Abstract:
A virtual filesystem for allowing a large number of data files to be created, deleted, and rearranged by a peripheral device incorporating the filesystem without requiring the host computer system to delete and rebuild its cache and other file system tables. Since the directory hierarchy of the virtual filesystem is predefined, data blocks in the directory and file allocation table regions of the filesystem&#39;s filespace do not need to be prestored in memory, but rather are calculable at the time a data block read request is processed by the virtual filesystem. Since the specific location in the filesystem of a file is determined only at the time that one of a set of lowest-level directories which contains the file is accessed, host caching problems can be avoided by predefining a large number of files in the filesystem and avoiding reuse of any previously-used file. The data blocks of a file can be dynamically generated upon request, or can be aliased to corresponding data blocks of an auxiliary file system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application relates to the subject matter disclosed in the co-pending U.S. application Ser. No. 09/686,553 (attorney docket 10003752-1), by Holcomb et al., filed Oct. 11, 2000, titled “Dynamically-Generated Web Pages”. This application is assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to file systems for mass storage devices, and pertains more particularly to a virtual file system for a peripheral device that provides a large block-oriented mass storage device interface to a host system while using a much smaller amount of memory in the peripheral device and dynamically generating the data blocks read by the host.  
         BACKGROUND OF THE INVENTION  
         [0003]    Mass storage peripherals, such as disk drives, are in common use in computer systems today for storing and retrieving data. This data is stored in blocks. Many different logical sets of data, commonly referred to as “files”, can be stored on the peripheral. While the data blocks are typically of a fixed size, a file can contain more data than can fit in a single data block, and so a file may consist of a number of data blocks. In addition, the various data blocks that make up a file do not need to be located together, but rather can be stored in different places on the peripheral. In order to facilitate access to files of data by the computer system, mass storage peripherals typically provide a file system which describes the organization on the peripheral of the data blocks for each file.  
           [0004]    With conventional mass storage peripherals, data blocks are read and written only under the explicit control of the computer system. In other words, the computer system can keep its own independent record of where certain data blocks and files are stored on the peripheral and be assured that, in fact, that is where the data blocks and files are located. In order to speed up the storage and retrieval of frequently used data blocks, computer systems also frequently implement a data block cache internal to the computer system itself that stores these frequently accessed data blocks. The computer system can read a data block from the cache much more quickly than it can from the peripheral, and so computer systems typically search their cache for a desired data block, and request it from the peripheral only if it is not found in the cache.  
           [0005]    The above-described caching mechanism works well in the case of traditional mass storage peripherals, such as disk drives, in which data blocks are only read and written under the explicit control of the computer system. Such disk drives also generally have large enough amounts of memory to store the tables that implement the file system, as well as the data blocks for all the files. However, certain intelligent mass storage peripherals, such as one described in the above-referenced co-pending and commonly owned patent application by Holcomb et al., may dynamically create, delete, or rearrange data blocks independent of the explicit control of the computer system. As a result, the computer system can no longer be assured that the data blocks in its cache match the data blocks on the peripheral, and incorrect operation can result when the computer system reads cached files rather than obtaining files from the peripheral. In order to ensure proper operation, the peripheral must inform the computer system each time it dynamically creates, deletes, or rearranges data blocks, and the computer system must delete and rebuild the cache and other file system tables. Some computer systems provide a mechanism for the peripheral to so inform the computer system; however, the actions to delete and rebuild the cache and other file system tables can be time-consuming and inefficient. Other computer systems do not support such a mechanism at all.  
           [0006]    A related limitation of non-traditional peripherals is that they may not contain sufficient memory to store all the data blocks needed to provide a large file system. As a result, the number and size of files stored on the peripheral may be disadvantageously limited, thus requiring the peripheral to dynamically delete certain files and create others, and giving rise to the previously discussed synchronization problem between the computer system and the peripheral.  
           [0007]    Accordingly, it would be highly desirable to have a new and improved file system and method of accessing a large number of data files which allows files to be created, deleted, and rearranged without requiring the host computer system to delete and rebuild its cache and other file system tables and without requiring a large amount of memory.  
