RTEMS Porting Guide
When an interrupt occurs, it is the responsibility of the interrupt dispatching software to save the context of the processor such that an ISR written in a high-level language (typically C) can be invoked without damaging the state of the task that was interrupted. In general, this results in the saving of registers which are NOT preserved across subroutine calls as well as any special interrupt state. A port should define the CPU_Interrupt_frame structure so that application code can examine the saved state.
typedef struct {
unsigned32 not_preserved_register_1;
unsigned32 special_interrupt_register;
} CPU_Interrupt_frame;
RTEMS Porting Guide
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