Abstract:
Embedded devices typically have an operating system, one or more file-systems, as well as a bootloader and other data components resident in flash memory. During software development and testing, there is frequently a need to selectively update a combination of such images. The described technique organizes the images in the flash memory such that one can speed up the update process by eliminating relocation of existing images. A command-driven update mechanism provides a flexible process—eg, one can upload the images back to a host, one can update the update code itself, etc. A start handshake is used that enables auto-detection of the embedded serial port that is used for the update.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to selectively updating flash memory, such as portions of code resident in flash memory for use in embedded devices.  
         BACKGROUND  
         [0002]    A typical configuration in many embedded devices is to store and run the operating system from the flash memory (or ROM), and store required data in a non-volatile RAM. However, many pervasive embedded devices have a full-fledged operating system, one or more file-systems, along with a bootloader and other data components, resident in flash memory.  
           [0003]    The life of flash memory storage is largely dictated by the number of accesses that occur to flash memory when updating flash memory. Any writes to a flash location are preceded by a corresponding erase. Erasing flash memory is a slow and time consuming process.  
           [0004]    During software development and testing, there is a frequent need to update a combination of selected images. Effective flash life time, and speed of development, can be adversely affected if existing images are relocated while performing such selective updates.  
           [0005]    In view of the above, a need clearly exists for improved method of updating code in embedded devices that at least attempts to address one or more of the above limitations.  
         SUMMARY  
         [0006]    The proposed technique involves an algorithm for performing updates on flash memory of, for example, an embedded device. The flash memory may contain a combination of images (for example, operating system image, filesystem(s), boot loader etc). Any combination of the images can be updated, without disturbing the images that are not intended to be modified. Replacement images can be bigger than those that they replace, limited only by available physical memory size.  
           [0007]    The described technique avoids moving existing images. Moving existing images slows down the updating process and reduces flash life.  
           [0008]    The described method of updating flash memory is implemented such that the update logic is itself able to be updated, thus allowing for “intelligent” functionality. This allows further functionality to be added to the update logic, after installation in the flash memory.  
           [0009]    Update logic resident in the flash memory responds to instructions from a program executing on an external host, connected to the embedded system through a serial line. Where the flash memory can be accessed through more than one serial port of an embedded device, the update logic automatically detects the serial port via which the external host is connected, using an initial handshake process. The host machine can alternatively communicate with the embedded device using other means, for example, a network connection. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic representation of the contents of a flash memory device.  
         [0011]    [0011]FIG. 2 is a schematic representation of the communication that occurs between update logic stored in the flash memory of FIG. 1, and a host program in a host machine operatively connected with the flash memory of FIG. 1.  
         [0012]    [0012]FIG. 3 is an alternative schematic representation of the contents of the flash memory of  
         [0013]    [0013]FIG. 1, in which multiple file systems are resident.  
         [0014]    [0014]FIG. 4 is a schematic representation of the contents of a scratch area of the flash memory of FIG. 3.  
         [0015]    [0015]FIG. 5 is a schematic representation of the sequence of communications between the host program and the update logic of FIG. 2 when the scratch area of FIG. 4 is updated.  
         [0016]    [0016]FIGS. 6A and 6B jointly represent a flowchart of the steps that occur when the contents of a flash memory is updated.  
         [0017]    [0017]FIG. 7 is a schematic representation of a host machine computer system which executes the host program of FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Selectively updating one or more portions of the contents of a flash memory (such as in an embedded device) is described herein. The described technique allows for selective updates of parts of embedded flash memory, which provides advantages in increasing the speed of embedded software development while minimising the number of erases and writes to the flash memory.  
         [0019]    The term “flash memory” is used herein to describe a type of non-volatile memory in which is an electrically erasable and programmable read-only memory (EEPROM) having a programmable operation which allows for the erasure of blocks of memory. Unless there is a clear and express indication to the contrary, any reference to a “flash memory” is taken to include any non-volatile storage memory in which (i) data can be written only in unwritten or erased physical memory locations and in which (ii) a zone of contiguous physical memory locations are simultaneously erased. For ease of reference, storage memory having such characteristics is referred to as “flash memory”.  
