20% Rule

Under Windows CE 3.0 32MB memory is provided per task (this is referred to as Slot, see Figure). In these 32 MB the Runtime (Variables, Functions, Pictures, Driver System Files etc.) have to be stored.

The operating system has the limitation for Windows CE 3.0: If the operating system (e.g. graphics driver or other application) loads a *.DLL file (Dynamic Link Library) this memory area will not be available in the other slots. This causes a considerable decrease of available memory for the Runtime. During Start of Windows CE, diverse driver- and system files are loaded into the memory, depending on the device. Therefore the Runtime can never use the whole 32 MB.

Windows CE NET (4.xx) and CE 5.0 is still limited to 32 MB memory per application. However, the memory that is in use by the operating system or other applications in other slots, can already be used.in the slot in which the runtime was started. Thanks to this update of CE.NET/CE 5.0, more memory is available for the Runtime.

Operating system and used hardware limit the maximum project size. Unlike with a PC, the memory is limited. To write a kilobyte data on a compact flash card for example, does not take some milliseconds (as usual for PC), but takes much more time.

Windows CE has been developed for low performance hardware that can be run without any moving parts. Only few CE devices have an active processor cooling. Often processors of CE devices take over all roles: Calculation, Graphics, TCP/IP Communication, Serial Communication etc.

On the PC these duties are done by various hardware components. Because of this essential difference the performance of a CE device can not be compared to the performance of a PC.

The maximum project size therefore depends on the device. No universally valid limit e.g. for a maximum number of variables or alarms can be defined.

Info

If after the Runtime start all pictures are called at the same time and the maximum number of alarms queue in the memory, 20% of the central memory have to be empty in order to guarantee a faultless function.

The available central memory can be found out with the system variable (system driver) Free disk space %.

Since CE 6.0, the limitations of the operating system mentioned above no longer apply. The available memory can be fully used by every application. However, as the memory is often limited to 128MB or 256MB RAM on conventional CE devices, there is still the chance of memory shortages. Especially when you use the Extended Trend & Archive Server Starter Edition. Therefore, the 20 % rule mentioned above still applies.