         SUMMARY OF THE INVENTION  
         [0008]    In a preferred embodiment, the present invention provides a virtual filesystem capable of including a large number of data files. The virtual filesystem has a predetermined structure of directories and files, and a linear filespace. The linear filespace includes a directory region and a file allocation table region in which, as a result of the predetermined nature of the directory structure, the data blocks are calculatable upon a read request and are not required to be stored in the filesystem. The linear filespace also includes a file region having a predefined number of predefined-size files. Each of a set of lowest-level directories of the directory structure contains a single file. The particular single file in the file region that is linked to a particular lowest-level directory is preferably determined at the time when the lowest-level directory is first accessed, thus allowing a previously-unused or least-recently-used one of the files to be allocated so as to avoid the need to delete and rebuild the host&#39;s cache. The data blocks of the file are preferably dynamically generated and provided to the virtual filesystem upon request, or aliased to a file stored in an auxiliary filesystem that is read via the virtual filesystem. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The above-mentioned features of the present invention and the manner of attaining them, and the invention itself, will be best understood by reference to the following detailed description of the preferred embodiment of the invention, taken in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a schematic representations of an exemplary predefined file structure for a virtual filesystem according to the present invention;  
         [0011]    [0011]FIG. 2 is a schematic representations of the filespace of the virtual filesystem of FIG. 1;  
         [0012]    [0012]FIG. 3 is a more detailed schematic representations of an exemplary directory region of the filespace of FIG. 2;  
         [0013]    [0013]FIG. 4 is a more detailed schematic representations of an exemplary file allocation table region of the filespace of FIG. 2;  
         [0014]    [0014]FIG. 5 is a flowchart of a method for reading data from the virtual file system of FIG. 1;  
         [0015]    [0015]FIG. 6 is a more detailed flowchart of a data block calculation portion of the flowchart of FIG. 5; and  
         [0016]    [0016]FIG. 7 is a block diagram of an apparatus for providing the virtual file system of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring now to the drawings, there is illustrated a virtual filesystem  10  (also referred to as a “VFS”) constructed in accordance with a novel method  100  of reading data from an apparatus  50  on which the filesystem  10  is implemented. The virtual filesystem  10  advantageously provides a large virtual filespace that can be implemented in only a relatively small amount of memory by predefining the directory hierarchy such that data blocks corresponding to the directories (and an associated file allocation table, also referred to herein as a “FAT”) can be algorithmically calculated, and by binding data files to the directory hierarchy only at or near the time of use such that data blocks corresponding to the data files can be dynamically generated at or near the time of access. The present invention is particularly applicable to FAT-based file systems such as the widely-used MS-DOS file system that is well known to those skilled in the art.  
         [0018]    As is understood by those skilled in the art, a filesystem is a structure of directories, files, and other internal information which an operating system uses to locate and access these directories and files. The directories are typically organized in a directory hierarchy in which some directories are related to other directories in a parent-and-child tree structure. Files are located within directories. The data structures that comprise the filesystem occupy a preferably linear filespace in memory. A virtual filesystem is one in which the directories and files appear to exist in a linear filespace when viewed by a host computer or other device which is external to the filesystem, but in actuality may not exist in linear filespace or may even not exist in physical memory at all until the host computer requests the filesystem to perform an operation on a directory or file.  
         [0019]    As is best understood with reference to the exemplary predefined file structure  15  of FIG. 1, the virtual filesystem  10  of the present invention has a predefined and fixed directory hierarchy  20 , which begins at a root directory “\”  21 . Below the root directory  21  is a set  22  of 1 st -level directories which are subdirectories to the root directory. The exemplary directory hierarchy  20  contains four  1   st -level directories, denoted D 1  through D 4 . The number of subdirectories contained below a higher-level directory is referred to as the “fanout”; the exemplary directory structure uses a fanout of four at all levels, but for clarity only one set of four subdirectories is illustrated in FIG. 1 for each level. 1 st -level directory D 2   23  contains four 2 nd -level directories, which are denoted D 9  through D 12 . 2 nd -level directory D 11   24  contains four 3 rd -level directories, which are denoted D 45  through D 48 . Both the 1 st -level directories and the 2 nd -level directories are considered to be intermediate-level directories in that they have both a higher-level (“parent”) directory and lower-level (“child”) directories. The Yth-level directories (in this exemplary hierarchy  20  the 3 rd -level directories) are the lowest-level directories in the directory hierarchy  20 . Each Yth-level directory according to the present invention contains one of a set  18  of predefined file placeholders, such as directory D 48   25  contains the file placeholder F 88   26 ; the lowest-level directories have a parent directory but no child directories. The significance of the directory and file labeling used in FIG. 1 will become apparent when FIG. 3 is discussed subsequently.  
