Special classes

There is an internal PXROS-HR MPU trap handler that is called first. This handler tries to solve the access problem. If it fails, the trap is not handled, and a user-trap handler is called if it was installed.

The internal PXROS-HR MPU trap handler that first tries to resolve the memory access does not change the ICR.CCPN value and therefore runs on task level. However, if the kernel does not resolve the memory access, the default PXROS-HR trap handler is called.

In the case of FCU, a user-defined trap handler is not called because there are no free CSAs. FCU traps will result in an endless trap recursion because saving the lower context will always trigger the next FCU trap. Therefore, it is recommended that the LCX is set up properly and the FCD trap is handled.

The SYSCALL instruction generates a system call trap, providing a secure mechanism for the user mode application code to request Supervisor mode services. If a handler is installed for SYSCALL it is never used — there is no calling of a user trap handler in this class.