Normally, RET is used to return in ordinary assembly subroutines, and RETI is used to return from interrupts in interrupt service subroutines. Both instructions can pop the breakpoint address from the stack and load it into the PC, allowing the CPU to return to the original breakpoint of the main program and continue running.
However, RET and RETI have essential differences: when an interrupt source is responded, one of the flag registers flag H and flag L in the microcontroller will be automatically set to prevent the triggering of interrupts of lower or same level than it. RETI can clear the "priority activation trigger" (i.e., flag register) to ensure timely response to subsequent interrupt sources.
So, when RETI is replaced by RET, the program may not fail the first time it is run, but it will fail the second time!
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