         [0020]    The directory fanout N and hierarchy depth Y can be advantageously chosen to provide a virtual filesystem having a desired number of files. The number of files is calculated according to the formula N Y . The exemplary hierarchy  20  with a fanout of 4 and a depth of 3 thus provides  4   3 =64 files. The preferred embodiment has a fanout of 32 and a depth of 4, and thus provides 32 4 =1,048,576 files. While the fanout N and the depth Y may be different at each level and branch of the directory hierarchy, in the preferred embodiment they are the same at all levels and branches, as will become apparent when the method of the present invention is discussed subsequently.  
         [0021]    As best understood with reference to FIG. 2, the virtual file system occupies a filespace  30  which contains B logical data blocks, such as data block  2 . Each data block contains b bytes. In the commonly-used FAT-32 file system well known to those skilled in the art, each block typically contains 4k bytes of data. a data block can be further divided, typically according to the type of physical media on which the filesystem is implemented, but such discussions will be omitted here since they are also well known in the art and are not germane to the present invention.  
         [0022]    The data blocks of the virtual filesystem  10  are preferably organized sequentially. The data blocks can be requested, typically by block number, from the virtual file system  10  by a host computer (not shown) or other device. The filespace  30  is logically divided into several contiguous regions. A directory region  32  of D blocks in size predefines the directory hierarchy of the virtual filesystem  10 , as explained heretofore.  
         [0023]    The filespace  30  also contains a number of file regions  34   a,b,c  which in total occupy F blocks of the filespace  30 . Each file region  34   a,b,c  predefines a quantity of data files of a predetermined number of data blocks in size. File region  34   a  defines a quantity Q 1  of data files, each of which is one block in size. File region  34   b  defines a quantity Q 2  of data files, each of which is two blocks in size. File region  34   c  defines a quantity Q M  of data files, each of which is M blocks in size. The size of each file region is determined by the quantity of files and the block size of each file. The number of file regions, the quantity of files in each region, and the block size of the files in each region are not limited by the present invention but rather are design choices available to the filesystem  10  designer.  
         [0024]    The filespace  30  also contains one or more FAT regions  36 . Each FAT region  36  predefines a file allocation table of T blocks in size for locating in the filespace  30  all the data blocks for the directories and the data files. According to the present invention, and as will discussed subsequently in greater detail, these regions are organized so as to allow the data blocks in the directory region  32  and the FAT region  36  to be algorithmically calculated when the filesystem  10  is read in order to reduce the amount of physical memory required to contain the filesystem  10 .  
