sashti srinivasan <svasn_tcpip@xxxxxxxxxxx> writes:
> > > The following is a line in rtems' timer driver
> > > source.
> > > asm volatile(".byte 0x0F, 0x31" : "=A"
> > (result));
...
> Lots of thanks for the clarifications. Sorry for
> further simple doubts. The rdtsc instruction stores
> the value(of time) in registers eax and edx, and you
> have said that compiler puts data from these registers
> into the variable 'result'. But we are not specifying
> the registers(eax,edx) in this statement. How the
> compiler knows that variable 'result' should be
> updated based on these registers?(eax,edx do not
> figure in this statement).
>
> If these are documented anywhere, please give me
> pointers.
The compiler knows that the results winds up in %eax/%edx due to use
of the constraint 'A' on the value to be stored into result. This is
documented, sort of, here:
http://gcc.gnu.org/onlinedocs/gccint/Machine-Constraints.html#Machine%20Constraints
Look for "Intel 386".
Ian
