STACK IMPLEMENTATION

You'll lose more than the value of A. You'll lose your job too probably. Think about what RET does, and from that guess whether you will ever get to call POP_A.

I don't think there will be jobs at stake - this is a homework question I reckon. Good question to ask students though.

It's a good interview question. For assembly programming it's not out of the realm of possibility to lose track of your stack, particularly if you're pushing and popping data in the middle of things. Knowing what happens would help debugging, particularly if the misalignment only happened every once in a while as a consequence of conditional execution.

Note that on some processors the call stack is separate from the data stack (I believe this is the case on the PIC18 and the 6805, but I haven't picked through either of them in detail). For these processors, pushing a register onto the data stack then returning from a subroutine wouldn't lose you your program counter, although it could certainly mess up your data stack if you weren't planning on that behavior.

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Hi Tim,

You have hit the nail on my head. Last Friday I had this question from my interview panel where in they tried to confuse me. As a result I landed up here to confirm myself what I have told is correct.

Thanks TIM and PAUL. for your time.

Regards,

-Dk

Just let me add, that many RISCs (PPC, ARM) do not push the return address on the stack automatically, so calling will not directly interfere with the stack (I do above things in my own code a lot :-)

And there are some processors, AVR for instance, where all the direct stack operations are to/from the hardware stack but many code implementations use a separate "software" stack.