Microsoft Explains the Advantages of Persistent Storage

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One of the most significant changes in the Pocket PC version of Windows Mobile 5.0 the inclusion of Persistent Storage.

Under this system, Pocket PCs will use RAM only for holding currently running applications, and nothing else. All Storage of applications and files will be done in Flash ROM.

This means that, if the handheld’s battery is completely depleted, the files and applications that the user has added to the device won’t be erased; this is the origin of the name “Persistent Storage.”

Windows Mobile Mike Calligaro has posted on the Windows Mobile Team Blog a discussion on the other advantages of Persistent Storage.

These include a greatly increased battery life and the ability for future Pocket PC to have far more built-in storage.

Doubling the Battery Life?

Although RAM is faster than ROM, it has one major disadvantage: power must be supplied to it constantly or it loses whatever data it is storing.

According to Calligaro, the Windows Mobile team feels that this is one of the worst things that can happen to a handheld. Therefore, when a Pocket PC says that its battery is critically low, it actually has plenty of capacity left. This is because Microsoft requires devices to still have enough power to keep the RAM from losing its contents for 72 hours after the low-battery warning has gone off.

With Persistent Storage, this isn’t necessary, because they won’t lose everything stored on them if the battery runs out. Devices running Windows Mobile 5.0 won’t display the low-battery warning until the battery is actually almost completely depleted. Calligaro says that this could increase the length of time a Pocket PC can be used on a single charge by a quarter or even a half.

A 1 GB Pocket PC?

The necessity to constantly keep supplying power to RAM hasn’t just shortened handhelds’ battery life, it has also limited how much built-in storage Pocket PCs can have.

Calligaro explains it this way: “A typical battery holds 1000 mAh of charge. 128 MB of RAM takes about 500 mAh to stay resident for 72 hours. 64 MB takes about 250. This is why you never saw a 256 MB WM 2003 device. It would have run for a minute then decided its batteries were critically low.”

However, this isn’t true of Flash ROM. A 128 MB ROM chip takes about the same amount of power as a 32 MB one. With Persistent Storage, Pocket PC makers will be able to create handhelds with far more Storage capacity.

“There’s no barrier to keeping you from seeing 256 MB, 1 GB, etc, devices,” says Calligaro.

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