in x86 inline assembly i can write something like this:
asm ("cpuid" : "=a" (_eax), "=b" (_ebx), "=c" (_ecx), "=d" (_edx) : "a" (op));
so in the matchin constraints instead of just writing "=r" and let the compiler chose the register, I can say which particular register i want to use (=a for example to use %eax)
how can I do this for ARM assembly? the ARM GCC assembly cookbook http://www.ethernut.de/en/documents/arm-inline-asm.html states that i can for example use the constraints "r" for one of the general purpose registers R0-R15 "w" for one of the VFP floating point registers S0-S31
but how can I constraint an operand for example exactly to s1? or to a particular general purpose register?