         [0025]    Considering now in further detail, and with reference to FIG. 3, the directory region  32 , the directory region  32  contains directory entries. Each directory entry contains different fields which identify (at a minimum) the name, the starting block number, the attributes of the entry (in particular, whether it represents a child directory or a file), and the size of each file or child directory. Directory entries describe the contents of each individual directory of the directory hierarchy  20 . In the preferred embodiment, the directory entries for each directory of the hierarchy  20  are contained in a separate data block whose position in the filespace  30  is calculable from the directory hierarchy  20 . To illustrate using the exemplary directory hierarchy  20  of FIG. 1, the directory entries for the root directory  21  are assigned to block  0  of directory region  32 . Block  0  contains four directory entries, which correspond to the directory entries for the four 1 st -level directories (D 1  through D 4 ) of the hierarchy  20 . The numbers shown within each block of the directory region  32  of FIG. 3 represent the value of the starting block field of the particular directory entry in the block, and correspond to the directory numbers used in the directory hierarchy  20 . For example, the second entry  38   a  of block  0  corresponds to 1 st -level directory D 2 , and indicates that the directory entries for directory D 2  may be found in block  2 . The third entry  38   b  of block  2  corresponds to 2 nd -level directory D 11 , and indicates that directory entries for directory D 11  may be found in block  11 . The fourth entry  38   c  of block  11  corresponds to Yth-level directory D 48 , and indicates that directory entries for directory D 48  may be found in block  48 . The first (and only) entry  38   d  of block  48  corresponds to a file placeholder. The file placeholder may or may not have an actual file associated with it at any particular point in time. The value of the starting block field for the file entry indicates the state of the file&#39;s existence. One predefined value indicates that no file has ever been created in directory D 48 , while a different predefined value indicates that a file was previously created in directory D 48  but the file is no longer in existence. If the starting block field contains a value between 0 and F-1, then it indicates that an associated file currently exists, and the starting block field value is the block number in the file region  34  at which the file data is located. As will be described subsequently in further detail, a file placeholder for a Yth-level directory is bound to a particular data file only when a particular Yth-level directory is first accessed.  
         [0026]    The above-described manner of storing N-ary (for example, binary, trinary, etc.) trees such as the directory hierarchy  20  in an array is well known to those skilled in the art. Because the directory hierarchy  20  is fixed and the positions in the filespace  30  of the data blocks for any particular directory are calculable, as will be described subsequently in further detail, a VFS  10  according to the present invention does not need to store the data for the directory region  32 , but rather can algorithmically calculate the data for any particular data block when requested by the host computer.  
         [0027]    Considering now in further detail the FAT region  36 , and with reference to FIG. 4, in many filesystems the data blocks of a file or directory which spans more than one block are not located contiguously in the filespace  30 . While a directory entry, as has been explained previously, indicates the starting block at which data for a directory or file will be found, it does not indicate the location of any additional data blocks. The FAT table provides this linkage between the starting block and any additional blocks. Because the directory hierarchy, and the number and size of files, are predetermined in the VFS  10  of the present invention, the contents of the FAT region are correspondingly calculable algorithmically.  
         [0028]    The size of the FAT region  36  is determined by the number of data blocks contained in the VFS  10 . The FAT  36  contains a FAT entry  42  for each data block in the system. Each FAT entry  42  must be of a size large enough to contain a value that represents the largest block number in the combined directory  32  and file  34  regions (i.e. the value of D+F). For example, the FAT- 32  file system uses a four byte FAT entry  42 , which is sufficient to address up to 2 32 =4,294,967,296 blocks.  
         [0029]    The FAT region  36  includes a directory FAT subregion  36   a  and a file FAT subregion  36   b . Since, as previously discussed, each directory occupies a single data block in the directory region  32 , the number of FAT entries  42  in the directory FAT subregion  36   a  is equal to the number of directories in the directory hierarchy  20 . For instance, in the exemplary directory hierarchy  20  of FIG. 1 in which N=4 and Y=3, there is one root directory, N 1 =4 1 st -level directories, N 2 =16 2 nd -level directories, and N 3 =64 Yth (3 rd )-level directories, for a total of 85 directories; therefore, there are 85 FAT entries  42  in the directory FAT subregion  36   a.    
         [0030]    With regard to the file FAT subregion  36   b , different files can occupy a different number of data blocks in the file region  34 . However, according to the filesystem  10  of the present invention, and as previously described, the number of files and the size of each file are predetermined, so the number of FAT entries  42  required for the FAT subregion  36   b  is algorithmically calculable. For instance, in the exemplary directory hierarchy  20  of FIG. 1 which contains N Y =43=64 files, assume that 32 (Q 1 ) of the files are 1-block files, 16 (Q 2 ) are 2-block files, 10 (Q 3 ) are 4-block files, and 6 (Q 4 ) are 8-block data files. Therefore, 32×1=32 blocks are required for the 1-block files, 16×2=32 blocks are required for the 2-block files, 10×4=40 blocks are required for the 4-block files, and 6×8=48 blocks are required for the M-block files (where M=8); for a total of 32+32+40+48=152 blocks. Therefore, the file FAT subregion  36   b  will contain 152 FAT entries  42 .  