         [0020]    This minimisation of erases/writes comes about due to a combination of organizing the images in flash memory suitably, and introducing fragmentation if necessary. Any fragmented image can be defragmented prior to product shipment of the embedded device in which the flash memory resides.  
         [0021]    A host machine contains the images that are to be updated in the flash memory. For the purposes of the following description, it is assumed that the host machine is to be connected to the embedded device through a serial line. Of course, this general approach is also valid for a network connection, though the initial handshake process will be different.  
         [0022]    The embedded device with which the technique is used preferably has a mechanism for determining when to initiate an update. This may be, for example, a jumper setting in the device, or some signal or other indication provided by the host machine etc.  
         [0023]    When the embedded device powers on, the boot-loader gets control of the device. If the boot-loader detects that an update indication (such as a software flag, or some form of hardware indication) is ON, the boot-loader copies the update logic to RAM and branches to the update logic. This procedure is needed as most flash memory chips do not support simultaneous writes and reads—which is required if the update logic writes to the flash memory, while also executing from the flash memory. If the boot-loader senses that the update indication is OFF, the boot-loader boots the system. The kernel flash-disk block driver subsequently mounts a file-system resident in flash memory of the embedded device as the root device.  
         [0024]    Flash memory is normally organized into banks and further into sectors. Erases can be done only at the granularity of a sector. A flash-write has to follow an erase on the corresponding sector.  
         [0025]    In the described arrangement, it is assumed that there is only one file-system image and one kernel image resident in flash memory. It is later explained how the described techniques differ for cases in which there are multiple images and file systems.  
         [0026]    FIG. 1  schematically represents a flash memory  100  used in connection with the techniques described. Physically, the flash memory  100  comprises N banks  112 ,  114 ,  116 . At the start of the flash memory  100  there is a boot loader  120 , followed by the update logic image  130 .  
         [0027]    Next, a scratch area  140  contains the start addresses and sizes of all the flash-resident images (as explained below in further detail, with reference to FIG. 4). The scratch area  140  is used by the boot-loader  120  to boot the device. The scratch area  140  is also used by the kernel flash-disk block driver to determine where the file-system  150  starts. The update logic  130  software also needs to use the contents of the scratch area  140  to perform selective updates. From a programming perspective, it is faster (though not necessary) if the scratch area  140  resides in a separate sector from the update logic  130  and the file system  150 . If the scratch area  140  is resident in a partly used sector, the remaining contents have to be buffered while re-programming the scratch area  140 . The kernel flash-disk driver emulates a disk in flash memory  100 , so that one or more file-systems can be resident on the flash memory  100 .  
         [0028]    [0028]FIG. 2 represents the start protocol, between the host resident program  210  executing on the host machine, and the update logic  130  executing on the flash memory  100  of the embedded device.  
         [0029]    As soon as the update logic  130  begins execution, it emits a UPDATE_START_CHAR  230  to inform the host resident program  210  on the host machine that it is ready to start the update. If the embedded device has multiple serial ports, the device sends the UPDATE_START_CHAR  230  on all ports. When the host machine receives the UPDATE_START_CHAR  230 , the host resident program  210  returns a UPDATE_ACK  240  to acknowledge receipt of the UPDATE_START_CHAR  230 . The update logic  130  polls each of the serial ports (using a timeout of, for example, 1 ms) to determine the PORT_NUMBER  250  on which the UPDATE_ACK  240  arrived.  
         [0030]    The update logic  130  now knows the serial port to which all reads and writes are to be directed. Now the update logic  130  sends the serial port number (that is, PORT_NUMBER  250 ) back to the host machine (through the serial port that it just detected), completing the three-way handshake. The host resident program  210  subsequently sends commands to the update logic  130  to configure the serial port that it thus detected, and to immediately switch the serial port to the same configuration.  
