Re: [AnyC-devel] Recursion

[Daniel W. <dan...@ro...>]

> I've been investigating the elusive problem of doing recursion on
> these chips.  I decided I'd look to see how other compilers did it.
> It turns out that NO compilers implement recursion on the PIC chips.
> It really isn't that practical in any case.  It could probably be
> done via some trickery, but the results would be rather useless.
> Recursion isn't really a demanded feature for such a small system in
> any case.  My current thoughts are to issue a warning if a recursive
> call is used, and otherwise basically not support recursion for the
> PIC.  Does this seem like a decent way to go?

  I have looked into this in the past.  You definitely can't do it easily=
,=20
because the PICs are designed to use the hardware stack.  To impliment a=20
software stack (which is what is needed for recursion), it is nice to be =
able=20
to read the program counter.  If you allow your code to be non-relocatabl=
e,=20
the compiler will know the absolute address of all the commands, and thus=
 the=20
program counter for each command.  It would then theoretically be possibl=
e to=20
save the high bits of the program counter somewhere.  This would be quite=
=20
tedious, it seems to me.

  The AVR line, on the other hand, seems to allow reading of the PC in an=
=20
indirect way: if you call a function, the address is pushed (in two bytes=
)=20
onto the stack.  You can then pop the stack into register space and play =
with=20
it.  This makes a true software stack for both code and data possible.

  Have you had a chance to look at the AVR datasheets yet?  I'm looking a=
t=20
the instruction set doc, and the 90S1200 and 90S8535 (doc is the same as=20
90S4433). =20

  I'm unsure how hard it would be to make the compiler work for both=20
situations... maybe not that hard.