Re: [AnyC-devel] Recursion

[Patrick E. <pa...@pa...>]

On Wednesday 18 July 2001 00:07, you wrote:
>   I have looked into this in the past.  You definitely can't do it
> easily, because the PICs are designed to use the hardware stack. 
> To impliment a software stack (which is what is needed for
> recursion), it is nice to be able to read the program counter.  If
> you allow your code to be non-relocatable, the compiler will know
> the absolute address of all the commands, and thus the program
> counter for each command.  It would then theoretically be possible
> to save the high bits of the program counter somewhere.  This would
> be quite tedious, it seems to me.
>
>   The AVR line, on the other hand, seems to allow reading of the PC
> in an indirect way: if you call a function, the address is pushed
> (in two bytes) onto the stack.  You can then pop the stack into
> register space and play with 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 at the instruction set doc, and the 90S1200 and 90S8535
> (doc is the same as 90S4433).
>
>   I'm unsure how hard it would be to make the compiler work for
> both situations... maybe not that hard.

I would be interested in making it possible to work with both 
situations.  I imagine it would be possible to do with some sort of 
generic variable request system.  If you need a variable, you ask it 
for code to get to that variable.  It decides if it's going to be 
local or global in scope based on if the processor supports local 
variables/recursion.  I haven't thought about it that much yet.

The AVR line sounds pretty good for compilers.  I wonder if they had 
compilers in mind when they were designed.  Perhaps to act more like 
regular computers.  I don't want to only support the AVR line though, 
especially since I don't even know if I can easily aquire them 
locally without special ordering them.  The PIC is a popular chip, 
and having a free compiler for it couldn't hurt at all. :)  I didn't 
look at the datasheets for the AVR chips.  I suppose they support 
20MHz as well?  That was the only question I had about them.  Perhaps 
I should take a look when thinking about these memory allocation 
issues.

        Patrick Earl