. Process  1400  includes steps  1402 - 1418 . The order in which the process is described is not intended to be construed as a limitation. Furthermore, the process can be implemented in any suitable hardware, software, firmware, or combination thereof. 
     In step  1402 , the X block counter  1304  and Y sector counter  1306  are initially set to zero. At this point it is assumed that no data resides on the medium  100 / 200 . 
     In step  1404 , the driver  306  receives a write request and the sector manager  402  is queried to send the next available free physical sector address to the logical-to-physical sector mapping module  404 . The write request may come from the file system  305  and/or internally from the compactor  406  for recycling sectors as shall be explained in more detail below. 
     In step  1406 , the data is written to the sector indicated by the write pointer  1302 . Since both counters are initially set to zero in this exemplary illustration, suppose that the write pointer  1302  points to sector zero, block zero. 
     In step  1408 , the sector counter  1306  is advanced one valid sector. For example, the write pointer advances to sector one of block zero, following the example from step  1406 . 
     Next, in decisional step  1410 , the sector manager  402  checks whether the sector counter  1306  exceeds the number of sectors K in a block. If the Y count does not exceed the maximum sector size of the block, then according to the NO branch of decisional step  1410 , steps  1404 - 1410  repeat for the next write request. 
     On the other hand, if the Y count does exceed the maximum sector size of the block, then the highest physical sector address of the block was written to and the block is full. Then according to the YES branch of step  1410 , in step  1412  the Y counter is reset to zero. Next, in step  1414 , X block counter  1304  is incremented by one, which advances the write pointer  1302  to the next block at the lowest valid physical sector address, zero, of that block. 
     Next, in decisional step  1416 , the compactor  406  checks whether the X block counter is pointing to a bad block. If it is, X block counter  1304  is incremented by one. In one implementation, the compactor  406  is responsible for checking this condition. As mentioned above, the sector manager stores all of the physical sector addresses that are free to handle a write request. Entire blocks of physical sector addresses are always added by the compactor during a compaction or during initialization. So, the sector manager  402  does not have to check to see if blocks are bad, although the sector manager could be implemented to do so. It should also be noted that in other implementations step  1416  could be performed at the start of process  1400 . 
     In step  1417 , the X block counter  1304  is incremented until it is pointing to a good block. To avoid a continuous loop, if all the blocks are bad, then process  1400  stops at step  1416  and provides an indication to a user that all blocks are bad. 
     Next in decisional step  1418 , the sector manager checks whether the X block counter  1304  exceeds the maximum numbers of blocks N. This would indicate that write pointer  1302  has arrived full circle (at the top of circle  1200 ). If that is the case, then according to the YES branch of step  1418 , the process  1400  repeats and the X and Y counter are reset to zero. Otherwise, according to the NO branch of step  1418 , the process  1400  returns to step  1404  and proceeds. 
     In this exemplary process  1400 , the write pointer  1302  initially starts with the lowest physical sector address of the lowest addressed block. The write pointer  1302  advances a sector at a time through to the highest physical sector address of the highest addressed block and then back to the lowest, and so forth. This continuous and circular process  1400  ensures that data is written to each sector of the medium  100 / 200  fairly and evenly. No particular block or sector is written to more than any other, ensuring even wear-levels throughout the medium  100 / 200 . Accordingly, process  1400  permits data to be written to the next available free sector extremely quickly without expensive processing algorithms used to determine where to write new data while maintaining even wear-levels. Such conventional algorithms can slow the write speed of a computer device. 
     In an alternative implementation, it is possible for the write pointer  1302  to move in a counter clock wise direction starting with highest physical sector address of the highest block address N and decrement its counters. In either case, bad blocks can be entirely skipped and ignored by the sector manager. Additionally, the counters can be set to any value and do not necessarily have to start with the highest or lowest values of for the counters. 
       FIG. 15  illustrates another view of media  100 / 200  viewed as a continuous circle  1200 . As shown in  FIG. 15 , the write pointer  1302  has advanced through blocks  0  through  7  and is approximately half way through circle  1200 . Accordingly, blocks  0  through  7  contain dirty, valid data, or bad blocks. That is, each good sector in blocks  0  through  7  is not free, and therefore, not available to receive new or modified data. Arrow  1504  represents that blocks  0  through  7  contain used sectors. Eventually, the write pointer  1302  will either run out of free sectors to write to unless sectors that are marked dirty or are not valid are cleared and recycled. To clear a sector means that sectors are reset to a writable state or in other words are “erased.” In order to free sectors it is necessary to erase at least a block at a time. Before a block can be erased, however, the contents of all good sectors are copied to the free sectors to a different portion of the media. The sectors are then later marked “dirty” and the block is erased. 
