PDAgold.com :: Article :: Memory in Pocket PC explained

PDAgold.com :: Article :: Memory in Pocket PC explained: Memory in Pocket PC explained
Author - Pavel Koza :: User rating - 4/5 gems (24 votes) :: Views - 563 July 07, 2005 :: A vast majority of PDA owners have some experience with personal computers and know at least something about the basic elements, such as the memory. However, if such a user tries to apply the knowledge on pocket computers, he may be rather confused. This article explains the basic terms, describes different types of memory used in pocket computers and main differences between them.

Memory is one of the basic components of a device called a pocket computer. It makes little difference how fast the processor, how large and colourful the display and how many wireless radio modules a device carries - without memory, it would be dead box of hardware. Nevertheless, even the less technically savvy users should read this article. A PDA's memory is a place where you keep the most valuable thing - you data. If you do not want to loose them, you should know at least the elementary issues.

As in real life, the computer memory, too, has to handle three main tasks - otherwise it would be useless. These include the possibility to write information, retain it and recall. All the information is stored in the binary form - as a combination of ones and zeros. There are many different types of memory but all of them are based on the electromagnetic principle (I have never seen a punch-card reader for Pocket PC:) Here, we shall discuss only the types of memory used in Pocket PCs. If you would like to learn more on memory technology in general, I recommend your local technical library or Google.

Even the simplest pocket computer with a Microsoft operating system integrates several types of memory that differ in purpose and speed. There are two basic types - ROM (read-only memory) and RAM (random-access memory). In addition, almost every device can be extended using a memory card or other memory media e.g. over a USB Host interface. However, we will stick to internal memory this time.
ROM / FlashROM

The abbreviation "ROM" stands for Read Only Memory. In pocket computers, this type of memory serves as a storage for all files of the operating system and basic applications supplied by the respective device manufacturer or Microsoft. Its main advantage is that it does not need to be powered permanently to retain the information it contains. The data are safe however long the memory remains without power. The first Pocket PC devices and almost all of their predecessors had the operating system loaded in the ROM. When Microsoft issued an upgrade of the operating system and a manufacturer decided to adopt it for older devices, it was necessary to physically replace the ROM module (which looks like the one in the image above). This was the only possible way of upgrading the system. The only advantage of this solution was that alongside a new ROM chip, the upgrade module often included also some RAM, so you received not only a new system but also a larger operating memory. On the other hand, such modules were quite expensive and required playing with the device's guts or visiting a service centre.
ROM upgrade for HP 620LX
ROM upgrade for HP 620LX

With the arrival of Compaq iPAQ and Pocket PC in general, ROM started to be replaced by its more convenient variant called Flash ROM. This type of memory, too, will retain data without power but it is possible to change its contents under certain conditions. This means that new versions of the operating system or partial patches can be loaded in the Flash ROM directly just using appropriate software; no hardware changes are needed. The latest innovation concerning Flash ROM in Pocket PC devices is a possibility to use its free part as an extra storage of up to several tens of megabytes for sensitive data and applications that the user wants to protect. This space is available as another folder in the memory structure and manufacturers call it different names (iPAQ FileStore, LOOXstore etc.), I called it generally Safe Storage. You can work with it as with any other folder; it only writes data significantly slower than standard RAM.

When speaking about Flash ROM, I should mention the terms NOR and NAND, which are the two dominant technologies used for producing Flash memory chips. The NOR technology was first introduced by Intel in 1988, NAND by Toshiba a year later. There are fundamental differences between them but a detailed description would go beyond the scope of this article (in addition, my technical knowledge does not reach far enough:) - if you are interested in a detailed comparison, click here. For most of us, ordinary mortals, it is sufficient o know that:

* NOR features smaller capacity (1 MB - 32 MB), shorter life (10-100 re-writes) and very slow erasing and writing. On the other hand, it supports a technology called XIP (Execute in Place). That means that binary code stored in the ROM can be executed straight away without the necessity to copy it to the RAM first. High price.
* NAND features larger capacity (16 MB - 512 MB, maybe more), longer life (100-1000 thousand re-writes) and fast erasing and writing. However, code needs to be copied to the RAM before execution (Code Shadowing). Low price.

As a user, you should be interested in what type of Flash ROM your device contains. For example, users of HP iPAQ h1915 could find out from the specifications that their device contains a Flash ROM but the memory being a NAND-type, they were limited in many ways as compared to users of other devices.

