Re: STACK IMPLEMENTATION

Tom Lucas wrote:
"Paul Burke" <paul@xxxxxxxxxx> wrote in message news:5h62puF2s3seiU1@xxxxxxxxxxxxxxxxxxxxx
dk wrote:

I am trying to understand if i implement something like this in my
micro what would happen. My understanding is that i will lose the the
value of 'A'


CALL PUSH_A
.
.
.
CALL POP_A


PUSH_A : PUSH A
RET
POP_A: POP A
RET
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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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