Abstract:
An apparatus and method for managing memory in low-end electronic devices is provided. The apparatus includes a memory management unit. The memory management unit configured to allocate a portion of random access memory and a portion of flash memory as swap areas. The memory management unit performs swapping operations by swapping pages of content between the random access memory swap area and one or more blocks of the flash memory swap area. Thereafter, a page of content can be loaded from the flash memory swap area. The memory management unit also allocates a portion of flash memory as a garbage collection area. The memory management unit transfers dirty pages from the flash swap area to the garbage collection unit to free up flash memory swap area blocks.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present application relates generally to computer systems and, more specifically, to flash memory management for computer systems. 
     BACKGROUND OF THE INVENTION 
     Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. Flash memory is primarily used in memory cards and USB flash drives for general storage and the transfer of data between computers and other digital products. 
     Flash memory is a specific type of Electrically Erasable Programmable Read-Only Memory (“EEPROM”) that is erased and programmed in large blocks. In early flash memory devices, the entire chip had to be erased at once. Flash memory costs far less than a byte-programmable EEPROM. Therefore, flash memory has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed. Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. Flash memory has also gained popularity in the game console market, where flash memory is often used instead of EEPROMs or battery-powered Static Random Access Memory (“SRAM”) for game save data. 
     Flash memory is non-volatile, which means that no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times; although not as fast as volatile Dynamic Random Access Memory (“DRAM”) used for main memory in PCs. Further, flash memory offers better kinetic shock resistance than hard disks. Another feature of flash memory is that when packaged in a “memory card,” flash memory is enormously durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water. 
     Although technically a type of EEPROM, the term “EEPROM” is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over old-style EEPROM when writing large amounts of data. 
     NAND Flash architecture is one of two flash technologies (the other being NOR) used in memory cards such as the CompactFlash cards. NAND gate flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB storage devices known as USB flash drives and most memory card formats available today. It is also used in MP3 players, and provides the image storage for digital cameras. NAND is best suited to flash devices requiring high capacity data storage. NAND flash devices offer storage space up to 512-MB and offers faster erase, write, and read capabilities over NOR architecture. 
     One limitation of flash memory is that, although flash memory can be read or programmed a byte or a word at a time in a random access fashion, flash memory must be erased a “block” at a time. This generally sets all bits in the block to one (1). Starting with a freshly erased block, any location within that block can be programmed. However, once a bit has been set to zero (0), only by erasing the entire block can it be changed back to one (1). In other words, flash memory offers random-access read and programming operations, but cannot offer arbitrary random-access rewrite or erase operations. In general the entire block is erased and rewritten at once. 
     Conventional memory management systems for NAND flash devices are used for features like file system or demand loading for code. This makes it very difficult to implement memory management for demand loading for Read Wright (RW) and Zero Initialized data (ZI) with efficient usage of flash memory (hereinafter “flash” or “NAND flash”) and Random-Access Memory (RAM). Conventional embedded systems do not consider swapping in memory management processes. In advanced systems, such as, but not limited to Linux®, static mapping between RAM and flash pages is used. When static mapping is used, the flash size must be the same as a size of the data from the swap area. Maintaining the flash size equal to the data from the swap area results in a reduction of the life cycle for NAND flash blocks. 
     SUMMARY OF THE INVENTION 
     An apparatus for managing memory in electronic devices is provided. The apparatus includes a memory management unit. The memory management unit configured to allocate a portion of random access memory and a portion of flash memory. The memory management unit performs swapping operations of pages of content between said random access memory and one or more blocks of said flash memory. 
     An electronic device is provided. The electronic device includes a processor, a random access memory, a flash memory, and a memory management unit. The memory management unit configured to allocate a portion of the random access memory and a portion of the flash memory. The memory management unit performs swapping operations of pages of content between the random access memory and one or more blocks of the flash memory. 
     A method for memory management is provided. The method includes allocating a portion of random access memory as a first swap area. The method also includes allocating a portion of flash memory as a second swap area. Further, the method includes swapping pages of content from the first swap area to the second swap area and loading a new page of content from the second swap area. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  illustrates a simple diagram for memory in a low-end device according to embodiments of the present disclosure; 
         FIG. 2  illustrates a memory swap operation according to embodiments of the present disclosure; 
         FIG. 3  illustrates a memory management swap process according to embodiments of the present disclosure; and 
         FIG. 4  illustrate a page fault process according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged memory management system. 
