Hi,

Just to re-iterate. All I ever said was that it can be "handy" to flush the
stack and that in a controlled environment can solve some difficult problems
like Abrupt Call Path exit which can be used for Exception Handling in a
controlled environment. I never said it was a pop, and only ever pointed it
out as a 'trick' to gain some control over the stack.

Regards,
James Caska
caska@virtualbreadboard.com
ujVM - 'The worlds smallest java virtual machine'




-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Olin Lathrop
Sent: Monday, 8 October 2001 11:47 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]:


> OKNowAbruptlyExitTheCallPath
>         MOVLW   .5      ;The trick is to know your call depth
(MaxDepth-CurrentDepth)
>         MOVWF   StackDepth
> FlushStackLoop
>         DECFSZ  StackDepth,F
>         CALL    FlushStackLoop
>         GOTO    MAIN.OnException.AbruptHandler

All you are really doing here is a GOTO to a fixed address, which has to be
in top level code.  All the stack diddling is just a waste of time.  It
doesn't matter what's on the stack as long as you don't intend to return to
any of it.  It seems you want to "reset" the stack pointer.  The stack is
circular, so you can "reset" the stack any time you like just be deciding
never to return to any called-from routine, which you have to do anyway if
you were able to "empty" the stack.  Your entire
OKNowAbruptlyExitTheCallPath routine could be replaced by the single GOTO
MAIN.OnException.AbruptHandler with no effect on the program.

Your other example was jumping to an exception handler, then continuing to
return to higher call levels from there.  That can't be done on a 16F877 by
stack manipulation alone.  The example here only works because the exception
handler is not nested inside a subroutine you intend to return from.

>         RETURN  ;Never Reach Here
>
> CallWithAbruptExitDepth3                        ;Build up a stack path
>         CALL    OKNowAbruptlyExitTheCallPath
>         RETURN  ;Never Reached
>
> CallWithAbruptExitDepth2
>         CALL    CallWithAbruptExitDepth3        ;Build up a stack path
>         RETURN  ;Never Reached
>
> CallWithAbruptExitDepth1
>         CALL    CallWithAbruptExitDepth2        ;Build up a stack path
>         RETURN  ;Never Reached
>
>
> CallWithRegularReturn
>         ;Code Here
>         RETURN
>
> MAIN
>         CALL    CallWithAbruptExitDepth1
> MAIN.OnException.AbruptHandler
>         CALL    CallWithRegularReturn
>
>         GOTO    MAIN
>
>         END


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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