[fpc-devel] AVR HeapSize and StackSize

Christo Crause christo.crause at gmail.com
Mon Mar 25 18:28:57 CET 2019

On Sat, Mar 23, 2019 at 9:37 PM Florian Klämpfl <florian at freepascal.org>

> Am 23.03.2019 um 11:52 schrieb Dimitrios Chr. Ioannidis via fpc-devel:
> > Hi,
> >
> >   I'm using the heapmgr unit and, for the atmega328p, AFAIU the default
> heap size is 80 bytes. Cause it's too small for
> > my needs, I changed to 512 bytes ( -Ch512 ). But now I'm worrying for
> the stack and heap collision.
> >
> >   Can anybody know any resources, docs regarding FPC stack and heap
> management for the AVR platform ? Any hints, tools ?
> The sources ;) For the embedded targets, the stack grows downward from the
> highest ram address. So set a suitable stack
> size so you do not exceed total memory, test with stack checking on, then
> you should be fine.

Out of interest I tried to see how stack checking worked.  One of the
puzzles is that function Sptr is empty, so it will return whatever values
happen to be in r24-r25. Is this correct behaviour?  I would have thought
(based on how Sptr is implemented for other architectures) that it should
return the value of the SP io register, something like this:
function Sptr : pointer; assembler; nostackframe;
  in r24, 0x3d // SPL
  in r25, 0x3e // SPH
 Any thoughts?
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