     Embodiments of the present disclosure illustrate a memory management system for NAND flash devices for use in low-end devices, such as, but not limited to ARM7 and ARM9 based devices with limited memory. ARM7 and ARM9 based devices, such as, but not limited to, an ARM7 based cellular telephone, can include thirty-two Mega-byte (32 MB) of flash, 2 MB of RAM and a 100 MHz Central Processing Unit (CPU). The memory management system is configured to use the NAND flash device to store changeable data. The memory management system is configured to utilize a smaller sized flash to perform the same functions as a large (e.g., high-end) device. The memory management system is configured to use flash to offset the size of RAM. 
       FIG. 1  illustrates a simple diagram for memory in a low-end device according to embodiments of the present disclosure. 
     The embodiment of the memory in the low-end device shown in  FIG. 1  is for illustration only. Other embodiments of the memory in the low-end device could be used without departing from the scope of this disclosure. 
     The memory for a low-end device  100  (hereinafter “device”) includes a RAM  105 , a flash  110  and a virtual memory  115 . 
     It will be understood that although the RAM  105 , flash  110  and memory  115  are illustrated together, the RAM  105 , flash  110  and memory  115  could be located in different portions in the device  100 . Further, one or more of the RAM  105 , flash  110  and memory  115  could be located external to the device  100 . 
     The RAM  105  includes a CODE/CONST execution area  120 , a swap area  125  and a Heap  130 . The flash  110  includes a CODE area  135 , a RW init Data area  140 , a flash swap area  145 , a garbage collection area  150  and a file system area  155 . The CODE area  135  includes instructions for the flash operation. The RW init data area  140  includes initialization data regarding what happens in the virtual memory  115 . A partition manager (not shown) partitions the flash for RW and ZI demand load. The virtual memory  115  includes a CODE area  160 , a CONST area  165 , a RW area  170  and a ZI area  175 . The virtual memory  115  can be much larger than RAM  105  and flash  110 . 
     It will be understood that the relative sizes of the areas illustrated in  FIG. 1  do not represent actual size relationships with respect to the areas. For example, the size of the flash swap area  145  actually may be larger than either the swap area  125  or the RW init Data area  140 . 
     The virtual memory  115  is configured such that an application running on the device  100  sees the RW area  170  and the ZI area  175  as available memory. Accordingly, the swap area  125  from the RAM  105  is allocated for operation. Additionally, the flash swap area  145  is allocated for updating memory pages (e.g., saving state data). The size of the swap area  125  and the flash swap area  145  equals the size of the RW area  170  and the ZI area  175 . For example, if the RW area  170  and ZI area  175  equals 4 MB, then the swap area  125  and the flash swap area  145  must equal at least 4 MB. Therefore, if the swap area  125  equals 1 MB, then the portion of flash (e.g., the flash swap area  145 ) must equal at least 3 MB. 
     The garbage collection area  150  is allocated for storage of “dirty page&#39;s”. Dirty pages are pages of information existing in RAM  105 . A Memory Management Unit (MMU)  180  of the device  100  is configured to perform garbage collection by removing dirty pages from flash swap area  145  and placing the dirty pages in garbage collection area  150 . In some embodiments, the MMU  180  is configured to perform garbage collection during idle time. The MMU  180  performs garbage collection to “clean” flash swap area  145  to allow for ongoing or future swap operations. 
     The MMU  180  can be a special processor responsive to a plurality of instructions stored in a computer readable medium, e.g., a portion of memory, such as RAM  105 . Additionally, MMU  180  can include the plurality of instructions stored in a computer readable medium within the MMU  180 . 
     When a page fault occurs, the MMU  180  saves a swapped page from RAM  105  into flash before loading new content from the flash  110 . For example, a data (e.g., a page) currently stored in swap area  125  is saved by the MMU  180  into the flash swap area  145 . 
     The MMU  180  uses a block in the flash swap area  145  that has been least used to store this data. In some embodiments, the MMU  180  saves the data in the garbage collection area  150 . The MMU  180  uses a block in the garbage collection area  150  that has been least used to store this data. 
     Further, the MMU  180  creates a database (not shown) for flash usage. The MMU  180  saves the database in flash  110 . 
     When loading page content from flash  110  into RAM  105 , the MMU  180  first checks if relevant content was referenced. If the content was not referenced, a Demand Load System (DLS)  185  initializes the data. If the content previously was referenced, the MMU  180  loads the content from a swapped page in the flash swap area  145 . 
