2.5.3.7.2 Exception Return

Exception return occurs when the processor is in Handler mode and execution of one of the following instructions attempts to set the PC to an EXC_RETURN value:

• An LDM or POP instruction that loads the PC
• An LDR instruction with PC as the destination
• A BX instruction using any register

The processor saves an EXC_RETURN value to the LR on exception entry. The exception mechanism relies on this value to detect when the processor has completed an exception handler. Bits[31:4] of an EXC_RETURN value are 0xFFFFFFF.

When the processor loads a value matching this pattern to the PC it detects that the operation is a not a normal branch operation and, instead, that the exception is complete. Therefore, it starts the exception return sequence. Bits[3:0] of the EXC_RETURN value indicate the required return stack and processor mode, as noted in the following table.