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By Chris De Herrera 
Copyright 1998-2007
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Windows Mobile 5.0 Memory Management
By Chris De Herrera, Copyright 2005, 2006
 Version 1.01  Revised 2/20/2006

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Introduction

With the introduction of Windows Mobile 5.0 for Pocket PCs, Microsoft has changed the way memory is handled in the Pocket PC.  Previous versions of the Pocket PC operating system used the flash rom to boot, store and execute programs while the ram was divided between executing programs and storing programs and data files, Also, data files stored in internal ram (not flash) are compressed at approximately a 2 to 1 ratio. 

Here is a screen shot of the Pocket PC 2002 memory management.  Note that this screen has not changed with Pocket PC 2000, Windows Mobile 2003, or Windows Mobile 2003 Second Edition.

Here is a screen shot of the Windows Mobile 5.0 memory management.

Implications of Memory Management in Windows Mobile 5.0 for Pocket PCs

Rom

Since Microsoft decided switch to storing all information in flash, there are benefits and trade offs.  First as a benefit, all your data is stored in flash and the flash is not erased if you run the battery of your Pocket PC dry.  This is now called persistent storage.  Second, the flash space in the Pocket PC is split between storage and rom.  So the amount of data or programs you install on your Pocket PC is limited by the amount of free storage (flash) on your device.  Also note that the storage in flash is not compressed like it was in prior versions so you cannot store as much information in flash.  Also due to this design, programs that are installed stored in flash are copied to ram to be executed thus they occupy twice the space.  Finally the directory of data stored in the boot rom is merged with the storage flash so the user sees the file system as one.  If a file in the boot rom is replaced by a file in storage flash the storage flash file is used.

RAM

So the ram inside the Pocket PC is now dedicated to running programs. So if you attempt to run multiple programs that are "hogs" like you used to in Windows Mobile 2003, they may not run or the operating system may close some of them.  Users should be aware that the 32mb size limitation for a single program still applies to Windows Mobile 5.0 applications. The memory management for programs still applies as well where the operating system flushes pages of programs that are no longer needed, thus optimizing the use of ram in the system for other programs to execute. 

RAM Cache for Flash Storage

A portion of the RAM is used as a write cache to increase the performance of the mobile device when writing to the flash.  This RAM cache is fixed in size by the OEM and the RAM is not usable for any other purpose.  So before performing a soft reset or removing the battery you must turn the unit off and wait 5-10 seconds.  If you fail to do this you may corrupt your internal flash storage.

Conclusion

Overall this new model of storage and execution space in Windows Mobile 5.0 more closely resembles the way your PC stores programs in rom (like your hard disk) and execute programs in ram.  I believe that the benefits of storing all information in flash outweigh the loss of space and ability to run large applications.  This is the same model that the Smartphone has used since it's introduction in 2002.

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