         [0031]    The start protocol, described immediately above, also enables the implementation of a multi-functional program  210  on the host machine. For instance, certain processor chips used in embedded devices have two boot-modes: (i) a first boot-mode that is used to load the boot-loader  120  and the other images for the first time (code-load), and (ii) a second boot-mode that boots from the top of the flash memory  100 . In this second mode, a boot-loader is expected to be resident at the top of the flash memory  100 . An example of such a processor chip is the EP7211 produced by Cirrus Logic of Austin, Tex. During different boot modes, different memory addresses obtain control. The software resident at these different memory addresses emits different start characters. By using different start characters for these different modes, the host resident program  210  executing on the host machine determines the boot mode that is active.  
         [0032]    The start address for the kernel in flash memory  100 , is computed as follows. The highest possible word-aligned address that accommodates the kernel in flash memory  100  is obtained. For this, one calculates backwards from the end address of the last flash memory bank  116 . The word-size depends on the flash chip-set used. Certain flash memory chip-sets support “page-write” commands. If the flash memory writes are done using this “page-write” mode, the computed address is the highest possible ‘page-aligned’ address.  
         [0033]    The start address for the file-system image  150  is the first word-aligned (or, “page-aligned”, as noted above) address following the scratch area  140 . The kernel  170  and the file-system  150  reside at different ends of the flash memory  100 . This facilitates selective update of the kernel  170  or the file system  150  for cases in which the replacement image is greater in size than the currently resident kernel  170  or the file-system  150 , without physical relocation of images within the flash memory  100 , and hence eliminates undesirable erases and writes to the flash memory  100 . The size of the updated image is thus limited only by the available capacity of the flash memory  100 .  
         [0034]    During updates, if the image start ad dress is not recomputed, there is a significant probability (especially while updating file-system images) that some of the sectors that are to be updated have data that has not changed. Only those sectors whose replacement data does not match the original data need be updated. Whether to perform this optimization or not, can be decided by the user at run time, via a special command supported by the update logic. In cases where the replacement image has large differences with the resident image, the above process might slow down the update, even though it could reduce the number of flash erases. This is described in more detail subsequently, in the general case where there are multiple file-system images. In the case of kernel images, revising the start address is preferable to fragmentation, especially if the embedded device executes the kernel in place; that is, runs the kernel directly from flash memory  100 .  
         [0035]    The kernel  170  is located at the end of the flash memory  100 , and the file-system  150  near the start of the flash memory  100 , rather than the other way around. This relative arrangement facilitates dynamic file-system extension, if the file-system  150  supports such a mechanism.  
         [0036]    A predetermined memory portion at the top of the flash memory  100  can be reserved for the boot-loader  120  and update logic  130  combination. An approach similar to that described above (in respect of the file-system  150  and kernel  170 ) can be used, wherein the boot-loader  120  and update logic  130  reside at different ends of this reserved memory portion. However, simpler approaches, as later described, can also be used.  
         [0037]    To support multiple file-systems, the scratch area  140  has to contain partition information. The partition area contains a set of null-terminated tuples. Each tuple set [(start bank i, start sector i, start offset i), (end bank i, end sector i, end offset i), . . . NULL] represents the different flash fragments where the corresponding file-system resides, the tuple ordering reflecting the fragment ordering. The number of resident file-systems and the index of the root file-system are also part of the partition area.  
         [0038]    In FIG. 3, “sect a” represents sector number “a”, and similar abbreviations are used for other sectors. For convenience, the offsets within the sectors are not shown. FIG. 4 is a schematic representation of the contents of the scratch area  140 , as represented in FIG. 3. The core of multiple resident file systems is described in more detail below.  
         [0039]    The computed addresses and the image sizes for the various images are stored in the scratch area  140 . FIG. 4 schematically indicates the contents of the scratch area  140  for the different images resident in the flash memory  100 , as represented in FIG. 3.  
         [0040]    [0040]FIG. 5 is a schematic representation of the sequence of steps that occur between the host resident program  210  and the update logic  130  in the flash memory  100  when the scratch area  140  is to be updated. The sequence of steps is progressively ordered from top to bottom. First, the host resident program  210  sends a “Z” character to the update logic  130 , denoting that the sector of the flash memory  100  in which the scratch area  140  is resident is to be erased under control of the update logic  130 . Once this step is performed by the update logic  130 , a “+” character is sent by the update logic  130  to the host resident program  210  to indicate that the scratch area  140  has been erased.  
