gnupic: Any way to address the stack?
Subject:
Re: Any way to address the stack?
From:
Byron A Jeff ####@####.####
Date:
8 Nov 2004 17:42:23 +0000
Message-Id: <20041108174216.GA2477@cleon.cc.gatech.edu>
On Mon, Nov 08, 2004 at 05:16:33PM +1300, David McNab wrote:
> Hi,
>
> Is there any way to access the hardware return stack on PIC 16F873A?
No.
>
> 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 ask, because I'm looking at ways of implementing a software
> call/return stack, and thus eliminating the ceiling of 8 call levels.
>
> As it is, a software call/return stack would be inefficient, because it
> would require something like:
>
> movlw low(return_address)
> movwf return_address_low
> movlw high(return_address)
> goto sw_stack_call
> return_address
That's the way to do it. There is no other way.
> ...
>
> This comes to 4 words per call, which will rapidly bloat out the
> application.
You could shorten it into a jump table of fixed return locations. It may
shave an instruction or two off the call. Something like:
movlw 0x43 ; Index to return location #43
goto sw_stack_call
return_address_43:
Then use a jump table to jump back:
movf return_index,W
addwf PCL,F ; I'm presuming that PCLATCH is properly set.
;... Jump return entries....
goto return_address_43 ; the 0x43 entry in the jump table.
Longer path but saves instructions on the call.
>
> If there were a way to 'hack' the return stack, I could cut this to one
> word per call.
Can't be done. Only the 18F parts have accessible return stacks. Deeper too
IIRC.
>
> Just asking, on the possibility that one of the 'unimplemented' FSRs (or
> some other hack) might let me access the stack.
Nope.
>
> Thoughts?
Easy call. Can't be done. You'll have to find some other way to do it.
BAJ
