gnupic: Any way to address the stack?
Subject:
Re: Any way to address the stack?
From:
"Robert J. Lee" ####@####.####
Date:
8 Nov 2004 22:01:04 +0000
Message-Id: <200411082201.06229.kiki@rjlee.dyndns.org>
On Monday 08 Nov 2004 07:41, Snail Instruments wrote:
> >Is there any way to access the hardware return stack on PIC 16F873A?
> >
> >In other words, to get the value of the current stack pointer, as well as
> >read (and possibly also write) the contents of the 8 stack levels
> > themselves?
>
> No.
I guess you could reverse-engineer how the chip works and then attach it to an
electron-microscope pointing at the call stack's registers, but that's
probably overkill.
> >I ask, because I'm looking at ways of implementing a software call/return
> >stack, and thus eliminating the ceiling of 8 call levels.
>
> Just a suggestion - if your application _really_ needs more then 8 levels
> of nested calls, use PIC18F442, pin compatible, but with 31 level stack.
> And software accessible on top of it.
>
> Josef
>
You can GOTO the procedure instead of calling it, storing a value in a
register to indicate which call you used. Then instead of RETURN/RETLW, you
can just jump back to the starting point using a lookup table:
; Load the call flag into W first
ADDWF PCL,1
GOTO callpoint1
GOTO callpoint2
GOTO callpoint3
etc.
You may need to put your return value in a register rather than in W if you
want RETLW-type behaviour
Using the indirection register, you can even create your own call stack. It's
very register-hungry, but if you've got the free registers and you can afford
the overhead then it does work.
Hope that helps,
— Robert J. Lee