         [0031]    The content of a FAT entry  42  for a particular data block in the directory region  32  or the file region  34  specifies whether the data for that directory or file continues in another data block and, if so, at which block the data continues. If the FAT entry  42  contains a value that corresponds to another block number in the filesystem  10 , then the data continues at that block number. If the FAT entry  42  contains a predefined “end-of-chain” value that does not correspond to a block number in the filesystem  10 , then the data does not continue in any additional block. With reference to the exemplary FAT table  36  of FIG. 4, the starting block of a 4-block data file is located at datafile-relative FAT entry # 64   42 ′. In the exemplary file FAT subregion  36   b  of FIG. 4, FAT entry # 64   42 ′ contains the value 65, indicating that the 4-block data file continues at data block # 65 . Similarly, FAT entry # 65  contains the value 66 indicating that the 4-block data file continues at data block # 66 , and FAT entry # 66  contains the value 67 indicating that the 4-block data file continues at data block # 67 . Since data block # 67  is the last of the four blocks in the chain, FAT entry # 67  contains the “end-of-chain” marker.  
         [0032]    Considering now a novel method  100  of reading data from the virtual filesystem  10 , and with reference to FIG. 5, the method  100  begins at  102  by defining a virtual file system  10  having a filespace  30  containing a logical linear arrangement of data blocks for a predetermined file structure  15  which includes a directory hierarchy  20  and a set of placeholders for predetermined file  18 . At  104 , a request to provide a data block  2  having a specified position in the VFS  10  is received, typically from the host computer connected to the VFS  10 . At  106 , the section of the VFS  10  in which the data block  2  is located is determined. If the data block  2  is located in the FAT region  36  or the directory region  32  (“FAT or Directory” branch of  106 ), then the method continues at  108 .  
         [0033]    If the data block  2  is in the FAT region  36 , or if the data block  2  is in the directory region  32  but corresponds to the root directory or one of the intermediate-level directories in the directory hierarchy  20  (“No” branch of  108 ), then at  114  the content of the data block  2  is calculated based on the predetermined directory hierarchy  20  and the specified block position, and at  116  the content is provided to the host computer in response to its request. The algorithm by which the data block content is calculated will be described subsequently; calculation of the data block content is advantageously enabled by the novel predefined organization of the file structure  15  and filespace  30  of the VFS  10 .  
         [0034]    If the data block  2  is in the directory region  32  and corresponds to one of the lowest-level directories in the directory hierarchy  20  (“Yes” branch of  108 ), then at  110  the actual amount of data to be contained by the file corresponding to the lowest-level directory is determined. This is preferably done by a process external to the virtual filesystem  10  which has previously associated the file placeholder with the data, as will be described subsequently with reference to the virtual filesystem apparatus  50 . Once this actual file size in bytes is known, at  112  the lowest-level directory is bound to an unused (or least recently used) predetermined file having a maximum file size which is greater than or equal to the amount of data, and the method continues at  114 . Typically the smallest predetermined file of sufficient size to contain the data will be bound to the particular lowest-level directory.  
         [0035]    By calculating data blocks  2  in the FAT region  36  and the directory region  32 , the present invention advantageously minimizes the amount of physical memory that is required by the virtual filesystem  10 , particularly as large numbers of files and large file sizes are included in the filesystem  10 .  
         [0036]    If the data block  2  is located in the file region  34  (“File” branch of  106 ), then at  116  the data source for the file is determined. If the data is to be dynamically generated (“Dynamic” branch of  116 ), then at  118  the content of the data block  2  is dynamically generated, preferably by a process external to the virtual filesystem  10 , and at  122  the content is provided to the host computer in response to its request. If the data is to be aliased from another data memory such as an auxiliary file system  70  external to the VFS  10 , then at  120  the content of the data block  2  is obtained from the auxiliary file system  70 , and at  122  the content is provided to the host computer in response to its request..  
         [0037]    By dynamically generating data blocks  2  as they are requested, or by obtaining data blocks  2  from an auxiliary file system  70  already present in an apparatus in which the virtual filesystem  10  is included, the present invention advantageously minimizes the amount of physical memory that is required by the virtual filesystem  10 , particularly as large numbers of files and large file sizes are included in the filesystem  10 .  
