[fpc-devel] AVR HeapSize and StackSize

[Christo Crause]

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

> 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.
>

I did some testing of stack checking on AVR (the long version is here:
https://github.com/ccrause/freepascal/wiki/Investigating-stack-and-heap-management-(avr-embedded)).
The short version is that I suspect there are a couple of problems with the
current code:
* The AVR implementation of function Sptr (which should return the stack
pointer I believe) is missing.
* The variable StackBottom (the stack limit) is not calculated correctly
because some of the required pointer declarations are not correctly linked
with corresponding external symbols.
* The current stack margin defined in system.inc of 16384 exceeds the SRAM
in most AVR 8 bit controllers (the only exception is atmega1284P), so a
smaller margin should be selected for the AVR target.

If anyone can confirm these issues I can log a bug with patches.  If I
missed the plot (most of the relevant RTL code was unchanged for 8 years or
longer) I would appreciate it if someone can point out where I made errors
or misinterpreted code or specifications.
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