The Operating System

PalmOS is a simple, single-tasking operating system with a GUI widget set that enables the application developer to build event driven, graphical applications.

The 2 to 8 MB of physical RAM is divided into two parts:

• 64 KBytes of ``dynamic memory'' in older version of PalmOS, and up to 256 KBytes in more recent versions. This is the amount of RAM in which a PalmOS application executes. Memory in this area can be acquired from the operating system through the MemPtrNew system call and then be freely written and read.

• The remaining memory - called ``static memory'' - is battery-backed. This area is maintained by the OS as secondary memory. It is organized as a list of ``databases'' with mostly untyped records of variable size to which the user can obtain a handle using the DmGetRecord call. By locking such a handle using the MemHandleLock call, a pointer to a record's location can be obtained. As long as a handle is locked the operating system guarantees not to move the associated record in memory. The pointer returned by MemHandleLock only allows read access to the record's memory area. Writing has to be performed using the DmWrite call - direct write access will result in a fatal exception and will eventually reset the device.

This partitioning of the device's physical memory can be avoided by using the undocumented system calls MemSemaphoreReserve/MemSemaphoreRelease. These will switch the memory protection of the ``static'' memory area off and on. When these calls are used, great care has to be taken when issuing other system calls as some may need the semaphore. This forces the programmer to release the memory semaphore before entering certain system calls.

As mentioned in the introduction, PalmOS has no automatic multi-tasking capabilities. If a user changes from one application to another, the first one will be asked to stop by an AppStop event. Once it has stopped, all dynamic memory will be reclaimed by the operating system. The second application will then be started using the same dynamic memory as the first. The operating system does not provide any implicit support for saving the context of an executing application. Instead, the application programmer is forced to save the state explicitly, in a database residing in static memory.