[avr-gcc-list] C<->asm docs/examples?

[David McNab]

Hi,

Can anyone please point me to some resources for learning about
interfacing C and assembler modules in gcc-avr?

I've been looking at the asmdemo code from avr-libc, but it only goes
part way with overall explanation.

I'm especially needing to learn how to write assembler (.S) modules
containing C-callable functions, and how to handle issues like:

 - how to access the arguments passed in by the calling C function
 - how to safely procure registers for temporary storage, without
   breaking (or being broken by) any neighbouring C code
 - how to allocate and access global variables, and make these
   available to C code
 - how to access C global variables
 - how to return values to the C caller
 - how to call C functions - how to pass arguments and retrieve
   the return value

I'd much rather steer away from using inline assembler inside C
functions. The syntax is not to my taste, to say the least.

Back in my Z80 and 8086 C/assembler hacking days, it was way simple. All
C funcs/vars were just prefixed with an underscore, arguments were
pushed onto the return stack along with the return address, and returned
values were passed back through clearly-defined registers.

The code in /doc/examples/asmdemo in the avr-libc tree is only limited
value, because all the assembler code seems to be just ISRs, with no
calling between C and assembler.

Can anyone point me to some simple examples?

Or should I just trawl through the lower layers of avr-libc source and
pick it up by osmosis?

Thanks in advance for any help.

Cheers
David

PS - my current project is a minimalistic dual-stack VM. The core VM
engine, written in straight C, weighs in at a hefty 150 bytes, which is
an embarrassing disgrace, and needs some serious liposuction, down to
about 50-80 bytes, if the VM is to be viable on the smaller ATtiny MCUs.