Persistent Storage on Windows Mobile

Starting with Windows Mobile 5, Microsoft introduced a new mechanism to store system and application data. It is now mandatory for OEMs like Dell, HTC that build Windows Mobile based PDAs or PDA phones to implement Persistent Storage. It means storing data on media which does not need continuous power source. Flash memory is one such media used in PDAs and it provides a way to avoid data loss even in the event of batteries getting fully drained out.

Windows Mobile 2003 and earlier had RAM based storage mechanism. There a backup battery was required to keep the RAM circuitry powered up even if the device was just suspended. In case main batteries have drained out, the backup battery would also drain out eventually unless it is recharged. Recharging must be done within a time limit otherwise all data would be lost. Microsoft had a stringent requirement that a backup battery must source power to RAM for at least 72 hours. With persistent storage this requirement is not relevant anymore.

Earlier if a device had 64 MB RAM, Operating system would partition into two logical parts: Storage Memory and Program Memory. Storage Memory is where the saved data would be maintained as files, databases or in the registry. Runtime memory requirements of application were fulfilled from Program memory area. Using a control panel applet, users could modify RAM allocated to these two regions.

Persistent Storage is not something invented by Microsoft. Infact Microsoft already had persistent storage supported on Smartphone but now they are supporting on Windows Mobile also.

By using devices with Persistent Storage, we have following advantages :
1. Data is not lost even if the batteries are not charged for months. As the device gets sufficient power to turn on, the data is available again and there will be no need to reinstall third party applications and their data will remain intact.
2. Since RAM is now not at all used for storage (object store), all of it is available as program memory. Applications can allocate more memory at runtime for the same RAM chip size.
3. Backup battery is not required to keep the RAM chip powered all the time. There may have some power savings.

There are some disadvantages also:
1. Flash Read and Write access times are way higher than those of RAM. Current Applications might seem to run sluggishly while reading and writing data. For better user experience, application developers can use the RAM as a cache and actually save (commit) the contents to flash when the device is idling. This approach can cause coherency issues which must be dealt with appropriately.

If you are thinking of getting a new PDA, this may be one good reason to go with Windows Mobile based PDA.