         [0041]    In response, the host resident program  210  sends a “W” character to the update logic  130 , indicating that the erased sector is to be replaced by a revised scratch area  140 . The host resident program  210  then writes the length of the scratch area data, followed by the actual data representing the contents of the scratch area. This is received by the update logic  130 , and used to write to the scratch area  140  of the flash memory  100 .  
         [0042]    Once the write process has been completed by the update logic  130 , a checksum representing the integrity of the scratch area data is returned by the update logic  130  to the host program. A checksum received from the update logic  130  by the host resident program  210  that agrees with that computed by the host resident program  210  indicates that the updating of the scratch area  140  has been successfully completed.  
         [0043]    In order to update only the kernel  170  resident in flash memory, the update logic  130  program performs the following steps: 
         [0044]    1. Detecting whether the new kernel  170 ′ will fit into the memory available (the free space available for the new kernel  170 ′ can be calculated from the information present in the scratch area  140 ). If sufficient capacity is not available, the update is stopped and the user is alerted accordingly.  
         [0045]    2. Computing the start address to load the replacement kernel  170  ′ as previously described in relation to the original kernel  170 .  
         [0046]    3. Computing the location of the sectors to be erased.  
         [0047]    4. Erasing the required sectors, located in step  3 .  
         [0048]    5. Writing the new kernel  170 ′ to flash memory  100 . Performing appropriate bank address translation, if the updated kernel  170 ′ spans banks.  
         [0049]    6. Computing and returning checksums to the host resident program  210 . The checksums are computed and sent for every block of data written to flash memory  100 . The host resident program  210  indicates the update progress whenever a checksum value is received, if it matches the value that it expects. If a checksum mismatch is detected, the update is stopped and the user is alerted accordingly.  
         [0050]    7. Reading the contents of the scratch area  140 . Erasing the scratch area  140  and updating the scratch area  140  using new values for kernel start and kernel size. 
         [0051]    If the kernel start-address is in the same sector as the end of the resident file-system  150 , special care is taken in updating this sector—the bytes used by the file-system  150  in this sector are temporarily saved before the erase, and then copied back as appropriate to maintain the integrity of the contents of the memory  100  that are not updated.  
         [0052]    The scratch sector erase should not be performed along with step  3 , because if the host program terminates in the middle of the selective update, we would end up effectively losing the file-system image also.  
         [0053]    In order to instead update only the flash resident file-system  150 , the update logic  130  program performs the following steps: 
         [0054]    1. Detecting whether the new file-system  150 ′ will fit into the memory available (the free space available for the new file-system  150 ′ can be calculated from the information present in the scratch area  140 ). If sufficient capacity is not available, the update is stopped and the user is alerted accordingly.  
         [0055]    2. Computing the start address for the replacement file-system image  150 ′ as the first word-aligned address following the scratch area, as previously described.  
         [0056]    3. Computing the location of the sectors to be erased. Only those sectors whose replacement data differs from the original data needs to be replaced, as previously described.  
         [0057]    4. Erasing the required sectors computed in step  3 .  
         [0058]    5. Writing the replacement file-system  150 ′ image to flash memory  100 . Performing appropriate bank address translation if the update spans banks.  
         [0059]    6. Computing and returning checksums to the host resident program  210  on the host machine. The checksums are computed and sent for every block of data written to flash. The host resident program  210  indicates the update progress whenever a checksum value is received, if it matches the value that it expects. If a checksum mismatch is detected, the update is stopped and the user is alerted accordingly.  
         [0060]    7. Reading the contents of the scratch area  140 . Erasing the scratch area  140  and updating the scratch area  140  using the newly computed values for file-system start and end addresses. 
