Abstract:
A method includes configuring a flash memory device including a first memory sector having a primary memory sector correspondence, a second memory sector having an alternate memory sector correspondence, and a third memory sector having a free memory sector correspondence, copying a portion of the primary memory sector to the free memory sector, erasing the primary memory sector, and changing a correspondence of each of the first memory sector, the second memory sector, and the third memory sector.

Description:
RELATED APPLICATIONS 
     The present application is related to U.S. application Ser. No. 11/317,998, filed Dec. 22, 2005, and assigned to the assignee of the present invention. 
     FIELD OF THE INVENTION 
     The present invention relates generally to a method for manipulating data on a flash storage medium. 
     BACKGROUND INFORMATION 
     It is known in the art to maintain state machines, typically consisting of state tables, defining the states of elements associated with a system. An example of such a state machine is the collection of state tables forming Postal State Tables (PSTs) stored in flash memory and utilized in postal printing devices. The PSTs maintain data related to the status of purchased postal indicia. 
     As noted, such state tables are typically stored in flash memory. It is an unfortunate attribute of flash memory that such memory possesses a relatively limited number of erase cycles. For example, flash memory internal to a processor may only allow one hundred erase cycles. The actual number of erase cycles that may be performed before experiencing a significant degradation in the operation of the memory varies. However, when such degradation does occur, the result is an increase in the amount of time to write to the flash memory and to retrieve data from the flash memory. As a result, it is desirable to minimize the number of erase cycles. 
     In a typical erase cycle, each bit in the flash memory device is set to logical “1”. In order to limit the number of erases performed on a flash memory, it is noted that any bit can be transitioned from a one to a zero between erase cycles (or from a zero to a one depending on the flash part). This fact allows multiple writes to occur in a flash memory device between erases. It is therefore preferable to manipulate data stored on a flash memory in a manner requiring only the transition of bits from one to zero. By so doing, one decreases the frequency with which the flash memory requires erasing. 
     In addition, it is preferable to employ an algorithm to efficiently clean the non-volatile memory (NVM), such as flash memory, such that erases occur only when required. When an erase cycle is needed, it is further preferable to engage in erasing flash memory in such a way that the entire flash memory experiences a generally uniform application of memory erasing. 
     SUMMARY OF THE INVENTION 
     In accordance with an exemplary embodiment of the invention, a method includes configuring a flash memory device including a first memory sector having a primary memory sector correspondence, a second memory sector having an alternate memory sector correspondence, and a third memory sector having a free memory sector correspondence, copying a portion of the primary memory sector to the free memory sector, erasing the primary memory sector, and changing a correspondence of each of the first memory sector, the second memory sector, and the third memory sector. 
     In accordance with an exemplary embodiment of the invention, a program of machine-readable instructions, tangibly embodied on an information bearing medium and executable by a digital data processor, performs actions directed toward managing a flash memory device the actions including configuring a flash memory device to include a first memory sector having a primary memory sector correspondence, a second memory sector having an alternate memory sector correspondence, and a third memory sector having a free memory sector correspondence, copying a portion of the primary memory sector to the free memory sector, erasing the primary memory sector, and changing a correspondence of each of the first memory sector, the second memory sector, and the third memory sector. 
     In accordance with another exemplary embodiment of the invention, a system includes a flash memory device including a first memory sector having a primary memory sector correspondence, a second memory sector having an alternate memory sector correspondence, and a third memory sector having a free memory sector correspondence, means for copying a portion of the primary memory sector to the free memory sector, means for erasing the primary memory sector, and means for changing the correspondence of each individual one of the first memory sector, the second memory sector, and the third memory sector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
         FIG. 1  is a diagram of an exemplary embodiment of a hardware configuration for practicing the invention. 
         FIG. 2  is a flowchart of an exemplary embodiment of a Postal State Table (PST) of the invention. 
         FIG. 3  is an exemplary embodiment of a PST file of the invention. 
         FIG. 4  is an exemplary embodiment of a memory sector of the invention. 
         FIG. 5  is an exemplary embodiment of a PST file of the invention illustrating both “dirty” and “active” PSTs. 
         FIG. 6  is an illustration of an exemplary embodiment of rules employed during a scrub operation according to the invention. 
         FIG. 7  is an illustration of an exemplary embodiment of scrub rules of the invention. 
         FIG. 8  is an illustration of an exemplary embodiment of a coalesce file according to the invention. 
         FIG. 9  is a flowchart of an exemplary embodiment of a scrubbing operation of the invention. 
         FIG. 10  is a diagram of an exemplary embodiment of a hardware configuration of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In exemplary embodiments of the invention, a method is provided for managing, and otherwise manipulating, data stored in flash memory. In particular, there is provided a method for storing data and coalescing stored data in a manner so as to reduce the need for erase cycles applied to the flash memory. In one exemplary embodiment, there is utilized the design and implementation of state table data requiring only the alteration of binary ones to binary zeros as the print states to which the state table data refers change from one to another. In addition, exemplary methodologies are provided for coalescing needed data so as to reduce the incidence of erase cycles. While described with reference to PSTs utilized in the operation of postal meters, the invention is not so limited. Rather, the invention is drawn broadly to cover any and all data stored on an electronic memory device, particularly a flash memory device. 
     Broadly stated, and described more fully below, exemplary embodiments of the invention operate to partition an NVM into a plurality of sectors and to clean such sectors in a manner that exercises each sector to an approximately equal extent. 
     With reference to  FIG. 1 , there is illustrated an exemplary embodiment of a postal state table (PST)  11 . As illustrated, each PST  11  is formed of a two byte (16 bit) Sequence ID, providing a unique value for accessing a particular PST  11 , followed by sixteen Postage State data elements  15  each of a size of three bits (48 bits in total). As a result, the exemplary PST  11  is 64 bits, or eight bytes, in size. As constructed, each Postage State data element  15  can be accessed as an offset from the starting memory location in which PST  11  is stored. Each Postage State data element  15  represents a single purchased postage with the bit pattern forming the three bits of the Postage State data element  15  indicating the status of the purchased postage. 
     As each Postage State data element  15  is formed of three bits, it is possible to represent up to eight separate states (binary 000 through binary 111). As noted above, the frequency of erase cycles can be reduced if the transition between states involves only the changing of bits with a value of “1” to a value of “0”. With reference to  FIG. 2 , there is illustrated an exemplary embodiment of the transitions between the different values for Postage State data elements  15  of a PST  11 . As illustrated the binary designations for each possible state are as follow: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Available 
                 111 
               