         [0038]    The method  100  is preferably implemented in firmware and/or software as a program of instructions contained on a program storage medium such as a ROM, CD-ROM, floppy disk, and the like, executable by a computing apparatus (not shown) incorporated within the virtual filesystem  10 . Such an implementation is well known to those skilled in the art. The program of instructions can be organized into a number of logical segments for performing the steps of the method  100  heretofore described to implement the file structure  15  and filespace  30  that have also been described.  
         [0039]    Considering now in further detail the calculating  114  of the contents of a data block  2  in the FAT region  36  or directory region  32  based on the predetermined directory hierarchy  20 , and as best understood with reference to FIGS. 2, 3,  4 , and  6 , at  130  the location of the requested data block within the FAT region  36  or directory region  32  of the filespace  30  is determined. If the requested data block is in the directory region  32  and corresponds to any directory other than a lowest-level (Yth level) directory, then at  132  the block number of the requested data block is converted to a directory-relative block number for the requested directory. At  134 , the directory-relative block numbers of all child subdirectories of the requested directory are determined, preferably by calculating the block numbers of the child subdirectories based on the depth and fanout of the predetermined directory hierarchy  20  according to a conventional algorithm known to those skilled in the art for traversing an N-ary tree stored in an array. For example, if the requested block corresponds to directory-relative block # 6 , child subdirectories # 25 , # 26 , # 27 , and # 28  are identified. Once the appropriate child subdirectories have been determined, then at  136 , directory entry data for these child subdirectories of the requested directory are created.  
         [0040]    If the requested data block is in the directory region  32  and corresponds to a lowest-level (Yth level) directory, then at  13   8  directory entry data representing a file is created. This data is derived from information contained in an entry  62  in the file shadow table  60  which corresponds to that lowest-level directory (the shadow table  60  and its entries  62  will be subsequently described in further detail with reference to FIG. 7). For example, if the requested block corresponds to directory-relative block # 48 , then directory entry data  38   d  representing the file contained in the lowest-level directory associated with block # 48  is created.  
         [0041]    If the requested data block is in the FAT region  36 , then at  140  the FAT entries contained in the requested data block are identified. In the preferred embodiment, these entries are readily calculated based on the size of the data block and the size of each FAT entry. Each FAT entry  42  refers to a data block  2  in the directory region  32  or the file region  34 . At  142 , it is determined which of these FAT entries correspond to blocks that are the last (or only) block in a block chain, and which correspond to blocks that are not the last (or only) block in a block chain. In the preferred embodiment, this can be readily calculated based on the organization of the filespace  30 , since all directories occupy one data block, and the number of data files of each size M are predetermined. At  144 , FAT entry data containing an end-of-chain marker is created for all FAT entries which correspond to the last (or the only) block in a block chain, while at  146  FAT entry data pointing to the next block in the chain is created for all FAT entries which do not correspond to the last (or the only) block in a block chain. For example, if the requested data block includes FAT entries for datafile-relative blocks # 64  through # 67 , the FAT entry created for block # 64  will point to block # 65 , while the FAT entry created for block # 67  will contain the end-of-chain marker.  
         [0042]    The present invention can also be embodied, as best understood with reference to FIG. 7, as an apparatus  50  which incorporates a virtual file system  10  to provide a block-oriented data source that can be accessed from a host computer (not shown). The VFS  10  further includes a region identifier  52  which receives “Read Block &lt;B&gt;” requests from the host computer, where &lt;B&gt; is the absolute data block number B in the VFS  10 . The region identifier  52  determines whether block B corresponds to a block in the FAT region  36  or directory region  32  and can therefore be calculated as previously described, or whether block B corresponds to a block in the file region  34  and therefore file data must be obtained as previously described. In the preferred embodiment, the region identifier  52  converts the absolute block number B into a block number relative to the region  32 , 34 , 36  to which the block belongs. If the block is in the FAT region  36  or the directory region  32 , a block calculator  54  which has knowledge of the predetermined file structure  15  along with the fanout N, depth Y, file quantums Q, and the number of blocks associated with each file quantum Q calculates the contents of the requested data block. If the block number corresponds to a Yth-level directory, a Yth-directory file binder  56  sends a request to a dynamic data generator  58  to bind a file shadow  62  in a file shadow table  60  to a particular predefined file of a given size in the filesystem  10 .  