         [0061]    If the kernel start-address is in the same sector as the end address of the file system, special care is taken in updating this sector—the bytes used by the kernel  170  in this sector are temporarily saved before the erase, and then copied back as appropriate, to maintain the integrity of the contents of the memory  100  that is not updated.  
         [0062]    The scratch sector erase should not be performed along with step  3 , because if the host resident program  210  terminates in the middle of the selective update, the kernel image  170  is effectively lost.  
         [0063]    An approach analogous to that used for the kernel  170 /file-system  150  combination described above, can also be used for the boot-loader  120 /update logic  130  combination. A predetermined size can be reserved for the boot-loader/update logic combination—both residing at different ends of the reserved memory portion of the flash memory  100 , as noted above. This technique can be simplified if it can be assumed that the boot-loader  120  and update logic  130  are updated together.  
         [0064]    Many flash memory chips have initial sectors whose sizes are small. In that case, it is realistic for the boot-loader  120  and update logic  130  to occupy separate predetermined sectors (say sector  0  and sector  1 ). In this case, selectively updating them is more convenient. As with the steps described above, the new start address and size information is updated in the scratch area  140  once the update is complete.  
         [0065]    It is described above how a combination of images are selective updated. For instance, one can update just the boot-loader  120  and the kernel  170  without disturbing the other images, obviating erases and writes in other parts of the flash memory  100 .  
         [0066]    A total update (of all the flash resident images) is relatively straight forward. The relevant steps are as follows: 
         [0067]    1. Erasing all sectors.  
         [0068]    2. Computing the start address for the images (boot-loader  120 , update logic  130 , kernel image  170  and file-system image  150 ), as described above. Updating the scratch area  140  with these new values.  
         [0069]    3. Writing the new images to the flash memory at the computed addresses. Performing appropriate bank address translation, if necessary.  
         [0070]    4. Computing and returning checksums to the host resident program  210 . The checksums are computed and sent for every block of data written to flash memory  100 . The host resident program  210  indicates the update progress whenever a checksum value is received, if it matches the value that it expects. If a checksum mismatch is detected, the update is stopped and the user is alerted accordingly. 
         [0071]    The update logic  130  also supports reverse updates (that is, copying combination of images from the flash memory  100  of the embedded device back to the host machine). This is useful for taking file-system backups, debugging crashes, etc.  
         [0072]    For example, if a file-system image  150  is to be uploaded from the embedded device to the host machine, the update logic  130  does the following (similar steps can be followed to upload other combinations of flash-resident images): 
         [0073]    1. Determining the file-system start and end addresses from the scratch area  140 .  
         [0074]    2. Sending the file-system size back to the host resident program  210 .  
         [0075]    3. Reading the file-system image  150  from the above-determined start address, and transmitting it back to the host resident program  210 .  
         [0076]    4. Computing (by the host resident program  210 ) the checksum, and sending the checksum back to the update logic  130 . The update logic  130  flags an error to the host resident program  210  if the checksum value received by the update logic  130  does not match the value that it expects. 
         [0077]    The embedded device may have multiple file-system images or kernel images resident in the flash memory  100 . It is now assumed for convenience and ease of illustration that only multiple file-system images are present. However, the described procedure in general holds for multiple kernel images also.  
         [0078]    The update logic  130 , as described above, supports selective updates of a file-system image  150 , without changing or relocating other resident image(s). Further, as an updated image can be bigger or smaller than the original one, image replacement can result in the file-systems becoming fragmented (that is, each file-system could end up occupying non-contiguous areas in the flash memory  100 ). This is because, the update logic  130  would use space available in disjointed (that is, non-contiguous) memory fragments in the flash memory  100  rather than physically move resident images between different memory locations within the flash memory  100 .  
         [0079]    Whenever the update logic  130  decides to use a fragment, the update logic  130  updates the partition information in the scratch area  140 . This process is described in further detail below. FIG. 3 is a schematic representation of an example of how a portion of the flash memory  100  may be occupied after a few selective updates to the memory  100  in which there are multiple file systems.  
