Hi

I gave you a good hint with the “-Cl” (static locals) option but I may help you more if you let me see your code.
By the way I am also writing a manual on how to code portable and optimal C code for vintage 8-bit systems.

Ideally I would recommend that you make a public GitHub (or equivalent) page.

Some portions of your code may even become part of CC65.

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Will you integrate some 6809 targets into your SDK? Which compiler will you be using?

I am using CMOC but GCC6809 is also an option. I chose CMOC because it is maintained more than GCC6809 and its usage is straightforward.

Including Z80 targets into your SDK should be very simple (at least for graphics and sounds).
Have you looked at Z88DK graphics and sound APIs?
Z88DK provides already everything to make univeral graphics and sounds on totally different systems (within the Z80 architecture).
In my universal library I have written abstract APIs to cover all dev-kits. You can take a look at my code to get some ideas.

Fabrizio