               
                   
                 Postage Loaded 
                 011 
               
               
                   
                 Printed 
                 010 
               
               
                   
                 Refund Pending 
                 001 
               
               
                   
                 Refund Confirmed 
                 000 
               
               
                   
                   
               
             
          
         
       
     
     Note that the transition from Block A to Block B, corresponding to the transition from “Available” to “Postage Loaded” requires changing only the first bit from one to zero. Likewise, transitioning from “Postage Loaded” to either “Printed” or “Refund Pending”, at Block C and Block D respectively, similarly requires changing only one bit from one to zero. Lastly, transitioning from “Refund Pending” to “Refund Confirmed” at Block E requires changing only one bit from one to zero. In this manner, the status, or state, of a purchased postage, stored in a Postage State data element  15 , can be transitioned from an initial status of “Available” to a final status of either “Printed” or “Refund Confirmed” without the need to change any bits comprising a Postage State data element  15  from zero to one. Specifically, none of the three bits defined as the data type  5  for a postage state  7  require being changed from a bit value of “0” to a bit value of “1” as the postage state  7  transitions from one state to the next. 
     With reference to  FIG. 3 , there is illustrated an exemplary embodiment of a PST file  31 . Each PST file  31  is formed of more than one PST  11 . Each PST file  31  is arranged such that that its component PSTs  11  are stored in contiguous memory of the memory medium on which they are stored. Typical, but non-limiting, sizes for PST files  31  utilized in conjunction with flash memory include 128 bytes, 256 bytes, and 1024 bytes formed of 16, 32, 128 PSTs  11  respectively. 
     Upon erasure and initial allocation of a PST file  31 , every bit is set to a value of “1”. In the exemplary embodiment illustrated, each state is thereby initially set to “Available”. As discussed more fully below, during the process of printing postage indicia, the state variables are updated as the value of each Postage State data element  15  is transitioned to reflect a current status. 
     With reference to  FIG. 4 , there is illustrated an exemplary embodiment of a memory sector  41 . Memory sector  41  is a portion of contiguous memory of a defined size. In operation, the size of a memory sector  41  can depend upon the physical structure of the memory device upon which memory sector  41  is defined, as well as the logical requirements attendant to the structure of the data to be stored upon it. Quite often, a memory sector  41  is of the minimum size that can be erased on the memory medium. As illustrated, memory sector  41  is of a size sufficient to store a plurality of PST files  31 ,  31 ′ as well as an overhead portion  43 . There is additionally illustrated unused space  45 . Overhead portion  43  can store any data not forming a part of a PST file  31 . Overhead portion  43  can include, but is not limited to, the value of parameters related to one or more PST file  31 ,  31 ′. Unused space  45  is formed of contiguous, unallocated data storage space in which there is not stored valid data. 
     With reference to  FIG. 5 , there is illustrated an exemplary embodiment of a PST file  31  wherein, over the course of operation, the status of individual Postage State data elements  15  has been changed to a value other than binary “111”. As illustrated, PSTs  11  wherein every component Postage State data element  15  has been transitioned to a status of either binary “010” or binary “000”, corresponding to “Printed” and “Refund Confirmed” respectively, are indicated by an “X” drawn through the status fields. As such, the “X” indicates that the PST  11  requires no further updates to any component Postage State data element  15 , and, as such, can be erased without the possibility of losing required data. As used herein, “dirty” refers to such PSTs  11  as can be erased. Note however, that, in this example, two PSTs  11 ′, 11 ″ each have at least one Postage State data element  15  in a transient state with a binary value indicative of “Postage Loaded” and which has neither been “Printed” nor had a “Refund Confirmed”. As used herein, “active” refers to such PSTs  11 ′, 11 ″ that cannot be erased without the loss of required data. 
     As illustrated, these two Postage State data elements  15 , comprising only six total bits, prohibit the erasure of the entire memory sector  41  upon which they are stored. As a result, a relatively large amount of memory is prevented from being freed up, via erasure, in order to maintain these six bits of residual data. While illustrated as a single memory sector  41 , it is possible to define a plurality of memory sectors  41  on a single memory device, such as a flash memory. 