         [0080]    [0080]FIGS. 6A and 6B jointly represent a flowchart of steps that occur for a generalized case in which there are multiple file-systems. In this instance, the algorithm for selectively updating a file-system becomes more complex than described above. With reference to FIGS. 6A and 6B, the steps involved are as follows: 
         [0081]    1. It is first determined in step  605  whether the size of the new file-system image is smaller than or equal to the size of the existing file-system image. If the size of the new image is smaller than or equal to the size of the existing image, the new file-system uses the needed fragments out of the ones owned by the original image, in step  645 .  
         [0082]    2. If the updated file-system image is larger than the existing size, it is determined in step  610  whether all the free flash fragments can together accommodate the extra size of the new file-system image. (The location of free flash fragments can be figured out from the tuple information in the partition table). If the physical memory capacity available is not sufficient, the update is stopped and the user is alerted in step  615 .  
         [0083]    3. Else if there is sufficient space, any free space following the existing image is used in step  620 , in addition to the original fragments, by recomputing the end address of the last component fragment accordingly.  
         [0084]    4. If that is insufficient or unavailable, a free flash fragment that best fits the remaining size is chosen in step  630 . If the largest free fragment is smaller than the needed size, that is used and the same procedure is continued for the remaining size. Fragments that do not have the end of another image directly above it are used in preference to the ones that do have the end of another image directly above it.  
         [0085]    5.  
         [0086]    (a) While writing data to pre-existing fragments, the following process is followed in step  635 . Sectors whose original and replacement data match, are left undisturbed. For this, bytes that are being received from the external host are buffered for the sector that is being currently updated. The comparison between the received data and the data present in the corresponding sector is stopped as soon as a mismatch is detected. Whether to perform the above optimization or not, can be controlled by the user at run time, via a special command supported by the update logic. This is because, in cases where the replacement image has large differences with the resident image, the above comparison might slow down the update process, even though it could reduce the number of flash erases and writes.  
         [0087]    (b) If the user does not want the above optimisation, the update logic first expands the component fragments wherever possible, before making use of the new free fragments described in step  4 . Fragments that do not have the end of another image directly preceding the fragment are enlarged in preference to the fragments that do have the end of another image directly above the fragment.  
         [0088]    (c) The necessary sectors are erased, data is written to flash, bank translation is performed if the fragment spans banks, and checksums are computed and sent to the host in step  640 . If the write is to a sector partly being used by another image, the relevant bytes are saved and copied back to their former position to maintain the integrity of the unaltered portions.  
         [0089]    6. Once the new image has been updated in step  645  or steps  610  to  640 , the partition table is also updated in step  650  with the new fragment information (start and end addresses of each fragment) for each updated file-system. 
         [0090]    The writes to the partition table (that is, involving the scratch area  140 ) are done onto a cached copy. The partition table is written back to the flash at the end of the update process.  
         [0091]    The update logic  130  also supports a “defrag” command (that is, one that defragments the contents of the flash memory  100 ). When the host resident program  210  issues this command, the update logic  130  makes each image reside in a physically contiguous area, using RAM for temporary storage.  
         [0092]    Selective file-system updates as described above will be used during embedded software development, and the ‘defrag’ command will be used prior to product shipment. ‘Defrag’ would eliminate the burden of extra translation logic inside the kernel flash-disk block device driver. If the file-systems in the flash are fragmented, the kernel flash-disk device driver will have to do extra translation on the offsets generated by the file-system, to locate the correct physical bank, sector and sector offset.  
         [0093]    The techniques described above are driven by a host resident program  210  resident on the host machine. The host machine sends a series of commands to the update logic  130 . In response, the update logic  130  processes these commands and returns the results back to the host resident program  210  on the host machine.  
         [0094]    For example, if the host machine wants the update logic  130  to erase the scratch area  140 , the host program  210 , sends a command to the update logic  130 . Erasing a sector typically takes a few milliseconds. The update logic  130  sends back an acknowledgment (ACK) when it completes the erase. The host waits till the ACK arrives, before sending the next command to the update logic  130 .  