       FIG. 2  illustrates a memory swap operation according to embodiments of the present disclosure. The embodiment of the memory swap operation  200  shown in  FIG. 2  is for illustration only. Other embodiments of the memory swap operation  200  could be used without departing from the scope of this disclosure. 
     Swap area  125  contains a plurality of blocks.  FIG. 2  illustrates one swap area block  205  of the plurality blocks contained in the swap area  125 . The swap area block  205  includes a number of sectors  210   a - 210   h  for storing a number of pages of content. 
     Flash swap area  145  also contains a plurality of blocks.  FIG. 2  illustrates a first flash swap area block  215 , a second flash swap area block  220  and a third flash swap area block  240 . Each of the flash swap area blocks  215 ,  220  includes a number of sectors  225   a - 225   h ,  230   a - 230   h , and  245   a - 245   h  respectively, for storing a number of pages of content. 
     In one example, sectors  210   a - 210   d  contain pages of content. Additionally, sectors  225   a - 225   e  contain pages of content. Further, sectors  230   g  and  230   h  and sectors  245   a - 245   f  also contain pages of content. 
     In some embodiments, an application is running on the device  100 . The application requires a specified amount of memory. 
     The device  100  may provide the required memory space as virtual memory. For example, the application may require memory space for four (4) pages of content. In some such embodiments, the device allocates virtual memory from the RW area  170  and the ZI area  175 . 
     The MMU  180  allocates sectors from flash  110  for use by the application. The MMU  180  determines that the first flash swap area block  215  only contains three (3) available sectors. Further, the MMU  180  determines that the second flash swap area block  220  contains six (6) available sectors. However, the MMU  180  further determines that allocating four (4) sectors from the second block  220  would result in three (3) free blocks in the first flash swap area block  215  and two (2) free blocks in the second flash swap area block  220 . Therefore, the MMU  180  determines that a memory swap operation needs to be performed prior to allocation of sectors from flash  110 . 
     Therefore, the MMU  180  copies the pages contained in sectors  230   g  and  230   h . The MMU  180  then saves the copied pages in sectors  225   f  and  225   g . After saving the pages in sectors  225   f  and  225   g , the MMU  180  erases the pages from sectors  230   g  and  230   h . The MMU  180  can erase the pages from sectors  230   g  and  230   h  by erasing the entire second flash swap area block  220 . Then, the MMU  180  can allocate memory from the first flash swap area block  215  for use by the application. 
     In some embodiments, the pages in sectors  230   g  and  230   h  contain the same content as found in any of sectors  210   a - 210   d , such as, for example,  210   c  and  210   d . As such, the pages contained in sectors  230   g  and  230   h  are referred to as dirty pages. 
     In such embodiments, the MMU  180  determines that the pages saved in sectors  230   g  and  230   h  are dirty pages. The MMU  180  can copy the dirty pages from the second flash swap area block  220 . 
     Then, the MMU  180  saves the copied dirty pages in a garbage collection area block  235 . The garbage collection area block  235  can be one of a plurality of blocks contained in the garbage collection area  150 . As such, illustration of one garbage area block  235  is for example purposes only. Further, one or more of the sectors included in the garbage collection area block  235  currently may contain pages of content. Thereafter, the MMU  180  erases the dirty pages from sectors  230   g  and  230   h  by erasing the entire second flash swap area block  220 . Then, the MMU  180  can allocate memory from the second flash swap area block  220  for use by the application. 
     In some embodiments, the MMU  180  swaps one or more pages from swap area block  205  to one of the first flash swap area block  215  and the second flash swap area block  220 . The MMU  180  identifies one or more blocks (e.g., the first flash swap area block  215  and the second flash swap area block  220 ) in flash swap area  145  that are least erased. Then, the MMU  180  saves the one or more pages, from swap area block  205 , to the least erased blocks. Thereafter, the MMU  180  can allocate memory from RAM  105  for use by the application. 
       FIG. 3  illustrates a memory management swap process according to embodiments of the present disclosure. The embodiment of the memory management swap process  300  shown in  FIG. 3  is for illustration only. Other embodiments of the memory management swap process  300  could be used without departing from the scope of this disclosure. 