     The compactor  406  is responsible for monitoring the condition of the medium  100 / 200  to determine when it is appropriate to erase blocks in order to recycle free sectors back to the sector manager  402 . The compactor  406  is also responsible for carrying out the clear operation. To complete the clear operation, the compactor  406 , like the sector manager  402 , maintains a pointer. In this case, the compactor  406  maintains a clear pointer  1502 , which is shown in  FIG. 15 . The clear pointer  1502  points to physical blocks and as will be explained enables the compactor  406  to keep track of sectors as the medium  100 / 200  as blocks are cleared. The compactor  406  can maintain a pointer to a block to compact next since an erase operation affects entire blocks. That is, when the compactor  406  is not compacting a block, the compactor  406  points to a block. 
       FIG. 16  is a flow chart illustrating a process  1600  used by the compactor to recycle sectors. Process  1600  includes steps  1602 - 1612 . The order in which the process is described is not intended to be construed as a limitation. Furthermore, the process can be implemented in any suitable hardware, software, firmware, or combination thereof. In step  1602 , the compactor  406  monitors how frequently the flash memory medium  100 / 200  is written to or updated by the file system. This is accomplished by specifically monitoring the quantities of free and dirty sectors on the medium  100 / 200 . The number of free sectors and dirty sectors can be determined counting free and dirty sectors stored in tables  600  and/or  900  described above. 
     In decisional step  1604 , the compactor  406  performs two comparisons to determine whether it is prudent to recycle sectors. The first comparison involves comparing the amount of free sectors to dirty sectors. If the amount of dirty sectors outnumbers the free sectors, then the compactor  406  deems it warranted to perform a recycling operation, which in this case is referred to as a “service compaction.” Thus a service compaction is indicated when the number of dirty sectors outnumbers the quantity of free sectors. 
     If a service compaction is deemed warranted, then in step  1606  the compactor waits for a low priority thread  1606 , before seizing control of the medium to carry out steps  1608 - 1612  to clear blocks of dirty data. The service compaction could also be implemented to occur at other convenient times when it is optional to recycle dirty sectors into free sectors. For instance, in an alternative implementation, when one third of the total sectors are dirty, the flash abstraction logic  308  can perform a service compaction. In either implementation, usually the compactor  406  waits for higher priority threads to relinquish control of the processor  302  and/or flash medium  100 / 200 . Once a low priority thread is available, the process proceeds to step  1608 . 
     Referring back to step  1604 , the second comparison involves comparing the amount of free sectors left on the medium, to determine if the write pointer  1302  is about to or has run out of free sectors to point to. If this is the situation, then the compactor  406  deems it warranted to order a “critical compaction” to recycle sectors. The compactor does not wait for a low priority thread and launches immediately into step  1608 . 
     In step  1608 , the compactor  406  operates at either a high priority thread or low priority thread depending on step  1604 . If operating at a high level thread (critical compaction), the compactor  1102  is limited to recycling a small number, e.g., 16 dirty sectors, into free sectors and return control of the processor back to computer device  300  to avoid monopolizing the processor  302  during such an interruption. 
     Thirty two sectors per block are commonly manufactured for flash media, but other numbers of sectors, larger or smaller, could be selected for a critical compaction. Regardless of these size characteristics, the number of sectors recycled during a critical compaction is arbitrary but must be at least 1 (in order to satisfy the current WRITE request). A critical compaction stalls the file system  305  from being able to complete a write; therefore, it is important to complete the compaction as soon as possible. In the case of a critical compaction, the compactor  406  must recycle at least one dirty sector into a free sector so that there is space on the medium to fulfill the pending write request. Having more than one sector recycled at a time, such as 16, avoids the situation where there are multiple pending write requests and multiple critical compactions that are performed back-to-back, effectively blocking control of the processor indefinitely. So, while the number of sectors recycled chosen for a critical compaction can vary, a number sufficient to prevent back-to-back critical compactions is implemented in the exemplary description. 
     So, in step  1608 , the compactor  406  will use the clear pointer  1502  to scan sectors for valid data, rewrite the data to free sectors, and mark a sector dirty after successfully moving data. Accordingly, when moving data, the compactor uses the same processes described with reference to process  700 , which is the same code that is used when the file system  305  writes new and/or updates data. The compactor  406  queries the sector manager  402  for free sectors when moving data, in the same fashion as described with reference to process  1400 . 
