ARM inline assembly - specify individual register as constraint

Question

Hi!

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?

thank you!

Answer 1

I don't think gcc for ARM allows you to use constraints to specify exactly which register to use. However, you can use explicit register variables to specify a register to store a variable in:

register int my_variable asm("r0");