     In an exemplary embodiment of the invention, a method is provided for manipulating data stored on a plurality of memory sectors that limits the number of required erases and minimizes the memory footprint needed to store required data. Broadly stated, and described more fully below, a method of the invention defines at least three memory sectors  41  and cleans each sector in a manner that serves to equally exercise each memory sector  41 . As defined herein, a “clean operation” refers to a procedure consisting of multiple “moves” wherein each move involves the movement of required data from one place in a memory device to another. As described more fully below, use is made of a PST coalescing function when performing such moves. More specifically, each “move” involves the transfer of a PST file  31 , one or more PSTs  11 , or any other data from one memory sector  41  to another memory sector  41 . Each move involves rewriting only in-use NVM files to a new sector. 
     In practice, three memory sectors  41 , 41 ′, 41 ″ are designated with one each being designated as primary, alternate, and free. When an attempt is made to add a PST file  31 , or other data, to a memory sector  41 , the three memory sectors  41 , 41 ′, 41 ″ are examined for available space as follows. First, the primary memory sector  41  is queried for space. If sufficient space is available on the primary memory sector  41 , the PST file  31  is added to the primary memory sector  41 . If insufficient space exists on the primary memory sector  41 , the alternate memory sector  41 ″ is queried for available space. If sufficient space is available on the alternate memory sector  41 ″, the PST file  31  is added to the alternate memory sector  41 ″. If insufficient space exists on the alternate memory sector  41 , 41 ′, 41 ″ the three memory sectors  41  undergo a scrub operation whereby sufficient memory space is sought. 
     During a scrub operation, the exemplary rules illustrated in  FIG. 6  are utilized to determine which operations are performed on which memory sectors  41 , 41 ′, 41 ″. First, in accordance with rule  1 , the primary memory sector  41  is cleaned to the free memory sector  41 . As described more fully below, this “cleaning” involves a process termed “coalescing”. Next, in accordance with rule  2 , the primary memory sector  41  is erased. As noted above, the process of erasing involves setting every bit in the primary memory sector  41  to binary value “1” and, hence, all data stored on primary memory sector  41  prior to erasure is lost. Next, in accordance with rule  3 , the scrub rules are updated. 
     With reference to  FIG. 7 , there is illustrated an exemplary embodiment of the scrub rules. At power up, a first memory sector, designated with a “1”, is defined to be the primary memory sector  41 . A second memory sector, designated with a “2”, is defined to be the alternate memory sector  41 ′. Lastly, a third memory sector, designated with a “3”, is defined to be the free memory sector  41 ″. After a scrub operation is performed, the designations of the memory sectors  41 , 41 ′, 41 ″ are changed. Specifically, the first memory sector is newly designated the free memory sector  41 ″, the second memory sector is newly designated the primary memory sector  41 , the third memory sector is newly designated the alternate memory sector  41 ′. As illustrated, this re-designation is continued after each performance of a scrub operation such that, after three such scrub operations, the original designations of each memory sector  41  are once again in force. 
     With continued reference to  FIG. 6 , in accordance with rule  4 , after updating the scrub rules, available space for storing the PST file  31  is sought, and, if found, the sector number upon which space was found is returned, such as to a processor coordinating the clean operation. Note that, in accordance with the scrub rules of  FIG. 7 , each memory sector  41  is erased only upon each third scrub. 
     As noted above, regarding the format of PST files  31 , each PST file  31  is a collection of PSTs  11 . During operation, PST files  31  are periodically cleaned such as when postage is purchased after the PST file  31  is uploaded to a server. During such an operation, a PST file  31  can be deleted if all of the postage values corresponding to the Postage State data elements  15  have been either printed or refunded. Once a PST file  31  has been deleted, it can be cleaned on the next scrub. 
     As noted above when discussing  FIG. 5 , it is often times the case that a PST file  31  occupies a substantial space while only a relatively few PSTs  11  contain Postage State data elements  15  corresponding to a postage value that has not been printed or refunded. During a clean operation, each PST file  31  is examined to determine if it should be deleted, left alone, or coalesced. The process of coalescing is illustrated with reference to  FIG. 8  wherein there is illustrated a coalesce PST file  81 . In an exemplary embodiment, coalesce PST file  81  is derived from the process of coalescing applied to the PST file  31  of  FIG. 5 . As illustrated, the active PSTs  11  from PST file  31  have been coalesced and transferred into coalesce PST file  81  such that coalesce PST file  81  consists only of active PSTs  11 ′, 11 ″. 
