2.5.3.7.1 Exception Entry

Exception entry occurs when there is a pending exception with sufficient priority and either:

The processor is in Thread mode
The new exception is of higher priority than the exception being handled, in which case the new exception preempts the exception being handled.

When one exception preempts another, the exceptions are nested.

Sufficient priority means the exception has greater priority than any limit set by the mask register, see 2.5.1.3.9 Exception Mask Registers. An exception with less priority than this is pending but is not handled by the processor.

When the processor takes an exception, unless the exception is a tail-chained or a late-arriving exception, the processor pushes information onto the current stack. This operation is referred to as stacking and the structure of eight data words is referred as a stack frame. The following figure illustrates the information contained in the stack frame.

Figure 2-13. Exception Entry Stack Contents

Immediately after stacking, the stack pointer indicates the lowest address in the stack frame. The alignment of the stack frame is controlled via the STKALIGN bit of the Configuration Control Register (CCR).

The stack frame includes the return address. This is the address of the next instruction in the interrupted program. This value is restored to the PC at exception return so that the interrupted program resumes.

The processor performs a vector fetch that reads the exception handler start address from the vector table. When stacking is complete, the processor starts executing the exception handler. At the same time, the processor writes an EXC_RETURN value to the LR. This indicates which stack pointer corresponds to the stack frame and what operation mode the processor was in before the entry occurred.

If no higher priority exception occurs during exception entry, the processor starts executing the exception handler and automatically changes the status of the corresponding pending interrupt to active.

If another higher priority exception occurs during exception entry, the processor starts executing the exception handler for this exception and does not change the pending status of the earlier exception. This is the late arrival case.