         [0043]    The file shadow  62  has typically been previously created by an asynchronous request made to the dynamic data generator  58  by another module (not shown) of the apparatus  50 . Examples of such requests are described in the above-referenced co-pending U.S. patent application to Holcomb et al. At the time the file shadow  62  is created, the amount of data to be associated with the shadow  62  may not be known, and therefore the file size (and in turn the file start block location in the file region  34 ) for the file cannot be known; the file shadow  62  thus typically contains only the Yth directory block number and a data source address or identifier at the time the shadow  62  is created. However, when the Yth directory is read, it is an advance notification that a subsequent request for the single file located in that Yth directory is in all probability imminent. In addition, in the preferred embodiment the size of the file must also be provided in the data block  2  returned for the Yth directory. Once the dynamic data generator  58  determines the file size, a particular predefined file that is at least as large as the file size can be selected, and therefore the file start block number and the file size fields in the shadow  62  are filled in and the particular file is bound to the Yth directory at that time.  
         [0044]    In order to eliminate or minimize any possibility that the host computer may think it already contains a particular data block in its cache memory (not shown) and thus not request it from the apparatus  50 , the apparatus  50  preferably does two things. First, the dynamic data generator  58 , when processing a request to create a file shadow  62 , establishes a pathname that has never been used before (by choosing a Yth directory block that has never been used before. And second, when processing a request to bind a file shadow  62  to a file, the dynamic data generator  58  selects a file that has never been used before. Because the physical memory size of the virtual filesystem  10  is largely insensitive to the number and sizes of the files that the filesystem  10  contains, the virtual filesystem  10  for a particular apparatus  50  can be chosen so as to minimize any chance that a Yth-level directory or a file need ever be used twice. However, the preferred embodiment of the apparatus also includes a block manager  64  which assigns Yth-level directories and files upon request. The block manager  64  keeps track of the usage of Yth-level directories and files by receiving notification from the Yth-directory file binder  56  and the file generator  66  as blocks are requested by the host computer. If the apparatus  50  runs out of unused directories or files, the block manager  64  ensures that a least recently used one is assigned in the expectation that any previous information about that directory or file has already expired from the host computer&#39;s cache. The algorithms used by the block manager  64  are similar to those used in memory management and garbage collection, and are well known to those skilled in the art.  
         [0045]    If the requested block B is in the file region  34 , a file generator  66  searches the file shadow table  60  to locate the file shadow  62  associated with the block. The data source field of the file shadow  62  is then used by the file generator  66  to determine the source of the data. If the source is the dynamic data generator  58 , the block is requested from the dynamic data generator  58 . If the source is a file on an auxiliary file system  70 , a file aliasing bridge  68  translates the requested block into the corresponding block of the file on the auxiliary file system  70 . One example of the auxiliary file system  70  is the file system utilized by a Compact Flash card or PCMCIA flash card which is used, for example, to store photographic image files recorded by a digital camera.  
         [0046]    From the foregoing it will be appreciated that the virtual filesystem provided by the present invention represents a significant advance in the art. Although several specific embodiments of the invention have been described and illustrated, the invention is not limited to the specific methods, forms, or arrangements of parts so described and illustrated. In particular, the filesystem may contain additional regions such as boot sectors, and may contain a redundant copy of the FAT region. It is also to be understood that the root directory and intermediate-level directories may contain additional predefined directories or files that have been omitted for clarity, including but not limited to a self-referencing directory (usually denoted as the“.” directory) and a parent directory (usually denoted as the “..” directory). Furthermore, while in the preferred embodiment the file size is fixed when a Yth-level directory is accessed, in some embodiments the binding may be deferred until the file itself is accessed, depending on the behavior of the host computer&#39;s operating system. The present invention may be advantageously used in a wide variety of traditional and embedded computer systems. The invention is limited only by the claims.