     While it is possible to apply the process of coalescing to any PST file  31  containing at least one active PST  11 , it is preferable to apply coalescing to PST files  31  in accordance a set of coalescing criteria. Examples of such criteria include that, prior to coalescing any PST files  31  on a memory sector  41 , all but one of the component PSTs  11  be marked “dirty” and there must be at least three PST files  31  in use on the memory sector  41 . An exception to such criteria is that no coalescing is to be performed if the last PST  11  in a PST file  31  is “active”. In normal usage, this condition is often the case and, thus, such an exception avoids unnecessary moving of data. Such criteria are presented for exemplary purposes only and can be altered or modified as desired to control the erasure of memory sectors  41   
     With continued reference to  FIG. 8 , coalesce PST file  81  is grouped with other PST files  31  on the memory sector  41  on which it resides. As such, coalesce PST file  81  requires no special handling, and memory management of the coalesce PST file  81  can be performed using operations employed when managing any other PST file  31 . As the coalesce PST file  81  may be only partially filled with state table data upon creation, the remaining empty, or unallocated, space forming coalesce PST file  81  serves as a place holder for other PSTs  11  to be added upon future applications of the coalesce process. As such, coalesce PST files  81  are created and deleted on an as needed basis. 
     With reference to  FIG. 9 , there is illustrated in detail an exemplary embodiment of a method of the invention showing a complete scrubbing cycle consisting of three scrub operations. As illustrated, there are a plurality of operation descriptions  91 - 91 ′″ each associated with a configuration of memory sectors  41  and their designations (primary, alternate, and free). Each operation description  91 - 91 ′″ defines an operation of memory allocation prior to the occurrence of a corresponding trigger condition  92 - 92 ′″. For example, operation description  91  specifies that data is added to sector one and then to sector two until the data to be added will longer fit on either sector as specified in trigger condition  92 . When trigger condition  92  is met, the rules embodied in rules  93  are performed and a new set of operations, defined by operation description  91 ′ is put into practice. This process repeats itself as shown. 
     With reference to  FIG. 10 , there is illustrated an exemplary embodiment of a hardware configuration for practicing the invention. A processing unit  1011  is coupled to an internal memory device  1013 . By “internal” it is meant that processing unit  1011  can communicate with internal memory device  1013  without the use of an external bus or other communication link permitting external examination of such communications. Processing unit  1011  can be, but is not limited to, a CPU fabricated to form a part of microprocessor  1001 . Internal memory device  1013  is preferably formed of flash memory. Processor  1011  can be additionally coupled to an external memory device  1015 . In operation, processor  1011  executes a program or programs, comprised of machine readable code embodied in a tangible, electronic format, to manipulate and otherwise manage the storage of data upon the memory devices  1013 , 1015  as described above. In an exemplary embodiment, processor  1011  and memory device  1015  form a part of a postal security device (PSD) operating to enable the secure printing of postage indicia. 
     While illustrated with application to flash memory devices, the invention is applicable to all other forms of memory devices, such as, for example, RAM. In addition, while described with reference to relatively small, embedded devices, the method of the invention is scale independent. In addition, the method of the invention can be parameterized to different trigger points so as to create the coalesce PST file  81  depending on a state of one or more PSTs  11 . In addition, the above described exemplary embodiments of the invention can be implemented as programs running on a processor  1011  that are run as background tasks. In addition, if such programs are stored in internal memory. 
     While certain of the embodiments have been described in terms of flash memory storage of program instructions, the embodiments can alternatively be utilized with other appropriate storage technology such as RAM storage, EEPROM storage, ROM storage or mirrored RAM storage that mirrors flash when running. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.