There is a lot to write about Flash ROM, this much just for a quick reference:)
RAM / SRAM / DRAM / SDRAM

RAM stands for Random Access Memory. Unlike ROM, the RAM memory needs to be powered to retain its contents. Even the shortest blackout wipes out all data irreversibly. There are many types of RAM but we shall focus only on some of them. Depending on whether a memory is static or dynamic, it is called either SRAM or DRAM. SRAM-type memory (Static RAM) retains data as long as it is powered. It is very fast but more sophisticated and therefore more expensive. It is used primarily as cache memory where speed is a crucial factor. On the other hand, DRAM (Dynamic RAM) retains information using electric charge from a capacitor. However, the charge tends to wither even if the memory is powered. In order to prevent complete discharge and subsequent loss of information, it is necessary to refresh memory cells periodically. It is simpler and therefore significantly cheaper, albeit slower than SRAM.

We shall discuss in detail only SDRAM (Synchronous DRAM) which we can find in virtually all Pocket PCs on the market. As you will guess correctly, it is based on the DRAM technology but is much faster than the original SDRAM due to certain enhancements, such as synchronisation with the frequency of the CPU bus. Favourable price makes it the preferred choice and only rarely can we encounter another type of memory in pocket devices.
How does it work?

You now know what types of memory are used in Pocket PCs, let's see how it all actually works and how a Pocket PC uses the memory. PDA beginners will probably be familiar with a desktop PC, so I will try to compare the two worlds.

All Pocket PCs I have ever seen have both RAM and ROM. Apart from some rare exceptions, there are no pocket computers with an integrated hard drive. The task of permanent storage of data even without power is assigned to the ROM. It contains the operating system, without which the device just would not work, as well as basic applications from Microsoft and other software vendors (it is the manufacturer of the device who decides what software you obtain with the device). In some devices, a part of their Flash ROM is accessible o the user as Safe Storage. When you switch the device on for the first time, a series of initial operations and settings will be performed. Besides other things, data that crate a user profile will be copied to the RAM (system registry, document templates, and bonus applications, if available). After you have calibrated the display and tried Tap&Hold, the device is ready to use.

Most Pocket PC first-timers will suffer a shock upon inspecting available memory. They usually believe that when they buy a device with 64 MB of RAM, they will have that amount of memory at their disposal for storing data and applications. In reality, the amount of free memory is significantly smaller for three reasons:

1. As I mentioned above, the system may occupy part of the RAM for its own purposes (such as code shadowing etc.), which reduces the original amount (down to the 55 MB).
2. Even a completely new device has some data in it (modules for the Today screen, document templates etc.), which eat up several more megs (that's the blue line)
3. The rest will be divided (and dynamically allocated) between storage memory and program memory where applications actually run (that's the slider)

As you can see in the image, a device boasting of 64 MB of RAM will eventually offer some 20 MB of memory space for your data and applications. You can adjust the storage/program memory allocation using the slider but if you run several applications simultaneously, the system will re-allocate the memory according to its needs. Having mentioned Safe Storage, the situation in Acer n35, for instance, is as follows:
Memory size (slightly) misleading In this case, the Safe Storage is less than 2 MB
Why is that?

There is a reason for this solution, naturally. It is assumed that a pocket device, which is dependent on batteries, will be left without power from time to time and all data in the RAM will be erased. Whereas a desktop computer would require a several-hour re-installation and recovery in such an event, a Pocket PC is ready for work within seconds after power has been restored. There are more advantages to it than that but I'll leave it up to curious users to find out:)

Note:: The latest system, Windows Mobile 5, will revolutionise the way a Pocket PCs use memory - the RAM will be used only for running applications, everything else, including the system, installed applications, user data and settings will be stored in the FlashROM, which will eliminate problems with flat batteries. More on that as soon as devices with the new system hit the market.
What is the difference between Soft a Hard reset?

One of the most frequently asked questions regarding pocket computer is the one I chose for the title of this paragraph. The difference is fundamental:

* A Soft Reset is an operation that forcibly closes all running applications. All UNSAVED data are lost but the contents of the RAM are available after the system recovers. A Soft Reset is usually performed by a press of a more or less hidden button.
* A Hard Reset will COMPLETELY ERASE the RAM. Data stored in the Safe Storage or on memory cards will remain intact. The device then runs the initial sequence as if just taken out of the box and is ready to use after a little while. The way of performing a Hard Reset differs from device to device: it may require a simultaneous press of several buttons, deactivation or removal of the main battery. Check your user manual for details.

Conclusion

That's about all a beginner should know about memory in pocket computers. As your experience grows, you will find that there are many exceptions from the general trend and the development is quite rapid. Nevertheless, the above should be enough to get a basic idea of memory types and usage in Pocket PCs. If you have any questions, send them to discussion forums. I'll try to answer and maybe eventually extend this article.