     In step  305 , the MMU  180  finds a block with a maximum number of dirty pages. The MMU  180  evaluates what blocks contain pages of content also located in RAM  105  (e.g., in swap area  125 ). 
     The MMU  180  selects a block with a maximum number of dirty pages in order to make available a maximum amount of space (e.g., sectors). The MMU  180  erases the dirty bits from the block with the maximum number of dirty pages. In some embodiments, the MMU  180  transfers the dirty pages to the garbage collection area  150 . In such embodiments, the MMU  180  saves a copy of the dirty pages to the garbage collection area  150  prior to erasing the dirty bits from the block with the maximum number of dirty pages. 
     For example, the MMU  180  may determine that all of the pages located in blocks  225   a - 225   e  are dirty (e.g., a copy of the content within those pages also resides in RAM  105 ). The MMU  180  may also determine that the pages located in blocks  230   g - 230   h  are dirty. Further, the MMU  180  may determine that one or more of the pages located in sectors  245   a - 245   c  previously have been swapped. 
     The MMU  180  selects the pages located in the first flash swap area block  215  as the pages to be erased. The MMU  180  selects the first flash swap area block  215  because erasure of the dirty pages located in the first flash swap area block  215  results in a freeing-up of the maximum amount of memory space. The MMU  180  does not select the second flash swap area block  220  because only two (2) blocks would be erased as opposed to the five (5) erased from the first flash swap area block  215 . The MMU  180  also does not select the third flash swap area block  240  because one or more of the blocks previously have been swapped. 
     In step  310 , the MMU  180  copies the valid pages from RAM  105  to flash  110 . The MMU  180  copies one or more pages from the swap area block  205 . The MMU  180  then saves the copied pages to one of the first flash swap area block  215 . In some embodiments, the MMU  180  selects either the second flash swap area block  220  or the third flash swap area block  240  based on the number of pages copied from the swap area block  205 . 
     Then, in step  315 , the MMU  180  erases the pages copied from the swap area block  205 . Thereafter, the MMU  180  updates a memory management table (not shown) in step  320 . 
       FIG. 4  illustrate a page fault process according to embodiments of the present disclosure. The embodiment of the page fault process  400  shown in  FIG. 4  is for illustration only. Other embodiments of the page fault process  400  could be used without departing from the scope of this disclosure. 
     A data abort occurs in step  405 . When a page fault (e.g., data abort) occurs, the MMU  180  saves a swapped page from RAM  105  into flash before loading new content from the flash  110 . For example, a data (e.g., a page) currently stored in swap area  125  is saved by the MMU  180  into the flash swap area  145 . The MMU  180  uses a block in the flash swap area  145  that has been least used to store this data. In some embodiments, the MMU  180  saves the data in the garbage collection area  150 . The MMU  180  uses a block in the garbage collection area  150  that has been least used to store this data. 
     In step  410 , the MMU  180  obtains a swap page in RAM  105 . 
     When loading page content from flash  110  into RAM  105 , the MMU  180  first checks if relevant content was referenced in step  415 . If the content was not referenced in step  415 , the DLS  185  initializes the data in step  420 . If the content previously was referenced, as identified in step  415 , the MMU  180  loads the content from a swapped page in the flash swap area  145  in step  430 . 
     The MMU  180  further is configured to perform garbage cleanup operations during idle times (e.g., periods of time wherein the application does not require use of memory and/or when the MMU  180  is not required to perform swap or page fault operations). The MMU  180 , periodically or during idle times, scans the plurality of blocks located in the flash swap area  145 . The MMU  180  identifies blocks containing dirty pages. The MMU  180  performs swapping operations (discussed in further detail with respect to  FIG. 2 ). 
     The MMU  180  relocates content (e.g., pages of content) from one of the flash swap area blocks to another of the flash swap area blocks if necessary. After moving the content, the MMU  180  erases flash swap area blocks containing only dirty pages. In some embodiments, the MMU  180  copies the dirty pages to the garbage collection area  150  prior to erasing the dirty pages. 
     Accordingly, the device  100  is configured to store most data flash  110  as opposed to RAM  105 . The MMU  180  can be configured to optimize memory management using an allocation of RAM and NAND flash, as discussed herein. The MMU  180  can be incorporated into any device  100  with limited RAM and limited flash, such as, but not limited to, cellular telephones, MP3 players, televisions, personal data assistants, navigation devices (such as global positioning system (GPS) devices), digital recorders, and ARM7 and ARM9 devices. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.