         [0095]    Computer Hardware  
         [0096]    The above described process involves a host machine from which the updated image originates. The host machine, and the host resident program  210  that executes on the host machine can be implemented using a computer program product in conjunction with a computer system  700  as shown in FIG. 3. In particular, the process performed by the host resident program  210  can be implemented as a computer software program, or some other form of programmed code, executing on the computer system  700 .  
         [0097]    The computer system  700  includes a computer  750 , a video display  710 , and input devices  730 ,  732 . The computer system  700  can have any of a number of other output devices including line printers, laser printers, plotters, and other reproduction devices connected to the computer  750 . The computer system  700  can be connected to one or more other similar computers via a communication input/output (I/O) interface  764  using an appropriate communication channel  740  such as a modem communications path, an electronic network, or the like. The network may include a local area network (LAN), a wide area network (WAN), an Intranet, and/or the Internet  720 , as represented.  
         [0098]    The computer  750  includes the control module  766 , a memory  770  that may include random access memory (RAM) and read-only memory (ROM), input output (I/O) interfaces  764 ,  772 , a video interface  760 , and one or more storage devices generally represented by the storage device  762 . The control module  766  is implemented using a central processing unit (CPU) that executes or runs a computer readable software program code that performs a particular function or related set of functions.  
         [0099]    The video interface  760  is connected to the video display  710  and provides video signals from the computer  750  for display on the video display  710 . User input to operate the computer  750  can be provided by one or more of the input devices  730 ,  732  via the I/O interface  772 . For example, a user of the computer  750  can use a keyboard as I/O interface  730  and or a pointing device such as a mouse as I/O interface  732 . The keyboard and the mouse provide input to the computer  750 . The storage device  762  can consist of one or more of the following: a floppy disk, a hard disk drive, a magneto-optical disk drive, CD-ROM, magnetic tape or any other of a number of existing non-volatile storage devices. Each of the elements in the computer system  750  is typically connected to other devices via a bus  780  that in turn can consist of data, address, and control buses.  
         [0100]    The software may be stored in a computer readable medium, including the storage device  762 , or downloaded from a remote location via the interface  764  and communications channel  740  from the Internet  720  or another network location or site. The computer system  700  includes the computer readable medium having such software or program code recorded such that instructions of the software or the program code can be carried out.  
         [0101]    The computer system  700  is provided for illustrative purposes and other configurations can be employed without departing from the scope and spirit of the invention. The foregoing is merely an example of the types of computers or computer systems with which the embodiments of the invention may be practised. Typically, the processes of the embodiments are resident as software or a computer readable program code recorded on a hard disk drive as the computer readable medium, and read and controlled using the control module  766 . Intermediate storage of the program code and any data may be accomplished using the memory  770 , possibly in conjunction with the storage device  761 .  
         [0102]    In some instances, the program may be supplied to the user encoded on a CD-ROM or a floppy disk (both generally depicted by the storage device  762 ), or alternatively could be read by the user from the network via a modem device connected to the computer  750 . Still further, the computer system  700  can load the software from other computer readable media. This may include magnetic tape, a ROM or integrated circuit, a magneto-optical disk, a radio or infra-red transmission channel between the computer and another device, a computer readable card such as a PCMCIA card, and the Internet  720  and Intranets including email transmissions and information recorded on Internet sites and the like. The foregoing are merely examples of relevant computer readable media. Other computer readable media may be used as appropriate.  
         [0103]    Further to the above, the described methods can be realised in a centralised fashion in one computer system  700 , or in a distributed fashion where different elements are spread across several interconnected computer systems.  
         [0104]    Computer program means, or computer program, in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation or b) reproduction in a different material form.  
         [0105]    Conclusion  
         [0106]    As described, the above techniques allow for selective updates of portions of the contents of a flash memory  100  (of, for example, an embedded device) to be performed with relative ease and speed, from a host machine onto the flash memory  100 . The method uses a combination of suitably organizing the images in flash memory  100  and introducing fragmentation if necessary, to minimize the number of flash operations, and hence speed up the update process.  
         [0107]    Various alterations and modifications can be made to the techniques and arrangements described herein, as would be apparent to one skilled in the relevant art.