[Coco] Calling convention used by older 6809 C compilers

[Pierre Sarrazin]

Hi. I'm in the process of adding a second calling convention to CMOC,
a C-like compiler that targets the 6809.

I have seen C programs for the 6809 that had some code written in
assembly that assumed a calling convention where the first parameter
of a function is received in register B or X, depending on its size,
instead of on the stack, as in CMOC's default convention.

platotermCoCo is such a program:
https://github.com/beretta42/platotermCoCo/tree/master/src/coco2

I'm looking for a detailed description of that calling convention,
with the goal of clarifying a few questions that I have on corner cases.

1. Is the first parameter still passed in B or X when the parameter is
   a 1- or 2-byte struct or union?

2. When the return value is one or two bytes, which register is it
   returned in?

3. When the function must return a struct, a union, a long, a float
   or a double, does it receive a hidden first parameter that points
   to the location to be filled with the return value?
   If so, is that hidden parameter received in X?
   What if the return type is a 1- or 2-byte struct or union?

Thanks for any leads...

The CMOC home page: http://sarrazip.com/dev/cmoc.html

-- 
Pierre Sarrazin <sarrazip @ sarrazip . com>