--- Ian Lance Taylor <ian@xxxxxxxxxxxxxxxxx> wrote: >
sashti srinivasan <svasn_tcpip@xxxxxxxxxxx> writes:
>
> > The following is a line in rtems' timer driver
> > source.
> > asm volatile(".byte 0x0F, 0x31" : "=A"
> (result));
> >
> > RTEMS mailing list clarified me saying that it
> is
> > encoding of pentium rdtsc instruction. Please
> tell me
> > how the above statement differs from the one
> below.
> >
> > asm("rdtsc result");
> > (Assuming the instruction mnemonic is correct)
>
> First I'll note that the assembly instruction rdtsc
> does not take an
> argument. The only legal asm would be asm
> ("rdtsc"). I assume that
> the RTEMS example uses ".byte" because it was
> written before all
> assemblers supported rdtsc. Or something like that.
>
> The difference between the plan asm and the one
> which appears above is
> that the one which appears above explicitly states
> that the
> instruction stores a result into %eax/%edx, and gcc
> will store that
> value into the variable "result".
>
> > Only thing I found from gnu documentation for
> gcc
> > that '=' specifies that result is a output of the
> > instruction. According to the document, there
> must
> > be an instruction pnemonic string first inside
> the
> > paranthesis. But there is the directive .byte
> > there. Does this mean the opcode directly?
>
> In this case .byte is the instruction mnemonic. The
> '=' does indeed
> specify the result of the instruction.
>
> > The document also says that the arguments can
> be
> > referred like %0, %1 etc. But there is nothing
> like
> > this here. Is it because, since there is only
> one
> > operand, it is assumed to follow the opcode?
>
> No. The instruction takes no operand. The
> instruction always stores
> its value into %eax/%edx. See the Intel
> documentation, which is
> available on the web.
>
> Ian
Hello,
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.
With Regards
Srinivasan
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