constraints for saving stack pointer

[peter novodvorsky]

Hello,

I need to run code with non-c calling conventiuons. I'm making run
function that jumps to the code setting stack pointer and frame
pointer to the places I need (stack is bootstrapped in another part of
code). What I want to do is to 
1). save general registers on stack
2). save stack pointer
3). run the code (actually it can smash all the general registers)
4). load stack pointer
5). load registers
The problem is that I don't know where to save stack pointer. I can't
save is in general register because it will get smashed. I want to
save it to variable with fixed address that can be inlined in asm. How
can I pass that variable to asm() (with which constraints) so GCC
would inline it's address as %[var]?

thanks,
Peter.