     In step  1610 , the compactor  406  moves the clear pointer  1502  sector-by-sector using a sector counter like the write counter  1306  shown in  FIG. 13 , except this sector counter pertains to the location of the clear pointer  1502 . The compactor  406  also keeps track of blocks through a counter in similar fashion as described with reference to the write pointer  1302 . However, the amount of blocks cleared is determined by the number of dirty sectors with the exception of a critical compaction. In a critical compaction, the compactor only compacts enough blocks to recycle a small number of physical sectors (i.e. 16 sectors). 
     In step  1612 , the compactor erases (clears) those blocks which contain good sectors that are fully marked dirty.  FIG. 17  shows exemplary results from process  1600 . In this example, blocks  0  and  1  were cleared and the clear pointer was moved to the first sector of block  2 , in the event another compaction is deemed warranted. As a result, the compactor  406  recycled two blocks worth of the sectors from blocks  0  and  1 , which provides more free sectors to the sector manager  402 . Used sectors  1504  forms a data stream (hereinafter a “data stream”  1504 ) that rotates in this implementation in a clockwise fashion. The write pointer  1302  remains at the head of the data stream  1504  and the clear pointer  1502  remains at the end or “tail” of the data stream  1504 . The data stream  1504  may shrink as data is deleted, or grow as new data is added, but the pointers always point to opposite ends of the data stream  1504 : head and tail. 
     Treating the flash memory medium as if the physical sector addresses form a continuous circle  1200 , and using the processes described above, enables the flash abstraction logic  308  to accomplish uniform wear-leveling throughout the medium  100 / 200 . The compactor  406  selects a given block the same number times for recycling of sectors through erasure. Since flash blocks have a limited write/erase cycle, the compactor as well as the sector manager distributes these operations across blocks  0 -N as evenly and as fairly as possible. In this regard, the data steam  1504  rotates in the circle  1200  (i.e. the medium  100 / 200 ) evenly providing perfect wear-levels on the flash memory medium  100 / 200 . 
     In the event of power failure, the flash abstraction logic  310  contains simple coded logic that scans the flash memory medium  100 / 200  and determines what locations are marked free and dirty. The logic is then able to deduce that the data stream  1504  resides between the locations marked free and dirty, e.g., the data stream  1106  portion of the circle  1200  described in  FIG. 17 . The head and tail of the data stream  1504  is easily determined by locating the highest of the physical sector addresses containing data for the head and by locating the lowest of the physical sector addresses containing data for the tail. 
     NOR Flash Devices 
     Although all the aforementioned sections in this Detailed Description section apply to NAND and NOR flash devices, if a NOR flash memory medium  200  is used, some additional implementation is needed for the flash medium logic to support the storing of data in each physical sector on the medium  200 . Each NOR block  0 ,  1 ,  2 , etc. can be treated like a NAND flash memory medium  100 , by the flash medium logic  310 . Specifically, each NOR block is subdivided into some number of pages where each page consists of a 512 byte “data area” for sector data and an 8 byte “spare area” for storing things like to the logical sector address, status bits, etc. (as described above). 
       FIG. 18  illustrates a logical representation of a NOR flash memory medium  200  divided in way to better support the processes and techniques implemented by the flash driver. In this implementation, sectors  1802  contain a 512 byte data area  1803  for the storage of sector related data and 8 bytes for a spare area  1804 . Sections  1806  represent unused portions of NOR blocks, because a NOR Flash block is usually a power of 2 in size, which is not evenly divisible. For instance, consider a 16 MB NOR flash memory device that has 128 flash blocks each 128 KB in size. Using a page size equal to 520 bytes, each NOR flash block can be divided into 252 distinct sectors with 32 bytes remaining unused. Unfortunately, these 32 bytes per block are “wasted” by the flash medium logic  310  in the exemplary implementation and are not used to store sector data. The tradeoff, however, is the enhanced write throughput, uniform wear leveling, data loss minimization, etc. all provided by the flash abstraction logic  308  of the exemplary flash driver  306  as described above. Alternative implementations could be accomplished by dividing the medium  200  into different sector sizes. 
     Computer Readable Media 
     An implementation of exemplary subject matter using a flash driver as described above may be stored on or transmitted across some form of computer-readable media. Computer-readable media can be any available media that can be accessed by a computer. By way of example, and not limitation, computer readable media may comprise “computer storage media” and “communications media.” 
     “Computer storage media” include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by a computer. 
     “Communication media” typically embodies computer readable instructions, data structures, program modules, or other data in a modulated data signal, such as carrier wave or other transport mechanism. Communication media also includes any information delivery media. 
     The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared, and other wireless media. Combinations of any of the above are also included within the scope of computer readable media. 
     CONCLUSION 
     Although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claimed invention.